process_builder/

process_builder.rs

// Copyright 2023 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

use crate::{elf_load, elf_parse, process_args, util};
use anyhow::{anyhow, Context};
use fidl::endpoints::{ClientEnd, Proxy};
use fuchsia_async::{self as fasync, TimeoutExt};
use fuchsia_runtime::{HandleInfo, HandleType};
use futures::prelude::*;
use std::ffi::{CStr, CString};
use std::{iter, mem};
use thiserror::Error;
use zx::{self as zx, AsHandleRef, HandleBased};
use {fidl_fuchsia_io as fio, fidl_fuchsia_ldsvc as fldsvc};

/// Error type returned by ProcessBuilder methods.
#[derive(Error, Debug)]
pub enum ProcessBuilderError {
    #[error("{}", _0)]
    InvalidArg(String),
    #[error("{}", _0)]
    BadHandle(&'static str),
    #[error("Failed to create process: {}", _0)]
    CreateProcess(zx::Status),
    #[error("Failed to create thread: {}", _0)]
    CreateThread(zx::Status),
    #[error("Failed to start process: {}", _0)]
    ProcessStart(zx::Status),
    #[error("Failed to parse ELF: {}", _0)]
    ElfParse(#[from] elf_parse::ElfParseError),
    #[error("Failed to load ELF: {}", _0)]
    ElfLoad(#[from] elf_load::ElfLoadError),
    #[error("{}", _0)]
    ProcessArgs(#[from] process_args::ProcessargsError),
    #[error("{}: {}", _0, _1)]
    GenericStatus(&'static str, zx::Status),
    #[error("{}: {}", _0, _1)]
    Internal(&'static str, #[source] anyhow::Error),
    #[error("Invalid PT_INTERP header: {}", _0)]
    InvalidInterpHeader(#[source] anyhow::Error),
    #[error("Failed to build process with dynamic ELF, missing fuchsia.ldsvc.Loader handle")]
    LoaderMissing(),
    #[error("Failed to load dynamic linker from fuchsia.ldsvc.Loader: {}", _0)]
    LoadDynamicLinker(#[source] fidl::Error),
    #[error("Timed out loading dynamic linker from fuchsia.ldsvc.Loader")]
    LoadDynamicLinkerTimeout(),
    #[error("Failed to write bootstrap message to channel: {}", _0)]
    WriteBootstrapMessage(zx::Status),
    #[error("Failed to destroy reservation VMAR: {}", _0)]
    DestroyReservationVMAR(zx::Status),
    #[error("Failed to duplicate handle of type {:?}: {}", _0, _1)]
    DuplicateHandle(HandleType, zx::Status),
}

impl ProcessBuilderError {
    /// Returns an appropriate zx::Status code for the given error.
    pub fn as_zx_status(&self) -> zx::Status {
        match self {
            ProcessBuilderError::InvalidArg(_)
            | ProcessBuilderError::InvalidInterpHeader(_)
            | ProcessBuilderError::LoaderMissing() => zx::Status::INVALID_ARGS,
            ProcessBuilderError::BadHandle(_) => zx::Status::BAD_HANDLE,
            ProcessBuilderError::CreateProcess(s)
            | ProcessBuilderError::CreateThread(s)
            | ProcessBuilderError::ProcessStart(s)
            | ProcessBuilderError::GenericStatus(_, s)
            | ProcessBuilderError::WriteBootstrapMessage(s)
            | ProcessBuilderError::DestroyReservationVMAR(s) => *s,
            ProcessBuilderError::ElfParse(e) => e.as_zx_status(),
            ProcessBuilderError::ElfLoad(e) => e.as_zx_status(),
            ProcessBuilderError::ProcessArgs(e) => e.as_zx_status(),
            ProcessBuilderError::Internal(_, _) => zx::Status::INTERNAL,
            ProcessBuilderError::LoadDynamicLinker(_) => zx::Status::NOT_FOUND,
            ProcessBuilderError::LoadDynamicLinkerTimeout() => zx::Status::TIMED_OUT,
            ProcessBuilderError::DuplicateHandle(_, s) => *s,
        }
    }
}

/// A container for a single namespace entry, containing a path and a directory handle. Used as an
/// input to [ProcessBuilder::add_namespace_entries()].
pub struct NamespaceEntry {
    /// Namespace path.
    pub path: CString,

    /// Namespace directory handle.
    pub directory: ClientEnd<fio::DirectoryMarker>,
}

/// The main builder type for this crate. Collects inputs and creates a new process.
///
/// See top-level crate documentation for a usage example.
pub struct ProcessBuilder {
    /// The ELF binary for the new process.
    executable: zx::Vmo,
    /// The fuchsia.ldsvc.Loader service to use for the new process, if dynamically linked.
    ldsvc: Option<fldsvc::LoaderProxy>,
    /// A non-default vDSO to use for the new process, if any.
    non_default_vdso: Option<zx::Vmo>,
    /// The contents of the main process_args message to be sent to the new process.
    msg_contents: process_args::MessageContents,
    /// Handles that are common to both the linker and main process_args messages, wrapped in an
    /// inner struct for code organization and clarity around borrows.
    common: CommonMessageHandles,
    /// Minimum size of the stack for the new process, in bytes.
    min_stack_size: usize,
    /// The default system vDSO.
    system_vdso_vmo: zx::Vmo,
}

struct CommonMessageHandles {
    process: zx::Process,
    thread: zx::Thread,
    root_vmar: zx::Vmar,
}

/// A container for a fully built but not yet started (as in, its initial thread is not yet
/// running) process, with all related handles and metadata. Output of [ProcessBuilder::build()].
///
/// You can use this struct to start the process with [BuiltProcess::start()], which is a simple
/// wrapper around the [zx_process_start] syscall. You can optionally use the handles and
/// information in this struct to manipulate the process or its address space before starting it,
/// such as when creating a process in a debugger.
///
/// [zx_process_start]: https://fuchsia.dev/fuchsia-src/reference/syscalls/process_start.md
pub struct BuiltProcess {
    /// The newly created process.
    pub process: zx::Process,

    /// The root VMAR for the created process.
    pub root_vmar: zx::Vmar,

    /// The process's initial thread.
    pub thread: zx::Thread,

    /// The process's entry point.
    pub entry: usize,

    /// The initial thread's stack pointer.
    pub stack: usize,

    /// The base address of the stack for the initial thread.
    pub stack_base: usize,

    /// The VMO of the stack for the initial thread.
    pub stack_vmo: zx::Vmo,

    /// The bootstrap channel, to be passed to the process on start as arg1 in zx_process_start /
    /// zx::Process::start.
    pub bootstrap: zx::Channel,

    /// The base address of the VDSO in the process's VMAR, to be passed to the process on start as
    /// arg2 in zx_process_start / zx::Process::start.
    pub vdso_base: usize,

    /// The base address where the ELF executable, or the dynamic linker if the ELF was dynamically
    /// linked, was loaded in the process's VMAR.
    pub elf_base: usize,

    /// The ELF headers of the main module of the newly created process.
    pub elf_headers: elf_parse::Elf64Headers,
}

struct StackInfo {
    /// The initial thread's stack pointer.
    pub stack_ptr: usize,

    /// The base address of the stack for the initial thread.
    pub stack_base: usize,

    /// The VMO of the stack for the initial thread.
    pub stack_vmo: zx::Vmo,
}

impl ProcessBuilder {
    /// Create a new ProcessBuilder that can be used to create a new process under the given job
    /// with the given name and ELF64 executable (as a VMO).
    ///
    /// This job is only used to create the process and thus is not taken ownership of. To provide
    /// a default job handle to be passed to the new process, use [ProcessBuilder::add_handles()]
    /// with [HandleType::DefaultJob].
    ///
    /// The provided VMO must have the [zx::Rights::EXECUTE] right.
    ///
    /// # Errors
    ///
    /// Returns Err([ProcessBuilderError::CreateProcess]) if process creation fails, such as if the
    /// process using this is disallowed direct process creation rights through job policy. See
    /// top-level crate documentation for more details.
    pub fn new(
        name: &CStr,
        job: &zx::Job,
        options: zx::ProcessOptions,
        executable: zx::Vmo,
        system_vdso_vmo: zx::Vmo,
    ) -> Result<ProcessBuilder, ProcessBuilderError> {
        if job.is_invalid_handle() {
            return Err(ProcessBuilderError::BadHandle("Invalid job handle"));
        }
        if executable.is_invalid_handle() {
            return Err(ProcessBuilderError::BadHandle("Invalid executable handle"));
        }

