selinux/policy/
view.rs

1// Copyright 2025 The Fuchsia Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style license that can be
3// found in the LICENSE file.
4
5use super::arrays::SimpleArrayView;
6use super::parser::{PolicyCursor, PolicyData, PolicyOffset};
7use super::{Counted, Parse, PolicyValidationContext, Validate};
8
9use hashbrown::hash_table::HashTable;
10use rapidhash::RapidHasher;
11use static_assertions::const_assert;
12use std::fmt::Debug;
13use std::hash::{Hash, Hasher};
14use std::marker::PhantomData;
15use zerocopy::{FromBytes, Immutable, KnownLayout, Unaligned};
16
17/// A trait for types that have metadata.
18///
19/// Many policy objects have a fixed-sized metadata section that is much faster to parse than the
20/// full object. This trait is used when walking the binary policy to find objects of interest
21/// efficiently.
22pub trait HasMetadata {
23    /// The Rust type that represents the metadata.
24    type Metadata: FromBytes + Sized;
25}
26
27/// A trait for types that can be walked through the policy data.
28///
29/// This trait is used when walking the binary policy to find objects of interest efficiently.
30pub trait Walk {
31    /// Walks the policy data to the next object of the given type.
32    ///
33    /// Returns an error if the cursor cannot be walked to the next object of the given type.
34    fn walk(policy_data: &PolicyData, offset: PolicyOffset) -> PolicyOffset;
35}
36
37/// A view into a policy object.
38///
39/// This struct contains the start and end offsets of the object in the policy data. To read the
40/// object, use [`View::read`].
41#[derive(Debug, Clone, Copy)]
42pub struct View<T> {
43    phantom: PhantomData<T>,
44
45    /// The start offset of the object in the policy data.
46    start: PolicyOffset,
47
48    /// The end offset of the object in the policy data.
49    end: PolicyOffset,
50}
51
52impl<T> View<T> {
53    /// Creates a new view from the start and end offsets.
54    pub fn new(start: PolicyOffset, end: PolicyOffset) -> Self {
55        Self { phantom: PhantomData, start, end }
56    }
57
58    /// The start offset of the object in the policy data.
59    fn start(&self) -> PolicyOffset {
60        self.start
61    }
62}
63
64impl<T: Sized> View<T> {
65    /// Creates a new view at the given start offset.
66    ///
67    /// The end offset is calculated as the start offset plus the size of the object.
68    pub fn at(start: PolicyOffset) -> Self {
69        let end = start + std::mem::size_of::<T>() as u32;
70        Self::new(start, end)
71    }
72}
73
74impl<T: FromBytes + Sized> View<T> {
75    /// Reads the object from the policy data.
76    ///
77    /// This function requires the object to have a fixed size and simply copies the object from
78    /// the policy data.
79    ///
80    /// For variable-sized objects, use [`View::parse`] instead.
81    pub fn read(&self, policy_data: &PolicyData) -> T {
82        debug_assert_eq!(self.end - self.start, std::mem::size_of::<T>() as u32);
83        let start = self.start as usize;
84        let end = self.end as usize;
85        T::read_from_bytes(&policy_data[start..end]).unwrap()
86    }
87}
88
89impl<T: HasMetadata> View<T> {
90    /// Returns a view into the metadata of the object.
91    ///
92    /// Assumes the metadata is at the start of the object.
93    pub fn metadata(&self) -> View<T::Metadata> {
94        View::<T::Metadata>::at(self.start)
95    }
96
97    /// Reads the metadata from the policy data.
98    pub fn read_metadata(&self, policy_data: &PolicyData) -> T::Metadata {
99        self.metadata().read(policy_data)
100    }
101}
102
103impl<T: Parse> View<T> {
104    /// Parses the object from the policy data.
105    ///
106    /// This function uses the [`Parse`] trait to parse the object from the policy data.
107    ///
108    /// If the object has a fixed size, prefer [`View::read`] instead.
109    pub fn parse(&self, policy_data: &PolicyData) -> T {
110        let cursor = PolicyCursor::new_at(policy_data, self.start);
111        let (object, _) =
112            T::parse(cursor).map_err(Into::<anyhow::Error>::into).expect("policy should be valid");
113        object
114    }
115}
116
117impl<T: Validate + Parse> Validate for View<T> {
118    type Error = anyhow::Error;
119
120    fn validate(&self, context: &PolicyValidationContext) -> Result<(), Self::Error> {
121        let object = self.parse(&context.data);
122        object.validate(context).map_err(Into::<anyhow::Error>::into)
123    }
124}
125
126/// A view into the data of an array of objects.
