wasmtime_environ/
address_map.rs

1//! Data structures to provide transformation of the source
2
3use object::{Bytes, LittleEndian, U32Bytes};
4use serde_derive::{Deserialize, Serialize};
5
6/// Single source location to generated address mapping.
7#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq)]
8pub struct InstructionAddressMap {
9    /// Where in the source wasm binary this instruction comes from, specified
10    /// in an offset of bytes from the front of the file.
11    pub srcloc: FilePos,
12
13    /// Offset from the start of the function's compiled code to where this
14    /// instruction is located, or the region where it starts.
15    pub code_offset: u32,
16}
17
18/// A position within an original source file,
19///
20/// This structure is used as a newtype wrapper around a 32-bit integer which
21/// represents an offset within a file where a wasm instruction or function is
22/// to be originally found.
23#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
24pub struct FilePos(u32);
25
26impl FilePos {
27    /// Create a new file position with the given offset.
28    pub fn new(pos: u32) -> FilePos {
29        assert!(pos != u32::MAX);
30        FilePos(pos)
31    }
32
33    /// Returns the offset that this offset was created with.
34    ///
35    /// Note that the `Default` implementation will return `None` here, whereas
36    /// positions created with `FilePos::new` will return `Some`.
37    pub fn file_offset(self) -> Option<u32> {
38        if self.0 == u32::MAX {
39            None
40        } else {
41            Some(self.0)
42        }
43    }
44}
45
46impl Default for FilePos {
47    fn default() -> FilePos {
48        FilePos(u32::MAX)
49    }
50}
51
52/// Parse an `ELF_WASMTIME_ADDRMAP` section, returning the slice of code offsets
53/// and the slice of associated file positions for each offset.
54fn parse_address_map(
55    section: &[u8],
56) -> Option<(&[U32Bytes<LittleEndian>], &[U32Bytes<LittleEndian>])> {
57    let mut section = Bytes(section);
58    // NB: this matches the encoding written by `append_to` above.
59    let count = section.read::<U32Bytes<LittleEndian>>().ok()?;
60    let count = usize::try_from(count.get(LittleEndian)).ok()?;
61    let (offsets, section) =
62        object::slice_from_bytes::<U32Bytes<LittleEndian>>(section.0, count).ok()?;
63    let (positions, section) =
64        object::slice_from_bytes::<U32Bytes<LittleEndian>>(section, count).ok()?;
65    debug_assert!(section.is_empty());
66    Some((offsets, positions))
67}
68
69/// Lookup an `offset` within an encoded address map section, returning the
70/// original `FilePos` that corresponds to the offset, if found.
71///
72/// This function takes a `section` as its first argument which must have been
73/// created with `AddressMapSection` above. This is intended to be the raw
74/// `ELF_WASMTIME_ADDRMAP` section from the compilation artifact.
75///
76/// The `offset` provided is a relative offset from the start of the text
77/// section of the pc that is being looked up. If `offset` is out of range or
78/// doesn't correspond to anything in this file then `None` is returned.
79pub fn lookup_file_pos(section: &[u8], offset: usize) -> Option<FilePos> {
80    let (offsets, positions) = parse_address_map(section)?;
81
82    // First perform a binary search on the `offsets` array. This is a sorted
83    // array of offsets within the text section, which is conveniently what our
84    // `offset` also is. Note that we are somewhat unlikely to find a precise
85    // match on the element in the array, so we're largely interested in which
86    // "bucket" the `offset` falls into.
87    let offset = u32::try_from(offset).ok()?;
88    let index = match offsets.binary_search_by_key(&offset, |v| v.get(LittleEndian)) {
89        // Exact hit!
90        Ok(i) => i,
91
92        // This *would* be at the first slot in the array, so no
93        // instructions cover `pc`.
94        Err(0) => return None,
95
96        // This would be at the `nth` slot, so we're at the `n-1`th slot.
97        Err(n) => n - 1,
98    };
99
100    // Using the `index` we found of which bucket `offset` corresponds to we can
101    // lookup the actual `FilePos` value in the `positions` array.
102    let pos = positions.get(index)?;
103    Some(FilePos(pos.get(LittleEndian)))
104}
105
106/// Iterate over the address map contained in the given address map section.
107///
108/// This function takes a `section` as its first argument which must have been
109/// created with `AddressMapSection` above. This is intended to be the raw
110/// `ELF_WASMTIME_ADDRMAP` section from the compilation artifact.
111///
112/// The yielded offsets are relative to the start of the text section for this
113/// map's code object.
114pub fn iterate_address_map<'a>(
115    section: &'a [u8],
116) -> Option<impl Iterator<Item = (u32, FilePos)> + 'a> {
117    let (offsets, positions) = parse_address_map(section)?;
118
119    Some(
120        offsets
121            .iter()
122            .map(|o| o.get(LittleEndian))
123            .zip(positions.iter().map(|pos| FilePos(pos.get(LittleEndian)))),
124    )
125}
wasmtime_environ/address_map.rs

wasmtime_environ/
address_map.rs