        // Creating the process immediately has the benefit that we fail fast if the calling
        // process does not have permission to create processes directly.
        let (process, root_vmar) = job
            .create_child_process(options, name.to_bytes())
            .map_err(ProcessBuilderError::CreateProcess)?;

        // Create the initial thread.
        let thread =
            process.create_thread(b"initial-thread").map_err(ProcessBuilderError::CreateThread)?;

        // Add duplicates of the process, VMAR, and thread handles to the bootstrap message.
        let msg_contents = process_args::MessageContents::default();
        let mut pb = ProcessBuilder {
            executable,
            ldsvc: None,
            non_default_vdso: None,
            msg_contents,
            common: CommonMessageHandles { process, thread, root_vmar },
            min_stack_size: 0,
            system_vdso_vmo,
        };
        pb.common.add_process_self(&mut pb.msg_contents)?;
        pb.common.add_thread_self(&mut pb.msg_contents)?;
        pb.common.add_root_vmar(&mut pb.msg_contents)?;
        Ok(pb)
    }

    /// Sets the fuchsia.ldsvc.Loader service for the process.
    ///
    /// The loader service is used to load dynamic libraries if the executable is a dynamically
    /// linked ELF file (i.e. if it contains a PT_INTERP header), and is required for such
    /// executables. It will only be provided to the new process in this case. Otherwise, it is
    /// unused and has no effect.
    ///
    /// If no loader service has been provided and it is needed, process creation will fail. Note
    /// that this differs from the automatic fallback behavior of previous process creation
    /// libraries, which would clone the loader of the current process. This fallback is likely to
    /// fail in subtle and confusing ways. An appropriate loader service that has access to the
    /// libraries or interpreter must be provided.
    ///
    /// Note that [ProcessBuilder::add_handles()] will automatically pass a handle with type
    /// [HandleType::LdsvcLoader] to this function.
    ///
    /// If called multiple times (e.g. if a loader was initially provided through
    /// [ProcessBuilder::add_handles()] and you want to replace it), the new loader replaces the
    /// previous and the handle to the previous loader is dropped.
    pub fn set_loader_service(
        &mut self,
        ldsvc: ClientEnd<fldsvc::LoaderMarker>,
    ) -> Result<(), ProcessBuilderError> {
        if ldsvc.is_invalid_handle() {
            return Err(ProcessBuilderError::BadHandle("Invalid loader service handle"));
        }
        self.ldsvc = Some(ldsvc.into_proxy());
        Ok(())
    }

    /// Sets the vDSO VMO for the process.
    pub fn set_vdso_vmo(&mut self, vdso: zx::Vmo) {
        self.non_default_vdso = Some(vdso);
    }

    /// Add arguments to the process's bootstrap message. Successive calls append (not replace)
    /// arguments.
    pub fn add_arguments(&mut self, mut args: Vec<CString>) {
        self.msg_contents.args.append(&mut args);
    }

    /// Add environment variables to the process's bootstrap message. Successive calls append (not
    /// replace) environment variables.
    pub fn add_environment_variables(&mut self, mut vars: Vec<CString>) {
        self.msg_contents.environment_vars.append(&mut vars);
    }

    /// Set the minimum size of the stack for the new process, in bytes.
    pub fn set_min_stack_size(&mut self, size: usize) {
        self.min_stack_size = size;
    }

    /// Add handles to the process's bootstrap message. Successive calls append (not replace)
    /// handles.
    ///
    /// Each [process_args::StartupHandle] contains a [zx::Handle] object accompanied by a [HandleInfo] object
    /// that includes the handle type and a type/context-dependent argument.
    ///
    /// A [HandleType::LdsvcLoader] handle will automatically be passed along to
    /// [ProcessBuilder::set_loader_service()] if provided through this function.
    ///
    /// # Errors
    ///
    /// [HandleType::NamespaceDirectory] handles should not be added through this function since
    /// they must be accompanied with a path. Use [ProcessBuilder::add_namespace_entries()] for
    /// that instead.
    ///
    /// The following handle types cannot be added through this, as they are added automatically by
    /// the ProcessBuilder:
    /// * [HandleType::ProcessSelf]
    /// * [HandleType::ThreadSelf]
    /// * [HandleType::RootVmar]
    /// * [HandleType::LoadedVmar]
    /// * [HandleType::StackVmo]
    /// * [HandleType::ExecutableVmo]
    pub fn add_handles(
        &mut self,
        startup_handles: Vec<process_args::StartupHandle>,
    ) -> Result<(), ProcessBuilderError> {
        // Do a bit of validation before adding to the bootstrap handles.
        for h in &startup_handles {
            if h.handle.is_invalid() {
                return Err(ProcessBuilderError::BadHandle("Invalid handle in startup handles"));
            }

            let t = h.info.handle_type();
            match t {
                HandleType::NamespaceDirectory => {
                    return Err(ProcessBuilderError::InvalidArg(
                        "Cannot add NamespaceDirectory handles directly, use add_namespace_entries"
                            .into(),
                    ));
                }
                HandleType::ProcessSelf
                | HandleType::ThreadSelf
                | HandleType::RootVmar
                | HandleType::LoadedVmar
                | HandleType::StackVmo
                | HandleType::ExecutableVmo => {
                    return Err(ProcessBuilderError::InvalidArg(format!(
                        "Cannot add a {:?} handle directly, it will be automatically added",
                        t,
                    )));
                }
                _ => {}
            }
        }

        // Intentionally separate from validation so that we don't partially add namespace entries.
        for h in startup_handles {
            match h.info.handle_type() {
                HandleType::LdsvcLoader => {
                    // Automatically pass this to |set_loader_service| instead.
                    self.set_loader_service(ClientEnd::from(h.handle))?;
                }
                HandleType::VdsoVmo => {
                    if h.info.arg() == 0 {
                        self.set_vdso_vmo(h.handle.into());
                    } else {
                        // Pass any additional vDSOs.
                        self.msg_contents.handles.push(h);
                    }
                }
                _ => {
                    self.msg_contents.handles.push(h);
                }
            }
        }
        Ok(())
    }

    /// Add directories to the process's namespace.
    ///
    /// Successive calls append new namespace entries, not replace previous entries.
    ///
    /// Each [NamespaceEntry] contains a client connection to a fuchsia.io.Directory FIDL service
    /// and a path to add that directory to the process's namespace as.
    ///
    /// # Errors
    ///
    /// Returns Err([ProcessBuilderError::InvalidArg]) if the maximum number of namespace entries
    /// (2^16) was reached and the entry could not be added. This is exceedingly unlikely, and most
    /// likely if you are anywhere near this limit [ProcessBuilder::build()] will fail because the
    /// process's process_args startup message is over its own length limit.
    pub fn add_namespace_entries(
        &mut self,
        mut entries: Vec<NamespaceEntry>,
    ) -> Result<(), ProcessBuilderError> {
        // Namespace entries are split into a namespace path, that is included in the bootstrap
        // message (as the so-called "namespace table"), plus a NamespaceDirectory handle, where the arg
        // value is the index of the path in the namespace table.
        //
        // Check that the namespace table doesn't exceed 2^16 entries, since the HandleInfo arg is
        // only 16-bits. Realistically this will never matter - if you're anywhere near this
        // many entries, you're going to exceed the bootstrap message length limit - but Rust
        // encourages us (and makes it easy) to be safe about the edge case here.
        let mut idx = u16::try_from(self.msg_contents.namespace_paths.len())
            .expect("namespace_paths.len should never be larger than a u16");
        let num_entries = u16::try_from(entries.len())
            .map_err(|_| ProcessBuilderError::InvalidArg("Too many namespace entries".into()))?;
        if idx.checked_add(num_entries).is_none() {
            return Err(ProcessBuilderError::InvalidArg(
                "Can't add namespace entries, limit reached".into(),
            ));
        }

        for entry in &entries {
            if entry.directory.is_invalid_handle() {
                return Err(ProcessBuilderError::BadHandle("Invalid handle in namespace entry"));
            }
        }

        // Intentionally separate from validation so that we don't partially add namespace entries=
        for entry in entries.drain(..) {
            self.msg_contents.namespace_paths.push(entry.path);
            self.msg_contents.handles.push(process_args::StartupHandle {
                handle: zx::Handle::from(entry.directory),
                info: HandleInfo::new(HandleType::NamespaceDirectory, idx),
            });
            idx += 1;
        }
        Ok(())
    }