127///
128/// This struct contains the start offset of the array and the number of objects in the array.
129/// To iterate over the objects, use [`ArrayDataView::iter`].
130#[derive(Debug, Clone, Copy)]
131pub struct ArrayDataView<D> {
132    phantom: PhantomData<D>,
133    start: PolicyOffset,
134    count: u32,
135}
136
137impl<D> ArrayDataView<D> {
138    /// Creates a new array data view from the start offset and count.
139    pub fn new(start: PolicyOffset, count: u32) -> Self {
140        Self { phantom: PhantomData, start, count }
141    }
142
143    /// Iterates over the objects in the array.
144    ///
145    /// The iterator returns views into the objects in the array.
146    ///
147    /// This function requires the policy data to be provided to the iterator because objects in
148    /// the array may have variable size.
149    pub fn iter(self, policy_data: &PolicyData) -> ArrayDataViewIter<D> {
150        ArrayDataViewIter::new(policy_data.clone(), self.start, self.count)
151    }
152}
153
154/// An iterator over the objects in an array.
155///
156/// This struct contains the cursor to the start of the array and the number of objects remaining
157/// to be iterated over.
158pub struct ArrayDataViewIter<D> {
159    phantom: PhantomData<D>,
160    policy_data: PolicyData,
161    offset: PolicyOffset,
162    remaining: u32,
163}
164
165impl<T> ArrayDataViewIter<T> {
166    /// Creates a new array data view iterator from the start cursor and remaining count.
167    fn new(policy_data: PolicyData, offset: PolicyOffset, remaining: u32) -> Self {
168        Self { phantom: PhantomData, policy_data, offset, remaining }
169    }
170}
171
172impl<D: Walk> std::iter::Iterator for ArrayDataViewIter<D> {
173    type Item = View<D>;
174
175    fn next(&mut self) -> Option<Self::Item> {
176        if self.remaining > 0 {
177            let start = self.offset;
178            self.offset = D::walk(&self.policy_data, start);
179            self.remaining -= 1;
180            Some(View::new(start, self.offset))
181        } else {
182            None
183        }
184    }
185}
186
187/// A view into the data of an array of objects.
188///
189/// This struct contains the start offset of the array and the number of objects in the array.
190/// To access the objects in the array, use [`ArrayView::data`].
191#[derive(Debug, Clone, Copy)]
192pub(super) struct ArrayView<M, D> {
193    phantom: PhantomData<(M, D)>,
194    start: PolicyOffset,
195    count: u32,
196}
197
198impl<M, D> ArrayView<M, D> {
199    /// Creates a new array view from the start offset and count.
200    pub fn new(start: PolicyOffset, count: u32) -> Self {
201        Self { phantom: PhantomData, start, count }
202    }
203}
204
205impl<M: Sized, D> ArrayView<M, D> {
206    /// Returns a view into the metadata of the array.
207    pub fn metadata(&self) -> View<M> {
208        View::<M>::at(self.start)
209    }
210
211    /// Returns a view into the data of the array.
212    pub fn data(&self) -> ArrayDataView<D> {
213        ArrayDataView::new(self.metadata().end, self.count)
214    }
215}
216
217fn parse_array_data<'a, D: Parse>(
218    cursor: PolicyCursor<'a>,
219    count: u32,
220) -> Result<PolicyCursor<'a>, anyhow::Error> {
221    let mut tail = cursor;
222    for _ in 0..count {
223        let (_, next) = D::parse(tail).map_err(Into::<anyhow::Error>::into)?;
224        tail = next;
225    }
226    Ok(tail)
227}
228
229impl<M: Counted + Parse + Sized, D: Parse> Parse for ArrayView<M, D> {
230    /// [`ArrayView`] abstracts over two types (`M` and `D`) that may have different [`Parse::Error`]
231    /// types. Unify error return type via [`anyhow::Error`].
232    type Error = anyhow::Error;
233
234    fn parse<'a>(cursor: PolicyCursor<'a>) -> Result<(Self, PolicyCursor<'a>), Self::Error> {
235        let start = cursor.offset();
236        let (metadata, cursor) = M::parse(cursor).map_err(Into::<anyhow::Error>::into)?;
237        let count = metadata.count();
238        let cursor = parse_array_data::<D>(cursor, count)?;
239        Ok((Self::new(start, count), cursor))
240    }
241}
242
243/// An iterator giving views of the objects in the underlying [`policy_data`] found from
244/// a single entry of a `HashedArrayView`.
