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    /// Get the null file position.
34    pub fn none() -> FilePos {
35        FilePos(u32::MAX)
36    }
37
38    /// Is this the null file position?
39    #[inline]
40    pub fn is_none(&self) -> bool {
41        *self == FilePos::none()
42    }
43
44    /// Returns the offset that this offset was created with.
45    ///
46    /// Note that positions created with `FilePos::none` and the `Default`
47    /// implementation will return `None` here, whereas positions created with
48    /// `FilePos::new` will return `Some`.
49    pub fn file_offset(self) -> Option<u32> {
50        if self.0 == u32::MAX {
51            None
52        } else {
53            Some(self.0)
54        }
55    }
56}
57
58impl Default for FilePos {
59    fn default() -> FilePos {
60        FilePos::none()
61    }
62}
63
64/// Parse an `ELF_WASMTIME_ADDRMAP` section, returning the slice of code offsets
65/// and the slice of associated file positions for each offset.
66fn parse_address_map(
67    section: &[u8],
68) -> Option<(&[U32Bytes<LittleEndian>], &[U32Bytes<LittleEndian>])> {
69    let mut section = Bytes(section);
70    // NB: this matches the encoding written by `append_to` in the
71    // `compile::address_map` module.
72    let count = section.read::<U32Bytes<LittleEndian>>().ok()?;
73    let count = usize::try_from(count.get(LittleEndian)).ok()?;
74    let (offsets, section) =
75        object::slice_from_bytes::<U32Bytes<LittleEndian>>(section.0, count).ok()?;
76    let (positions, section) =
77        object::slice_from_bytes::<U32Bytes<LittleEndian>>(section, count).ok()?;
78    debug_assert!(section.is_empty());
79    Some((offsets, positions))
80}
81
82/// Lookup an `offset` within an encoded address map section, returning the
83/// original `FilePos` that corresponds to the offset, if found.
84///
85/// This function takes a `section` as its first argument which must have been
86/// created with `AddressMapSection` above. This is intended to be the raw
87/// `ELF_WASMTIME_ADDRMAP` section from the compilation artifact.
88///
89/// The `offset` provided is a relative offset from the start of the text
90/// section of the pc that is being looked up. If `offset` is out of range or
91/// doesn't correspond to anything in this file then `None` is returned.
92pub fn lookup_file_pos(section: &[u8], offset: usize) -> Option<FilePos> {
93    let (offsets, positions) = parse_address_map(section)?;
94
95    // First perform a binary search on the `offsets` array. This is a sorted
96    // array of offsets within the text section, which is conveniently what our
97    // `offset` also is. Note that we are somewhat unlikely to find a precise
98    // match on the element in the array, so we're largely interested in which
99    // "bucket" the `offset` falls into.
100    let offset = u32::try_from(offset).ok()?;
101    let index = match offsets.binary_search_by_key(&offset, |v| v.get(LittleEndian)) {
102        // Exact hit!
103        Ok(i) => i,
104
105        // This *would* be at the first slot in the array, so no
106        // instructions cover `pc`.
107        Err(0) => return None,
108
109        // This would be at the `nth` slot, so we're at the `n-1`th slot.
110        Err(n) => n - 1,
111    };
112
113    // Using the `index` we found of which bucket `offset` corresponds to we can
114    // lookup the actual `FilePos` value in the `positions` array.
115    let pos = positions.get(index)?;
116    Some(FilePos(pos.get(LittleEndian)))
117}
118
119/// Iterate over the address map contained in the given address map section.
120///
121/// This function takes a `section` as its first argument which must have been
122/// created with `AddressMapSection` above. This is intended to be the raw
123/// `ELF_WASMTIME_ADDRMAP` section from the compilation artifact.
124///
125/// The yielded offsets are relative to the start of the text section for this
126/// map's code object.
127pub fn iterate_address_map<'a>(
128    section: &'a [u8],
129) -> Option<impl Iterator<Item = (u32, FilePos)> + 'a> {
130    let (offsets, positions) = parse_address_map(section)?;
131
132    Some(
133        offsets
134            .iter()
135            .map(|o| o.get(LittleEndian))
136            .zip(positions.iter().map(|pos| FilePos(pos.get(LittleEndian)))),
137    )
138}
wasmtime_environ/address_map.rs

wasmtime_environ/
address_map.rs