    /// Build the new process using the data and handles provided to the ProcessBuilder.
    ///
    /// The return value of this function is a [BuiltProcess] struct which contains the new process
    /// and all the handles and data needed to start it, but the process' initial thread is not yet
    /// started. Use [BuiltProcess::start()] or the [zx_process_start] syscall to actually start
    /// it.
    ///
    /// # Errors
    ///
    /// There are many errors that could result during process loading and only some are listed
    /// here. See [ProcessBuilderError] for the various error types that can be returned.
    ///
    /// Returns Err([ProcessBuilderError::LoaderMissing]) if the ELF executable is dynamically
    /// linked (has a PT_INTERP program header) but no loader service has been provided through
    /// [ProcessBuilder::set_loader_service()] or [ProcessBuilder::add_handles()].
    ///
    /// [zx_process_start]: https://fuchsia.dev/fuchsia-src/reference/syscalls/process_start.md
    pub async fn build(mut self) -> Result<BuiltProcess, ProcessBuilderError> {
        // Parse the executable as an ELF64 file, reading in the headers we need. Done first since
        // this is most likely to be invalid and error out.
        let elf_headers = elf_parse::Elf64Headers::from_vmo(&self.executable)?;

        // Create bootstrap message channel.
        let (bootstrap_rd, bootstrap_wr) = zx::Channel::create();

        // Check if main executable is dynamically linked, and handle appropriately.
        let loaded_elf;
        let mut reserve_vmar = None;
        let dynamic;
        if let Some(interp_hdr) =
            elf_headers.program_header_with_type(elf_parse::SegmentType::Interp)?
        {
            // Dynamically linked so defer loading the main executable to the dynamic
            // linker/loader, which we load here instead.
            dynamic = true;

            // Check that a ldsvc.Loader service was provided.
            let ldsvc = self.ldsvc.take().ok_or(ProcessBuilderError::LoaderMissing())?;

            // A process using PT_INTERP might be loading a libc.so that supports sanitizers;
            // reserve the low address region for sanitizers to allocate shadow memory.
            //
            // The reservation VMAR ensures that the initial allocations & mappings made in this
            // function stay out of this area. It is destroyed below before returning and the
            // process's own allocations can use the full address space.
            //
            // !! WARNING: This makes a specific address VMAR allocation, so it must come before
            // any elf_load::load_elf calls. !!
            reserve_vmar =
                Some(ReservationVmar::reserve_low_address_space(&self.common.root_vmar)?);

            // Get the dynamic linker and map it into the process's address space.
            let ld_vmo = get_dynamic_linker(&ldsvc, &self.executable, interp_hdr).await?;
            let ld_headers = elf_parse::Elf64Headers::from_vmo(&ld_vmo)?;
            loaded_elf = elf_load::load_elf(&ld_vmo, &ld_headers, &self.common.root_vmar)?;

            // Build the dynamic linker bootstrap message and write it to the bootstrap channel.
            // This message is written before the primary bootstrap message since it is consumed
            // first in the dynamic linker.
            let executable = mem::replace(&mut self.executable, zx::Handle::invalid().into());
            let msg = self.build_linker_message(ldsvc, executable, loaded_elf.vmar)?;
            msg.write(&bootstrap_wr).map_err(ProcessBuilderError::WriteBootstrapMessage)?;
        } else {
            // Statically linked but still position-independent (ET_DYN) ELF, load directly.
            dynamic = false;

            loaded_elf =
                elf_load::load_elf(&self.executable, &elf_headers, &self.common.root_vmar)?;
            self.msg_contents.handles.push(process_args::StartupHandle {
                handle: loaded_elf.vmar.into_handle(),
                info: HandleInfo::new(HandleType::LoadedVmar, 0),
            });
        }

        // Load the vDSO - either the default system vDSO, or the user-provided one - into the
        // process's address space and a handle to it to the bootstrap message.
        let vdso_base = self.load_vdso()?;

        // Calculate initial stack size.
        let mut stack_size;
        let stack_vmo_name;
        if dynamic {
            // Calculate the initial stack size for the dynamic linker. This factors in the size of
            // an extra handle for the stack that hasn't yet been added to the message contents,
            // since creating the stack requires this size.
            stack_size = calculate_initial_linker_stack_size(&mut self.msg_contents, 1)?;
            stack_vmo_name = format!("stack: msg of {:#x?}", stack_size);
        } else {
            // Set stack size from PT_GNU_STACK header, if present, or use the default. The dynamic
            // linker handles this for dynamically linked ELFs (above case).
            const ZIRCON_DEFAULT_STACK_SIZE: usize = 256 << 10; // 256KiB
            let mut ss = ("default", ZIRCON_DEFAULT_STACK_SIZE);
            if let Some(stack_hdr) =
                elf_headers.program_header_with_type(elf_parse::SegmentType::GnuStack)?
            {
                if stack_hdr.memsz > 0 {
                    ss = ("explicit", stack_hdr.memsz as usize);
                }
            }

            // Stack size must be page aligned.
            stack_size = util::page_end(ss.1);
            stack_vmo_name = format!("stack: {} {:#x?}", ss.0, stack_size);
        }
        if stack_size < self.min_stack_size {
            stack_size = util::page_end(self.min_stack_size);
        }

        // Allocate the initial thread's stack, map it, and add a handle to the bootstrap message.
        let stack_vmo_name =
            zx::Name::new(&stack_vmo_name).expect("Stack VMO name must be less than 31 bytes");
        let stack_info = self.create_stack(stack_size, &stack_vmo_name)?;

        // Build and send the primary bootstrap message.
        let msg = process_args::Message::build(self.msg_contents)?;
        msg.write(&bootstrap_wr).map_err(ProcessBuilderError::WriteBootstrapMessage)?;

        // Explicitly destroy the reservation VMAR before returning so that we can be sure it is
        // gone (so we don't end up with a process with half its address space gone).
        if let Some(mut r) = reserve_vmar {
            r.destroy().map_err(ProcessBuilderError::DestroyReservationVMAR)?;
        }

        Ok(BuiltProcess {
            process: self.common.process,
            root_vmar: self.common.root_vmar,
            thread: self.common.thread,
            entry: loaded_elf.entry,
            stack: stack_info.stack_ptr,
            stack_base: stack_info.stack_base,
            stack_vmo: stack_info.stack_vmo,
            bootstrap: bootstrap_rd,
            vdso_base: vdso_base,
            elf_base: loaded_elf.vmar_base,
            elf_headers,
        })
    }

    /// Build the bootstrap message for the dynamic linker, which uses the same process_args
    /// protocol as the message for the main process but somewhat different contents.
    ///
    /// The LoaderProxy provided must be ready to be converted to a Handle with into_channel(). In
    /// other words, there must be no other active clones of the proxy, no open requests, etc. The
    /// intention is that the user provides a handle only (perhaps wrapped in a ClientEnd) through
    /// [ProcessBuilder::set_loader_service()], not a Proxy, so the library can be sure this
    /// invariant is maintained and a failure is a library bug.
    fn build_linker_message(
        &self,
        ldsvc: fldsvc::LoaderProxy,
        executable: zx::Vmo,
        loaded_vmar: zx::Vmar,
    ) -> Result<process_args::Message, ProcessBuilderError> {
        // Don't need to use the ldsvc.Loader anymore; turn it back into into a raw handle so
        // we can pass it along in the dynamic linker bootstrap message.
        let ldsvc_hnd =
            ldsvc.into_channel().expect("Failed to get channel from LoaderProxy").into_zx_channel();

        // The linker message only needs a subset of argv and envvars.
        let args = extract_ld_arguments(&self.msg_contents.args);
        let environment_vars =
            extract_ld_environment_variables(&self.msg_contents.environment_vars);

        let mut linker_msg_contents = process_args::MessageContents {
            // Argument strings are sent to the linker so that it can use argv[0] in messages it
            // prints.
            args,
            // Environment variables are sent to the linker so that it can see vars like LD_DEBUG.
            environment_vars,
            // Process namespace is not set up or used in the linker.
            namespace_paths: vec![],
            // Loader message includes a few special handles needed to do its job, plus a set of
            // handles common to both messages which are generated by this library.
            handles: vec![
                process_args::StartupHandle {
                    handle: ldsvc_hnd.into_handle(),
                    info: HandleInfo::new(HandleType::LdsvcLoader, 0),
                },
                process_args::StartupHandle {
                    handle: executable.into_handle(),
                    info: HandleInfo::new(HandleType::ExecutableVmo, 0),
                },
                process_args::StartupHandle {
                    handle: loaded_vmar.into_handle(),
                    info: HandleInfo::new(HandleType::LoadedVmar, 0),
                },
            ],
        };
        self.common.add_process_self(&mut linker_msg_contents)?;
        self.common.add_root_vmar(&mut linker_msg_contents)?;
        Ok(process_args::Message::build(linker_msg_contents)?)
    }