245struct HashedArrayViewEntryIter<'a, D: HasMetadata> {
246    policy_data: &'a PolicyData,
247    limit: PolicyOffset,
248    metadata: D::Metadata,
249    offset: Option<PolicyOffset>,
250}
251
252/// An unsigned integer in the range [0, 0x1000000) stored in three unaligned bytes.
253//
254// TODO: https://fxbug.dev/479180246 - it would be better to get this type from a library
255// somewhere. Probably not https://docs.rs/u24 (because of "The type has the same size,
256// alignment, and memory layout as a little-endian encoded u32" in that implementation's
257// specification). But maybe from somewhere else? Maybe BUintD8<3> in https://docs.rs/bnum?
258#[derive(Clone, Copy, Debug, KnownLayout, FromBytes, Immutable, PartialEq, Unaligned)]
259#[repr(C, packed)]
260pub(super) struct U24 {
261    low: u8,
262    middle: u8,
263    high: u8,
264}
265
266// U24's space-efficiency is its reason for existence.
267const_assert!(std::mem::size_of::<U24>() == 3);
268const_assert!(std::mem::align_of::<U24>() == 1);
269
270impl TryFrom<u32> for U24 {
271    type Error = ();
272
273    fn try_from(value: u32) -> Result<Self, Self::Error> {
274        if 0x1000000 <= value {
275            Err(())
276        } else {
277            Ok(Self {
278                low: (value & 0xff) as u8,
279                middle: ((value >> 8) & 0xff) as u8,
280                high: ((value >> 16) & 0xff) as u8,
281            })
282        }
283    }
284}
285
286impl From<U24> for u32 {
287    fn from(value: U24) -> u32 {
288        ((value.high as u32) << 16) + ((value.middle as u32) << 8) + (value.low as u32)
289    }
290}
291
292impl<'a, D: HasMetadata + Walk> Iterator for HashedArrayViewEntryIter<'a, D>
293where
294    D::Metadata: Eq,
295{
296    type Item = View<D>;
297
298    fn next(&mut self) -> Option<Self::Item> {
299        if let Some(offset) = self.offset
300            && offset < self.limit
301        {
302            let element = View::<D>::at(offset);
303            let metadata = element.read_metadata(&self.policy_data);
304            if metadata == self.metadata {
305                self.offset = Some(D::walk(&self.policy_data, offset));
306                Some(element)
307            } else {
308                self.offset = None;
309                None
310            }
311        } else {
312            None
313        }
314    }
315}
316
317/// A view into the data of an array of objects, with efficient lookup based on metadata hash.
318///
319/// This struct contains only a vector of offsets into the policy data, to allow efficient lookup
320/// of vector elements with matching metadata.
321#[derive(Debug, Clone)]
322pub(super) struct HashedArrayView<D: HasMetadata> {
323    phantom: PhantomData<D>,
324    index: HashTable<U24>,
325    /// The offset in the policy data at which the elements indexed by this [`HashedArrayView`]
326    /// end. Iteration of elements by this [`HashedArrayView`] must not look for an element at
327    /// or beyond this offset.
328    limit: PolicyOffset,
329}
330
331impl<D: HasMetadata> HashedArrayView<D>
332where
333    D::Metadata: Hash,
334{
335    fn metadata_hash(metadata: &D::Metadata) -> u64 {
336        let mut hasher = RapidHasher::default();
337        metadata.hash(&mut hasher);
338        hasher.finish()
339    }
340}
341
342impl<D: Parse + HasMetadata + Walk> HashedArrayView<D>
343where
344    D::Metadata: Eq + PartialEq + Hash + Debug,
345{
346    /// Looks up the entry with the specified metadata `key`, and parses and returns the value.
347    /// This method is only appropriate to call when expecting to find at most one entry for
348    /// `key`; if there is a possibility of more than one element in the underlying
349    /// `policy_data` being associated with `key`, call `find_all` instead.
350    pub fn find(&self, key: D::Metadata, policy_data: &PolicyData) -> Option<D> {
351        let key_hash = Self::metadata_hash(&key);
352        let offset = self.index.find(key_hash, |&offset| {
353            let element = View::<D>::at(u32::from(offset));
354            key == element.read_metadata(policy_data)
355        })?;
356        let element = View::<D>::at(u32::from(*offset));
357        Some(element.parse(policy_data))
358    }
359
360    /// Returns an iterator over the entries with the specified metadata `key` and parses and
361    /// emits those values.