    /// Load the vDSO VMO into the process's address space and a handle to it to the bootstrap
    /// message. If a vDSO VMO is provided, loads that one, otherwise loads the default system
    /// vDSO, invaliding the duplicate default system vDSO handle stored in this object.
    /// Returns the base address that the vDSO was mapped into.
    fn load_vdso(&mut self) -> Result<usize, ProcessBuilderError> {
        let vdso = match self.non_default_vdso.take() {
            Some(vmo) => vmo,
            None => mem::replace(&mut self.system_vdso_vmo, zx::Handle::invalid().into()),
        };
        let vdso_headers = elf_parse::Elf64Headers::from_vmo(&vdso)?;
        let loaded_vdso = elf_load::load_elf(&vdso, &vdso_headers, &self.common.root_vmar)?;

        self.msg_contents.handles.push(process_args::StartupHandle {
            handle: vdso.into_handle(),
            info: HandleInfo::new(HandleType::VdsoVmo, 0),
        });

        Ok(loaded_vdso.vmar_base)
    }

    /// Allocate the initial thread's stack, map it, and add a handle to the bootstrap message.
    /// Returns the initial stack pointer for the process.
    ///
    /// Note that launchpad supported not allocating a stack at all, but that only happened if an
    /// explicit stack size of 0 is set. ProcessBuilder does not support overriding the stack size
    /// so a stack is always created.
    fn create_stack(
        &mut self,
        stack_size: usize,
        vmo_name: &zx::Name,
    ) -> Result<StackInfo, ProcessBuilderError> {
        let stack_vmo = zx::Vmo::create(stack_size as u64).map_err(|s| {
            ProcessBuilderError::GenericStatus("Failed to create VMO for initial thread stack", s)
        })?;
        stack_vmo
            .set_name(vmo_name)
            .map_err(|s| ProcessBuilderError::GenericStatus("Failed to set stack VMO name", s))?;
        let stack_flags = zx::VmarFlags::PERM_READ | zx::VmarFlags::PERM_WRITE;
        let stack_base =
            self.common.root_vmar.map(0, &stack_vmo, 0, stack_size, stack_flags).map_err(|s| {
                ProcessBuilderError::GenericStatus("Failed to map initial stack", s)
            })?;
        let stack_ptr = compute_initial_stack_pointer(stack_base, stack_size);
        let dup_stack_vmo = stack_vmo.duplicate_handle(zx::Rights::SAME_RIGHTS).map_err(|s| {
            ProcessBuilderError::GenericStatus("Failed to duplicate initial stack", s)
        })?;

        // Pass the stack VMO to the process. Our protocol with the new process is that we warrant
        // that this is the VMO from which the initial stack is mapped and that we've exactly
        // mapped the entire thing, so vm_object_get_size on this in concert with the initial SP
        // value tells it the exact bounds of its stack.
        self.msg_contents.handles.push(process_args::StartupHandle {
            handle: dup_stack_vmo.into_handle(),
            info: HandleInfo::new(HandleType::StackVmo, 0),
        });

        Ok(StackInfo { stack_ptr, stack_base, stack_vmo })
    }
}

/// Calculate the size of the initial stack to allocate for the dynamic linker, based on the given
/// process_args message contents.
///
/// The initial stack is used just for startup work in the dynamic linker and to hold the bootstrap
/// message, so we only attempt to make it only as big as needed. The size returned is based on the
/// stack space needed to read the bootstrap message with zx_channel_read, and thus includes the
/// message data itself plus the size of the handles (i.e. the size of N zx_handle_t's).
///
/// This also allows the caller to specify an number of "extra handles" to factor into the size
/// calculation. This allows the size to be calculated before all the real handles have been added
/// to the contents, for example if the size is needed to create those handles.
pub fn calculate_initial_linker_stack_size(
    msg_contents: &mut process_args::MessageContents,
    extra_handles: usize,
) -> Result<usize, ProcessBuilderError> {
    // Add N placeholder handles temporarily to factor in the size of handles that are not yet
    // added to the message contents.
    msg_contents.handles.extend(
        iter::repeat_with(|| process_args::StartupHandle {
            handle: zx::Handle::invalid(),
            info: HandleInfo::new(HandleType::User0, 0),
        })
        .take(extra_handles),
    );

    // Include both the message data size and the size of the handles since we're calculating the
    // stack space required to read the message.
    let num_handles = msg_contents.handles.len();
    let msg_stack_size = process_args::Message::calculate_size(msg_contents)?
        + num_handles * mem::size_of::<zx::sys::zx_handle_t>();
    msg_contents.handles.truncate(num_handles - extra_handles);

    // PTHREAD_STACK_MIN is defined by the C library in
    // //zircon/third_party/ulib/musl/include/limits.h. It is tuned enough to cover the dynamic
    // linker and C library startup code's stack usage (up until the point it switches to its own
    // stack in __libc_start_main), but leave a little space so for small bootstrap message sizes
    // the stack needs only one page.
    const PTHREAD_STACK_MIN: usize = 3072;
    Ok(util::page_end(msg_stack_size + PTHREAD_STACK_MIN))
}

/// Extract only the arguments that are needed for a linker message.
fn extract_ld_arguments(arguments: &[CString]) -> Vec<CString> {
    let mut extracted = vec![];

    if let Some(argument) = arguments.get(0) {
        extracted.push(argument.clone())
    }

    extracted
}

/// Extract only the environment variables that are needed for a linker message.
fn extract_ld_environment_variables(envvars: &[CString]) -> Vec<CString> {
    let prefixes = ["LD_DEBUG=", "LD_TRACE="];

    let mut extracted = vec![];
    for envvar in envvars {
        for prefix in &prefixes {
            let envvar_bytes: &[u8] = envvar.to_bytes();
            let prefix_bytes: &[u8] = prefix.as_bytes();
            if envvar_bytes.starts_with(prefix_bytes) {
                extracted.push(envvar.clone());
                continue;
            }
        }
    }

    extracted
}

impl CommonMessageHandles {
    fn add_process_self(
        &self,
        msg: &mut process_args::MessageContents,
    ) -> Result<(), ProcessBuilderError> {
        Self::add_to_message(msg, self.process.as_handle_ref(), HandleType::ProcessSelf)
    }

    fn add_thread_self(
        &self,
        msg: &mut process_args::MessageContents,
    ) -> Result<(), ProcessBuilderError> {
        Self::add_to_message(msg, self.thread.as_handle_ref(), HandleType::ThreadSelf)
    }

    fn add_root_vmar(
        &self,
        msg: &mut process_args::MessageContents,
    ) -> Result<(), ProcessBuilderError> {
        Self::add_to_message(msg, self.root_vmar.as_handle_ref(), HandleType::RootVmar)
    }

    /// Add a handle to the procargs message with `0` for its `arg`.
    fn add_to_message(
        msg: &mut process_args::MessageContents,
        handle: zx::HandleRef<'_>,
        handle_type: HandleType,
    ) -> Result<(), ProcessBuilderError> {
        let dup = handle
            .duplicate(zx::Rights::SAME_RIGHTS)
            .map_err(|s| ProcessBuilderError::DuplicateHandle(handle_type, s))?;
        msg.handles.push(process_args::StartupHandle {
            handle: dup,
            info: HandleInfo::new(handle_type, 0),
        });
        Ok(())
    }
}

/// Given the base and size of the stack block, compute the appropriate initial
/// SP value for an initial thread according to the C calling convention for the
/// machine.
///
/// Copied from, and must be kept in sync with:
/// //zircon/system/ulib/elf-psabi/include/lib/elf-psabi/sp.h
pub fn compute_initial_stack_pointer(base: usize, size: usize) -> usize {
    // Assume stack grows down.
    let mut sp = base.checked_add(size).expect("Overflow in stack pointer calculation");

    // The x86-64 and AArch64 ABIs require 16-byte alignment.
    // The 32-bit ARM ABI only requires 8-byte alignment, but 16-byte alignment is preferable for
    // NEON so use it there too.
    // RISC-V ABIs also require 16-byte alignment.
    sp &= 16usize.wrapping_neg();

    // The x86-64 ABI requires %rsp % 16 = 8 on entry.  The zero word at (%rsp) serves as the
    // return address for the outermost frame.
    #[cfg(target_arch = "x86_64")]
    {
        sp -= 8;
    }

    // The ARMv7 and ARMv8 ABIs both just require that SP be aligned, so just catch unknown archs.
    #[cfg(not(any(
        target_arch = "x86_64",
        target_arch = "arm",
        target_arch = "aarch64",
        target_arch = "riscv64"
    )))]
    {
        compile_error!("Unknown target_arch");
    }

    sp
}

/// Load the dynamic linker/loader specified in the PT_INTERP header via the fuchsia.ldsvc.Loader
/// handle.
pub async fn get_dynamic_linker<'a>(
    ldsvc: &'a fldsvc::LoaderProxy,
    executable: &'a zx::Vmo,
    interp_hdr: &'a elf_parse::Elf64ProgramHeader,
) -> Result<zx::Vmo, ProcessBuilderError> {
    // Read the dynamic linker name from the main VMO, based on the PT_INTERP header.
    let mut interp = vec![0u8; interp_hdr.filesz as usize];
    executable
        .read(&mut interp[..], interp_hdr.offset as u64)
        .map_err(|s| ProcessBuilderError::GenericStatus("Failed to read from VMO", s))?;
    // Trim null terminator included in filesz.
    match interp.pop() {
        Some(b'\0') => Ok(()),
        _ => Err(ProcessBuilderError::InvalidInterpHeader(anyhow!("Missing null terminator"))),
    }?;
    let interp_str = std::str::from_utf8(&interp)
        .context("Invalid UTF8")
        .map_err(ProcessBuilderError::InvalidInterpHeader)?;

    // Retrieve the dynamic linker as a VMO from fuchsia.ldsvc.Loader
    const LDSO_LOAD_TIMEOUT_SEC: i64 = 30;
    let load_fut =
        ldsvc.load_object(interp_str).map_err(ProcessBuilderError::LoadDynamicLinker).on_timeout(
            fasync::MonotonicInstant::after(zx::MonotonicDuration::from_seconds(
                LDSO_LOAD_TIMEOUT_SEC,
            )),
            || Err(ProcessBuilderError::LoadDynamicLinkerTimeout()),
        );
    let (status, ld_vmo) = load_fut.await?;
    zx::Status::ok(status).map_err(|s| {
        ProcessBuilderError::GenericStatus(
            "Failed to load dynamic linker from fuchsia.ldsvc.Loader",
            s,
        )
    })?;
    Ok(ld_vmo.ok_or(ProcessBuilderError::GenericStatus(
        "load_object status was OK but no VMO",
        zx::Status::INTERNAL,
    ))?)
}

impl BuiltProcess {
    /// Start an already built process.
    ///
    /// This is a simple wrapper around the [zx_process_start] syscall that consumes the handles
    /// and data in the BuiltProcess struct as needed.
    ///
    /// [zx_process_start]: https://fuchsia.dev/fuchsia-src/reference/syscalls/process_start.md
    pub fn start(self) -> Result<zx::Process, ProcessBuilderError> {
        self.process
            .start(
                &self.thread,
                self.entry,
                self.stack,
                self.bootstrap.into_handle(),
                self.vdso_base,
            )
            .map_err(ProcessBuilderError::ProcessStart)?;
        Ok(self.process)
    }
}

struct ReservationVmar(Option<zx::Vmar>);

impl ReservationVmar {
    /// Reserve the lower half of the address space of the given VMAR by allocating another VMAR.
    ///
    /// The VMAR wrapped by this reservation is automatically destroyed when the reservation
    /// is dropped.
    fn reserve_low_address_space(vmar: &zx::Vmar) -> Result<ReservationVmar, ProcessBuilderError> {
        let info = vmar
            .info()
            .map_err(|s| ProcessBuilderError::GenericStatus("Failed to get VMAR info", s))?;

        // Reserve the lower half of the full address space, not just half of the VMAR length.
        // (base+len) represents the full address space, assuming this is used with a root VMAR and
        // length extends to the end of the address space, including a region the kernel reserves
        // at the start of the space.
        // TODO(https://fxbug.dev/42099306): Clean up address space reservation to avoid unnecessary
        // reservations, which should also avoid the "fake" reservation in the else-clause.
        if let Some(reserve_size) =
            util::page_end((info.base + info.len) / 2).checked_sub(info.base)
        {
            let (reserve_vmar, reserve_base) =
                vmar.allocate(0, reserve_size, zx::VmarFlags::SPECIFIC).map_err(|s| {
                    ProcessBuilderError::GenericStatus("Failed to allocate reservation VMAR", s)
                })?;
            assert_eq!(reserve_base, info.base, "Reservation VMAR allocated at wrong address");

            Ok(ReservationVmar(Some(reserve_vmar)))
        } else {
            // The VMAR does not intersect the "bottom half," so return a success but without an
            // actual reservation.
            Ok(ReservationVmar(None))
        }
    }

    /// Destroy the reservation. The reservation is also automatically destroyed when
    /// ReservationVmar is dropped.
    ///
    /// VMARs are not destroyed when the handle is closed (by dropping), so we must explicit destroy
    /// it to release the reservation and allow the created process to use the full address space.
    fn destroy(&mut self) -> Result<(), zx::Status> {
        match self.0.take() {
            Some(vmar) => {
                // This is safe because there are no mappings in the region and it is not a region
                // in the current process.
                unsafe { vmar.destroy() }
            }
            None => Ok(()),
        }
    }
}

// This is probably unnecessary, but it feels wrong to rely on the side effect of the process's
// root VMAR going away. We explicitly call destroy if ProcessBuilder.build() succeeds and returns
// a BuiltProcess, in which case this will do nothing, and if build() fails then the new process
// and its root VMAR will get cleaned up along with this sub-VMAR.
impl Drop for ReservationVmar {
    fn drop(&mut self) {
        _ = self.destroy();
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use anyhow::Error;
    use assert_matches::assert_matches;
    use fidl::prelude::*;
    use fidl_test_processbuilder::{UtilMarker, UtilProxy};
    use lazy_static::lazy_static;
    use vfs::file::vmo::read_only;
    use vfs::pseudo_directory;
    use zerocopy::Ref;
    use {fidl_fuchsia_io as fio, fuchsia_async as fasync};

    extern "C" {
        fn dl_clone_loader_service(handle: *mut zx::sys::zx_handle_t) -> zx::sys::zx_status_t;
    }

    fn get_system_vdso_vmo() -> Result<zx::Vmo, ProcessBuilderError> {
        lazy_static! {
            static ref VDSO_VMO: zx::Vmo = {
                zx::Vmo::from(
                    fuchsia_runtime::take_startup_handle(HandleInfo::new(HandleType::VdsoVmo, 0))
                        .expect("Failed to take VDSO VMO startup handle"),
                )
            };
        }

        let vdso_dup = VDSO_VMO
            .duplicate_handle(zx::Rights::SAME_RIGHTS)
            .map_err(|s| ProcessBuilderError::GenericStatus("Failed to dup vDSO VMO handle", s))?;
        Ok(vdso_dup)
    }

    // Clone the current loader service to provide to the new test processes.
    fn clone_loader_service() -> Result<ClientEnd<fldsvc::LoaderMarker>, zx::Status> {
        let mut raw = 0;
        let status = unsafe { dl_clone_loader_service(&mut raw) };
        zx::Status::ok(status)?;

        let handle = unsafe { zx::Handle::from_raw(raw) };
        Ok(ClientEnd::new(zx::Channel::from(handle)))
    }

    fn connect_util(client: &zx::Channel) -> Result<UtilProxy, Error> {
        let (proxy, server) = zx::Channel::create();
        fdio::service_connect_at(&client, UtilMarker::PROTOCOL_NAME, server)
            .context("failed to connect to util service")?;
        Ok(UtilProxy::from_channel(fasync::Channel::from_channel(proxy)))
    }

    fn create_test_util_builder() -> Result<ProcessBuilder, Error> {
        const TEST_UTIL_BIN: &'static str = "/pkg/bin/process_builder_test_util";
        let file = fdio::open_fd(TEST_UTIL_BIN, fio::PERM_READABLE | fio::PERM_EXECUTABLE)?;
        let vmo = fdio::get_vmo_exec_from_file(&file)?;
        let job = fuchsia_runtime::job_default();

        let procname = CString::new(TEST_UTIL_BIN.to_owned())?;
        Ok(ProcessBuilder::new(
            &procname,
            &job,
            zx::ProcessOptions::empty(),
            vmo,
            get_system_vdso_vmo().unwrap(),
        )?)
    }