362    pub(super) fn find_all(
363        &self,
364        key: D::Metadata,
365        policy_data: &PolicyData,
366    ) -> impl Iterator<Item = D> {
367        let key_hash = Self::metadata_hash(&key);
368        let offset = self.index.find(key_hash, |&offset| {
369            let element = View::<D>::at(u32::from(offset));
370            key == element.read_metadata(policy_data)
371        });
372        (HashedArrayViewEntryIter {
373            policy_data: policy_data,
374            limit: self.limit,
375            metadata: key,
376            offset: offset.map(|offset| u32::from(*offset)),
377        })
378        .map(|element| element.parse(policy_data))
379    }
380
381    /// Returns an iterator that emits a view for each reachable element.
382    pub(super) fn iter(&self, policy_data: &PolicyData) -> impl Iterator<Item = View<D>> {
383        self.index
384            .iter()
385            .map(|offset| {
386                let element = View::<D>::at(u32::from(*offset));
387                HashedArrayViewEntryIter {
388                    policy_data: policy_data,
389                    limit: self.limit,
390                    metadata: element.read_metadata(policy_data),
391                    offset: Some(u32::from(*offset)),
392                }
393            })
394            .flatten()
395    }
396}
397
398impl<D: Parse + HasMetadata + Walk> Parse for HashedArrayView<D>
399where
400    D::Metadata: Eq + Debug + PartialEq + Parse + Hash,
401{
402    type Error = anyhow::Error;
403
404    fn parse<'a>(cursor: PolicyCursor<'a>) -> Result<(Self, PolicyCursor<'a>), Self::Error> {
405        let (array_view, cursor) = SimpleArrayView::<D>::parse(cursor)?;
406
407        // Allocate a hash table sized appropriately for the array size.
408        let mut index = HashTable::with_capacity(array_view.count as usize);
409
410        // Record the offset at which the last array element ends.
411        let limit = cursor.offset();
412
413        // Iterate over the elements inserting the first offset at which each is
414        // seen into a hash bucket.
415        for view in array_view.data().iter(cursor.data()) {
416            let metadata = view.read_metadata(cursor.data());
417
418            index
419                .entry(
420                    Self::metadata_hash(&metadata),
421                    |&offset| {
422                        let element = View::<D>::at(u32::from(offset));
423                        metadata == element.read_metadata(cursor.data())
424                    },
425                    |&offset| {
426                        let element = View::<D>::at(u32::from(offset));
427                        Self::metadata_hash(&element.read_metadata(cursor.data()))
428                    },
429                )
430                .or_insert(U24::try_from(view.start()).expect("Policy offsets ought fit in U24!"));
431        }
432
433        Ok((Self { phantom: PhantomData, index, limit }, cursor))
434    }
435}
436
437impl<D: Validate + Parse + HasMetadata + Walk> Validate for HashedArrayView<D>
438where
439    D::Metadata: Eq,
440{
441    type Error = anyhow::Error;
442
443    fn validate(&self, context: &PolicyValidationContext) -> Result<(), Self::Error> {
444        let policy_data = context.data.clone();
445        for element in self
446            .index
447            .iter()
448            .map(|offset| {
449                let element = View::<D>::at(u32::from(*offset));
450                HashedArrayViewEntryIter::<D> {
451                    policy_data: &policy_data,
452                    limit: self.limit,
453                    metadata: element.read_metadata(&policy_data),
454                    offset: Some(u32::from(*offset)),
455                }
456            })
457            .flatten()
458        {
459            element.validate(context)?;
460        }
461
462        Ok(())
463    }
464}
465
466#[cfg(test)]
467mod tests {
468    use super::U24;
469
470    #[test]
471    fn to_and_from_u24() {
472        for i in 0u32..0x10000 {
473            let u24_result = U24::try_from(i);
474            assert!(u24_result.is_ok());
475            let u24 = u24_result.unwrap();
476            assert_eq!(i >> 16, u24.high as u32);
477            assert_eq!((i >> 8) & 0xff, u24.middle as u32);
478            assert_eq!(i & 0xff, u24.low as u32);
479            let j = u32::from(u24);
480            assert_eq!(i, j);
481        }
482
483        assert!(U24::try_from(0x1000000).is_err());
484    }
485}
selinux/policy/view.rs

selinux/policy/
view.rs