    // Common builder setup for all tests that start a test util process.
    fn setup_test_util_builder(set_loader: bool) -> Result<(ProcessBuilder, UtilProxy), Error> {
        let mut builder = create_test_util_builder()?;
        if set_loader {
            builder.add_handles(vec![process_args::StartupHandle {
                handle: clone_loader_service()?.into_handle(),
                info: HandleInfo::new(HandleType::LdsvcLoader, 0),
            }])?;
        }

        let (dir_client, dir_server) = zx::Channel::create();
        builder.add_handles(vec![process_args::StartupHandle {
            handle: dir_server.into_handle(),
            info: HandleInfo::new(HandleType::DirectoryRequest, 0),
        }])?;

        let proxy = connect_util(&dir_client)?;
        Ok((builder, proxy))
    }

    fn check_process_running(process: &zx::Process) -> Result<(), Error> {
        let info = process.info()?;
        const STARTED: u32 = zx::ProcessInfoFlags::STARTED.bits();
        assert_matches!(
            info,
            zx::ProcessInfo {
                return_code: 0,
                start_time,
                flags: STARTED,
            } if start_time > 0
        );
        Ok(())
    }

    async fn check_process_exited_ok(process: &zx::Process) -> Result<(), Error> {
        fasync::OnSignals::new(process, zx::Signals::PROCESS_TERMINATED).await?;

        let info = process.info()?;
        const STARTED_AND_EXITED: u32 =
            zx::ProcessInfoFlags::STARTED.bits() | zx::ProcessInfoFlags::EXITED.bits();
        assert_matches!(
            info,
            zx::ProcessInfo {
                return_code: 0,
                start_time,
                flags: STARTED_AND_EXITED,
            } if start_time > 0
        );
        Ok(())
    }

    // These start_util_with_* tests cover the most common paths through ProcessBuilder and
    // exercise most of its functionality. They verify that we can create a new process for a
    // "standard" dynamically linked executable and that we can provide arguments, environment
    // variables, namespace entries, and other handles to it through the startup process_args
    // message. The test communicates with the test util process it creates over a test-only FIDL
    // API to verify that arguments and whatnot were passed correctly.
    #[fasync::run_singlethreaded(test)]
    async fn start_util_with_args() -> Result<(), Error> {
        let test_args = vec!["arg0", "arg1", "arg2"];
        let test_args_cstr =
            test_args.iter().map(|&s| CString::new(s)).collect::<Result<_, _>>()?;

        let (mut builder, proxy) = setup_test_util_builder(true)?;
        builder.add_arguments(test_args_cstr);
        let process = builder.build().await?.start()?;
        check_process_running(&process)?;

        // Use the util protocol to confirm that the new process was set up correctly. A successful
        // connection to the util validates that handles are passed correctly to the new process,
        // since the DirectoryRequest handle made it.
        let proc_args = proxy.get_arguments().await.context("failed to get args from util")?;
        assert_eq!(proc_args, test_args);

        mem::drop(proxy);
        check_process_exited_ok(&process).await?;
        Ok(())
    }

    #[fasync::run_singlethreaded(test)]
    async fn start_util_with_huge_args() -> Result<(), Error> {
        // This test is partially designed to probe the stack usage of
        // code processing the initial loader message. Such processing
        // is on a stack of limited size, a few pages, and well
        // smaller than a maximally large channel packet. Each
        // instance of "arg" takes 4 bytes (counting the separating
        // '\0' byte), so let's send 10k of them to be well larger
        // than the initial stack but well within the 64k channel size.
        let test_args = vec!["arg"; 10 * 1000];
        let test_args_cstr =
            test_args.iter().map(|&s| CString::new(s)).collect::<Result<_, _>>()?;

        let (mut builder, proxy) = setup_test_util_builder(true)?;
        builder.add_arguments(test_args_cstr);
        let process = builder.build().await?.start()?;
        check_process_running(&process)?;

        // Use the util protocol to confirm that the new process was set up correctly. A successful
        // connection to the util validates that handles are passed correctly to the new process,
        // since the DirectoryRequest handle made it.
        // We can't use get_arguments() here because the FIDL response will be bigger than the
        // maximum message size[1] and cause the process to crash. Instead, we just check the number
        // of environment variables and assume that if that's correct we're good to go.
        // Size of each vector entry: (length = 8, pointer = 8) = 16 + (string size = 8) = 24
        // Message size = (10k * vector entry size) = 240,000 > 65,536
        let proc_args =
            proxy.get_argument_count().await.context("failed to get arg count from util")?;

        assert_eq!(proc_args, test_args.len() as u64);

        mem::drop(proxy);
        check_process_exited_ok(&process).await?;
        Ok(())
    }

    // Verify that the lifecycle channel can be passed through the bootstrap
    // channel. This test checks by creating a channel, passing it through,
    // asking the remote process for the lifecycle channel's koid, and then
    // comparing that koid to the one the test recorded.
    #[fasync::run_singlethreaded(test)]
    async fn start_util_with_lifecycle_channel() -> Result<(), Error> {
        let (mut builder, proxy) = setup_test_util_builder(true)?;
        let (lifecycle_server, _lifecycle_client) = zx::Channel::create();
        let koid = lifecycle_server
            .as_handle_ref()
            .basic_info()
            .expect("error getting server handle info")
            .koid
            .raw_koid();
        builder.add_handles(vec![process_args::StartupHandle {
            handle: lifecycle_server.into_handle(),
            info: HandleInfo::new(HandleType::Lifecycle, 0),
        }])?;
        let process = builder.build().await?.start()?;
        check_process_running(&process)?;

        // Use the util protocol to confirm that the new process received the
        // lifecycle channel
        let reported_koid =
            proxy.get_lifecycle_koid().await.context("failed getting koid from util")?;
        assert_eq!(koid, reported_koid);
        mem::drop(proxy);
        check_process_exited_ok(&process).await?;
        Ok(())
    }

    // Verify that if no lifecycle channel is sent via the bootstrap channel
    // that the remote process reports ZX_KOID_INVALID for the channel koid.
    #[fasync::run_singlethreaded(test)]
    async fn start_util_with_no_lifecycle_channel() -> Result<(), Error> {
        let (builder, proxy) = setup_test_util_builder(true)?;
        let process = builder.build().await?.start()?;
        check_process_running(&process)?;

        // Use the util protocol to confirm that the new process received the
        // lifecycle channel
        let reported_koid =
            proxy.get_lifecycle_koid().await.context("failed getting koid from util")?;
        assert_eq!(zx::sys::ZX_KOID_INVALID, reported_koid);
        mem::drop(proxy);
        check_process_exited_ok(&process).await?;
        Ok(())
    }

    #[fasync::run_singlethreaded(test)]
    async fn start_util_with_big_stack() -> Result<(), Error> {
        let stack_size: usize = zx::system_get_page_size() as usize * 10;

        let (mut builder, proxy) = setup_test_util_builder(true)?;
        builder.set_min_stack_size(stack_size);
        let built = builder.build().await?;
        assert!(built.stack_vmo.get_size()? >= stack_size as u64);

        let process = built.start()?;
        check_process_running(&process)?;
        mem::drop(proxy);
        check_process_exited_ok(&process).await?;
        Ok(())
    }

    #[fasync::run_singlethreaded(test)]
    async fn elf_headers() -> Result<(), Error> {
        let (builder, _) = setup_test_util_builder(true)?;
        let built = builder.build().await?;
        assert!(
            built.elf_headers.file_header().phnum
                == built.elf_headers.program_headers().len() as u16
        );
        Ok(())
    }

    // Verify that a loader service handle is properly handled if passed directly to
    // set_loader_service instead of through add_handles. Otherwise this test is identical to
    // start_util_with_args.
    #[fasync::run_singlethreaded(test)]
    async fn set_loader_directly() -> Result<(), Error> {
        let test_args = vec!["arg0", "arg1", "arg2"];
        let test_args_cstr =
            test_args.iter().map(|&s| CString::new(s)).collect::<Result<_, _>>()?;

        let (mut builder, proxy) = setup_test_util_builder(false)?;
        builder.set_loader_service(clone_loader_service()?)?;
        builder.add_arguments(test_args_cstr);
        let process = builder.build().await?.start()?;
        check_process_running(&process)?;

        // Use the util protocol to confirm that the new process was set up correctly. A successful
        // connection to the util validates that handles are passed correctly to the new process,
        // since the DirectoryRequest handle made it.
        let proc_args = proxy.get_arguments().await.context("failed to get args from util")?;
        assert_eq!(proc_args, test_args);

        mem::drop(proxy);
        check_process_exited_ok(&process).await?;
        Ok(())
    }

    // Verify that a vDSO handle is properly handled if passed directly to set_vdso_vmo instead of
    // relying on the default value.
    // Note: There isn't a great way to tell here whether the vDSO VMO we passed in was used
    // instead of the default (because the kernel only allows use of vDSOs that it created for
    // security, so we can't make a fake vDSO with a different name or something), so that isn't
    // checked explicitly. The failure tests below make sure we don't ignore the provided vDSO VMO
    // completely.
    #[fasync::run_singlethreaded(test)]
    async fn set_vdso_directly() -> Result<(), Error> {
        let test_args = vec!["arg0", "arg1", "arg2"];
        let test_args_cstr =
            test_args.iter().map(|&s| CString::new(s)).collect::<Result<_, _>>()?;

        let (mut builder, proxy) = setup_test_util_builder(true)?;
        builder.set_vdso_vmo(get_system_vdso_vmo()?);
        builder.add_arguments(test_args_cstr);
        let process = builder.build().await?.start()?;
        check_process_running(&process)?;

        // Use the util protocol to confirm that the new process was set up correctly.
        let proc_args = proxy.get_arguments().await.context("failed to get args from util")?;
        assert_eq!(proc_args, test_args);

        mem::drop(proxy);
        check_process_exited_ok(&process).await?;
        Ok(())
    }

    // Verify that a vDSO handle is properly handled if passed directly to set_vdso_vmo instead of
    // relying on the default value, this time by providing an invalid VMO (something that isn't
    // ELF and will fail to parse). This also indirectly tests that the reservation VMAR cleanup
    // happens properly by testing a failure after it has been created.
    #[fasync::run_singlethreaded(test)]
    async fn set_invalid_vdso_directly_fails() -> Result<(), Error> {
        let bad_vdso = zx::Vmo::create(1)?;

        let (mut builder, _) = setup_test_util_builder(true)?;
        builder.set_vdso_vmo(bad_vdso);

        let result = builder.build().await;
        match result {
            Err(ProcessBuilderError::ElfParse(elf_parse::ElfParseError::InvalidFileHeader(_))) => {}
            Err(err) => {
                panic!("Unexpected error type: {}", err);
            }
            Ok(_) => {
                panic!("Unexpectedly succeeded to build process with invalid vDSO");
            }
        }
        Ok(())
    }

    // Verify that a vDSO handle is properly handled if passed through add_handles instead of
    // relying on the default value, this time by providing an invalid VMO (something that isn't
    // ELF and will fail to parse). This also indirectly tests that the reservation VMAR cleanup
    // happens properly by testing a failure after it has been created.
    #[fasync::run_singlethreaded(test)]
    async fn set_invalid_vdso_fails() -> Result<(), Error> {
        let bad_vdso = zx::Vmo::create(1)?;

        let (mut builder, _) = setup_test_util_builder(true)?;
        builder.add_handles(vec![process_args::StartupHandle {
            handle: bad_vdso.into_handle(),
            info: HandleInfo::new(HandleType::VdsoVmo, 0),
        }])?;

        let result = builder.build().await;
        match result {
            Err(ProcessBuilderError::ElfParse(elf_parse::ElfParseError::InvalidFileHeader(_))) => {}
            Err(err) => {
                panic!("Unexpected error type: {}", err);
            }
            Ok(_) => {
                panic!("Unexpectedly succeeded to build process with invalid vDSO");
            }
        }
        Ok(())
    }

    #[fasync::run_singlethreaded(test)]
    async fn add_additional_vdso() -> Result<(), Error> {
        let mut builder = create_test_util_builder()?;
        builder.set_loader_service(clone_loader_service()?)?;
        builder.add_handles(vec![process_args::StartupHandle {
            handle: get_system_vdso_vmo().unwrap().into_handle(),
            info: HandleInfo::new(HandleType::VdsoVmo, 1),
        }])?;
        let built = builder.build().await?;

        // Ignore linker message handles.
        let mut msg_buf = zx::MessageBuf::new();
        built.bootstrap.read(&mut msg_buf)?;

        // Validate main message handles.
        let mut msg_buf = zx::MessageBuf::new();
        built.bootstrap.read(&mut msg_buf)?;
        let handle_info = parse_handle_info_from_message(&msg_buf)?
            .drain(..)
            .filter(|info| info.handle_type() == HandleType::VdsoVmo)
            .collect::<Vec<_>>();
        assert_eq!(2, handle_info.len());
        for (i, info) in handle_info.iter().rev().enumerate() {
            assert_eq!(HandleType::VdsoVmo, info.handle_type());
            assert_eq!(i as u16, info.arg());
        }
        Ok(())
    }

    #[fasync::run_singlethreaded(test)]
    async fn start_util_with_env() -> Result<(), Error> {
        let test_env = vec![("VAR1", "value2"), ("VAR2", "value2")];
        let test_env_cstr = test_env
            .iter()
            .map(|v| CString::new(format!("{}={}", v.0, v.1)))
            .collect::<Result<_, _>>()?;

        let (mut builder, proxy) = setup_test_util_builder(true)?;
        builder.add_environment_variables(test_env_cstr);
        let process = builder.build().await?.start()?;
        check_process_running(&process)?;

        let proc_env = proxy.get_environment().await.context("failed to get env from util")?;
        let proc_env_tuple: Vec<(&str, &str)> =
            proc_env.iter().map(|v| (&*v.key, &*v.value)).collect();
        assert_eq!(proc_env_tuple, test_env);

        mem::drop(proxy);
        check_process_exited_ok(&process).await?;
        Ok(())
    }

    #[fasync::run_singlethreaded(test)]
    async fn start_util_with_huge_env() -> Result<(), Error> {
        // This test is partially designed to probe the stack usage of
        // code processing the initial loader message. Such processing
        // is on a stack of limited size, a few pages, and well
        // smaller than a maximally large channel packet. Each
        // instance of "a=b" takes 4 bytes (counting the separating
        // '\0' byte), so let's send 10k of them to be well larger
        // than the initial stack but well within the 64k channel size.
        let test_env = vec!["a=b"; 10 * 1000];
        let test_env_cstr = test_env.iter().map(|&s| CString::new(s)).collect::<Result<_, _>>()?;

        let (mut builder, proxy) = setup_test_util_builder(true)?;
        builder.add_environment_variables(test_env_cstr);
        let process = builder.build().await?.start()?;
        check_process_running(&process)?;

        // We can't use get_environment() here because the FIDL response will be bigger than the
        // maximum message size and cause the process to crash. Instead, we just check the number
        // of environment variables and assume that if that's correct we're good to go.
        let proc_env =
            proxy.get_environment_count().await.context("failed to get env from util")?;
        assert_eq!(proc_env, test_env.len() as u64);

        mem::drop(proxy);
        check_process_exited_ok(&process).await?;
        Ok(())
    }

    #[fasync::run_singlethreaded(test)]
    async fn start_util_with_namespace_entries() -> Result<(), Error> {
        let mut randbuf = [0; 8];
        zx::cprng_draw(&mut randbuf);
        let test_content1 = format!("test content 1 {}", u64::from_le_bytes(randbuf));
        zx::cprng_draw(&mut randbuf);
        let test_content2 = format!("test content 2 {}", u64::from_le_bytes(randbuf));

        let dir1 = pseudo_directory! {
            "test_file1" => read_only(test_content1.clone()),
        };
        let dir1_client = vfs::directory::serve_read_only(dir1).into_client_end().unwrap();

        let dir2 = pseudo_directory! {
            "test_file2" => read_only(test_content2.clone()),
        };
        let dir2_client = vfs::directory::serve_read_only(dir2).into_client_end().unwrap();

        let (mut builder, proxy) = setup_test_util_builder(true)?;
        builder.add_namespace_entries(vec![
            NamespaceEntry { path: CString::new("/dir1")?, directory: dir1_client },
            NamespaceEntry { path: CString::new("/dir2")?, directory: dir2_client },
        ])?;
        let process = builder.build().await?.start()?;
        check_process_running(&process)?;

        let namespace_dump = proxy.dump_namespace().await.context("failed to dump namespace")?;
        assert_eq!(namespace_dump, "/dir1, /dir1/test_file1, /dir2, /dir2/test_file2");

        let dir1_contents =
            proxy.read_file("/dir1/test_file1").await.context("failed to read file via util")?;
        assert_eq!(dir1_contents, test_content1);
        let dir2_contents =
            proxy.read_file("/dir2/test_file2").await.context("failed to read file via util")?;
        assert_eq!(dir2_contents, test_content2);

        mem::drop(proxy);
        check_process_exited_ok(&process).await?;
        Ok(())
    }

    // Trying to start a dynamically linked process without providing a loader service should
    // fail. This verifies that nothing is automatically cloning a loader.
    #[fasync::run_singlethreaded(test)]
    async fn start_util_with_no_loader_fails() -> Result<(), Error> {
        let (builder, _) = setup_test_util_builder(false)?;

        let result = builder.build().await;
        match result {
            Err(ProcessBuilderError::LoaderMissing()) => {}
            Err(err) => {
                panic!("Unexpected error type: {}", err);
            }
            Ok(_) => {
                panic!("Unexpectedly succeeded to build process without loader");
            }
        }
        Ok(())
    }

    // Checks that, for dynamically linked binaries, the lower half of the address space has been
    // reserved for sanitizers.
    #[fasync::run_singlethreaded(test)]
    async fn verify_low_address_range_reserved() -> Result<(), Error> {
        let (builder, _) = setup_test_util_builder(true)?;
        let built = builder.build().await?;

        // This ends up being the same thing ReservationVmar does, but it's not reused here so that
        // this catches bugs or bad changes to ReservationVmar itself.
        let info = built.root_vmar.info()?;
        let lower_half_len = util::page_end((info.base + info.len) / 2) - info.base;
        built
            .root_vmar
            .allocate(0, lower_half_len, zx::VmarFlags::SPECIFIC)
            .context("Unable to allocate lower address range of new process")?;
        Ok(())
    }

    // Parses the given channel message as a process_args message and returns the HandleInfo's
    // contained in it.
    fn parse_handle_info_from_message(message: &zx::MessageBuf) -> Result<Vec<HandleInfo>, Error> {
        let bytes = message.bytes();
        let (header, _) = Ref::<&[u8], process_args::MessageHeader>::from_prefix(bytes)
            .map_err(|_| anyhow!("Failed to parse process_args header"))?;

        let offset = header.handle_info_off as usize;
        let len = mem::size_of::<u32>() * message.n_handles();
        let info_bytes = &bytes[offset..offset + len];
        let raw_info = Ref::<&[u8], [u32]>::from_bytes(info_bytes)
            .map_err(|_| anyhow!("Failed to parse raw handle info"))?;

        Ok(raw_info.iter().map(|raw| HandleInfo::try_from(*raw)).collect::<Result<_, _>>()?)
    }

    const LINKER_MESSAGE_HANDLES: &[HandleType] = &[
        HandleType::ProcessSelf,
        HandleType::RootVmar,
        HandleType::LdsvcLoader,
        HandleType::LoadedVmar,
        HandleType::ExecutableVmo,
    ];

    const MAIN_MESSAGE_HANDLES: &[HandleType] = &[
        HandleType::ProcessSelf,
        HandleType::ThreadSelf,
        HandleType::RootVmar,
        HandleType::VdsoVmo,
        HandleType::StackVmo,
    ];

    #[fasync::run_singlethreaded(test)]
    async fn correct_handles_present() -> Result<(), Error> {
        let mut builder = create_test_util_builder()?;
        builder.set_loader_service(clone_loader_service()?)?;
        let built = builder.build().await?;

        for correct in &[LINKER_MESSAGE_HANDLES, MAIN_MESSAGE_HANDLES] {
            let mut msg_buf = zx::MessageBuf::new();
            built.bootstrap.read(&mut msg_buf)?;
            let handle_info = parse_handle_info_from_message(&msg_buf)?;

            assert_eq!(handle_info.len(), correct.len());
            for correct_type in *correct {
                // Should only be one of each of these handles present.
                assert_eq!(
                    1,
                    handle_info.iter().filter(|info| &info.handle_type() == correct_type).count()
                );
            }
        }
        Ok(())
    }

    // Verify that [ProcessBuilder::add_handles()] rejects handle types that are added
    // automatically by the builder.
    #[fasync::run_singlethreaded(test)]
    async fn add_handles_rejects_automatic_handle_types() -> Result<(), Error> {
        // The VMO doesn't need to be valid since we're not calling build.
        let vmo = zx::Vmo::create(1)?;
        let job = fuchsia_runtime::job_default();
        let procname = CString::new("test_vmo")?;
        let mut builder = ProcessBuilder::new(
            &procname,
            &job,
            zx::ProcessOptions::empty(),
            vmo,
            get_system_vdso_vmo().unwrap(),
        )?;

        // There's some duplicates between these slices but just checking twice is easier than
        // deduping these.
        for handle_type in LINKER_MESSAGE_HANDLES.iter().chain(MAIN_MESSAGE_HANDLES) {
            if *handle_type == HandleType::LdsvcLoader {
                // Skip LdsvcLoader, which is required in the linker message but is not added
                // automatically. The user must supply it.
                continue;
            }

            if *handle_type == HandleType::VdsoVmo {
                // Skip VdsoVmo, which may be supplied by the user.
                continue;
            }

            // Another VMO, just to have a valid handle.
            let vmo = zx::Vmo::create(1)?;
            let result = builder.add_handles(vec![process_args::StartupHandle {
                handle: vmo.into_handle(),
                info: HandleInfo::new(*handle_type, 0),
            }]);
            match result {
                Err(ProcessBuilderError::InvalidArg(_)) => {}
                Err(err) => {
                    panic!("Unexpected error type, should be invalid arg: {}", err);
                }
                Ok(_) => {
                    panic!("add_handle unexpectedly succeeded for type {:?}", handle_type);
                }
            }
        }
        Ok(())
    }

    // Verify that invalid handles are correctly rejected.
    #[fasync::run_singlethreaded(test)]
    async fn rejects_invalid_handles() -> Result<(), Error> {
        let invalid = || zx::Handle::invalid();
        let assert_invalid_arg = |result| match result {
            Err(ProcessBuilderError::BadHandle(_)) => {}
            Err(err) => {
                panic!("Unexpected error type, should be BadHandle: {}", err);
            }
            Ok(_) => {
                panic!("API unexpectedly accepted invalid handle");
            }
        };

        // The VMO doesn't need to be valid since we're not calling build with this.
        let vmo = zx::Vmo::create(1)?;
        let job = fuchsia_runtime::job_default();
        let procname = CString::new("test_vmo")?;

        assert_invalid_arg(
            ProcessBuilder::new(
                &procname,
                &invalid().into(),
                zx::ProcessOptions::empty(),
                vmo,
                get_system_vdso_vmo().unwrap(),
            )
            .map(|_| ()),
        );
        assert_invalid_arg(
            ProcessBuilder::new(
                &procname,
                &job,
                zx::ProcessOptions::empty(),
                invalid().into(),
                get_system_vdso_vmo().unwrap(),
            )
            .map(|_| ()),
        );

        let (mut builder, _) = setup_test_util_builder(true)?;

        assert_invalid_arg(builder.set_loader_service(invalid().into()));
        assert_invalid_arg(builder.add_handles(vec![process_args::StartupHandle {
            handle: invalid().into(),
            info: HandleInfo::new(HandleType::User0, 0),
        }]));
        assert_invalid_arg(builder.add_handles(vec![process_args::StartupHandle {
            handle: invalid().into(),
            info: HandleInfo::new(HandleType::User0, 0),
        }]));
        assert_invalid_arg(builder.add_namespace_entries(vec![NamespaceEntry {
            path: CString::new("/dir")?,
            directory: invalid().into(),
        }]));

        Ok(())
    }

    #[fasync::run_singlethreaded]
    #[test]
    async fn start_static_pie_binary() -> Result<(), Error> {
        const TEST_BIN: &'static str = "/pkg/bin/static_pie_test_util";
        let file = fdio::open_fd(TEST_BIN, fio::PERM_READABLE | fio::PERM_EXECUTABLE)?;
        let vmo = fdio::get_vmo_exec_from_file(&file)?;
        let job = fuchsia_runtime::job_default();

        let procname = CString::new(TEST_BIN.to_owned())?;
        let mut builder = ProcessBuilder::new(
            &procname,
            &job,
            zx::ProcessOptions::empty(),
            vmo,
            get_system_vdso_vmo().unwrap(),
        )?;

        // We pass the program a channel with handle type User0 which we send a message on and
        // expect it to echo back the message on the same channel.
        let (local, remote) = zx::Channel::create();
        builder.add_handles(vec![process_args::StartupHandle {
            handle: remote.into_handle(),
            info: HandleInfo::new(HandleType::User0, 0),
        }])?;

        let mut randbuf = [0; 8];
        zx::cprng_draw(&mut randbuf);
        let test_message = format!("test content 1 {}", u64::from_le_bytes(randbuf)).into_bytes();
        local.write(&test_message, &mut vec![])?;

        // Start process and wait for channel to have a message to read or be closed.
        builder.build().await?.start()?;
        let signals = fasync::OnSignals::new(
            &local,
            zx::Signals::CHANNEL_READABLE | zx::Signals::CHANNEL_PEER_CLOSED,
        )
        .await?;
        assert!(signals.contains(zx::Signals::CHANNEL_READABLE));

        let mut echoed = zx::MessageBuf::new();
        local.read(&mut echoed)?;
        assert_eq!(echoed.bytes(), test_message.as_slice());
        assert_eq!(echoed.n_handles(), 0);
        Ok(())
    }
}