Add reference element reads

src/file_format/elf.rs

@@ -1074,7 +1074,7 @@ struct SymTab64 {
 ///
 /// By using this function, the user must be aware of the following limitations:
 /// - Only allocatable symbols and symbols used by the dynamic linker are exported
-/// (.symtab is not loaded in memory at runtime)
+///   (.symtab is not loaded in memory at runtime)
 /// - Only 64-bit ELFs are supported (an empty iterator will be returned for 32-bit ELFs)
 pub fn symbols(
     process: &Process,
@@ -1157,3 +1157,210 @@ pub fn symbols(
     })
     .fuse()
 }
+
+/// The GNU build ID of an ELF module, read from its `NT_GNU_BUILD_ID` note.
+/// The linker derives it from the built binary, so it names one exact build.
+#[derive(Copy, Clone, PartialEq, Eq, Hash)]
+pub struct BuildId {
+    bytes: [u8; 32],
+    len: u8,
+}
+
+impl BuildId {
+    /// The bytes of the build ID.
+    pub fn as_bytes(&self) -> &[u8] {
+        &self.bytes[..self.len as usize]
+    }
+}
+
+impl fmt::Debug for BuildId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        for byte in self.as_bytes() {
+            write!(f, "{byte:02x}")?;
+        }
+        Ok(())
+    }
+}
+
+/// Reads the GNU build ID from the notes of the ELF module starting at the
+/// given address. Returns [`None`] if the module carries no such note, which
+/// not every module does. Only little-endian ELFs are supported.
+pub fn build_id(process: &Process, module_address: Address) -> Option<BuildId> {
+    #[derive(Debug, Copy, Clone, Pod, Zeroable)]
+    #[repr(C)]
+    struct NoteHeader {
+        n_namesz: u32,
+        n_descsz: u32,
+        n_type: u32,
+    }
+
+    const NT_GNU_BUILD_ID: u32 = 3;
+
+    let header = process.read::<Header>(module_address).ok()?;
+    let info = Info::parse(bytemuck::bytes_of(&header))?;
+
+    if info.endian != Endian::Little {
+        return None;
+    }
+
+    let (e_phoff, e_phentsize, e_phnum) = if info.bitness.is_64() {
+        let header = process.read::<Elf64>(module_address).ok()?;
+        (header.e_phoff, header.e_phentsize as u64, header.e_phnum)
+    } else {
+        let header = process.read::<Elf32>(module_address).ok()?;
+        (
+            header.e_phoff as u64,
+            header.e_phentsize as u64,
+            header.e_phnum,
+        )
+    };
+
+    (0..e_phnum).find_map(|index| {
+        let at = module_address + e_phoff + e_phentsize.wrapping_mul(index as u64);
+
+        let (p_type, p_vaddr, p_filesz) = if info.bitness.is_64() {
+            let program_header = process.read::<ProgramHeader64>(at).ok()?;
+            (
+                program_header.p_type,
+                program_header.p_vaddr,
+                program_header.p_filesz,
+            )
+        } else {
+            let program_header = process.read::<ProgramHeader32>(at).ok()?;
+            (
+                program_header.p_type,
+                program_header.p_vaddr as u64,
+                program_header.p_filesz as u64,
+            )
+        };
+
+        if SegmentType(p_type) != SegmentType::PT_NOTE {
+            return None;
+        }
+
+        // A note is its header, the name, then the data, the latter two padded
+        // to four bytes.
+        let segment = module_address + p_vaddr;
+        let mut offset = 0;
+        // The walk is bounded so corrupt sizes can't turn it into a scan.
+        for _ in 0..0x10 {
+            if offset + size_of::<NoteHeader>() as u64 > p_filesz {
+                return None;
+            }
+
+            let note = process.read::<NoteHeader>(segment + offset).ok()?;
+            let name = offset + size_of::<NoteHeader>() as u64;
+            let desc = name + (note.n_namesz as u64).next_multiple_of(4);
+
+            if note.n_type == NT_GNU_BUILD_ID
+                && note.n_namesz == 4
+                && (1..=32).contains(&note.n_descsz)
+                && desc + note.n_descsz as u64 <= p_filesz
+                && process.read::<[u8; 4]>(segment + name).ok()? == *b"GNU\0"
+            {
+                let mut bytes = [0; 32];
+                process
+                    .read_into_buf(segment + desc, &mut bytes[..note.n_descsz as usize])
+                    .ok()?;
+                return Some(BuildId {
+                    bytes,
+                    len: note.n_descsz as u8,
+                });
+            }
+
+            offset = desc + (note.n_descsz as u64).next_multiple_of(4);
+        }
+
+        None
+    })
+}
+
+#[cfg(all(test, not(target_family = "wasm")))]
+mod tests {
+    use super::build_id;
+    use crate::runtime::mock::with_process;
+
+    use std::{format, vec, vec::Vec};
+
+    const BASE: u64 = 0x7F12_3450_0000;
+
+    // The build ID of the 2019.1 mono runtime, a sha1 the linker computed.
+    const BUILD_ID: [u8; 20] = [
+        0xE6, 0xAA, 0x00, 0x0A, 0x9A, 0x52, 0x01, 0x63, 0x57, 0x43, 0xC6, 0xA1, 0xB2, 0x78, 0x1E,
+        0x05, 0x7D, 0x65, 0x1D, 0xAD,
+    ];
+
+    fn put(image: &mut [u8], at: usize, bytes: &[u8]) {
+        image[at..at + bytes.len()].copy_from_slice(bytes);
+    }
+
+    // Builds a minimal mapped ELF by hand from the spec, so the walk is
+    // checked against the format rather than against itself: the header, a
+    // load and a note program header, and a note segment holding an ABI tag
+    // note followed by the build ID note.
+    fn image(wide: bool) -> Vec<u8> {
+        let mut image = vec![0; 0x400];
+        put(&mut image, 0x00, b"\x7fELF");
+        image[0x04] = if wide { 2 } else { 1 };
+        image[0x05] = 1;
+        image[0x06] = 1;
+        put(&mut image, 0x10, &3_u16.to_le_bytes());
+        if wide {
+            put(&mut image, 0x20, &0x40_u64.to_le_bytes());
+            put(&mut image, 0x36, &56_u16.to_le_bytes());
+            put(&mut image, 0x38, &2_u16.to_le_bytes());
+            put(&mut image, 0x40, &1_u32.to_le_bytes());
+            put(&mut image, 0x78, &4_u32.to_le_bytes());
+            put(&mut image, 0x88, &0x200_u64.to_le_bytes());
+            put(&mut image, 0x98, &0x44_u64.to_le_bytes());
+        } else {
+            put(&mut image, 0x1C, &0x34_u32.to_le_bytes());
+            put(&mut image, 0x2A, &32_u16.to_le_bytes());
+            put(&mut image, 0x2C, &2_u16.to_le_bytes());
+            put(&mut image, 0x34, &1_u32.to_le_bytes());
+            put(&mut image, 0x54, &4_u32.to_le_bytes());
+            put(&mut image, 0x5C, &0x200_u32.to_le_bytes());
+            put(&mut image, 0x64, &0x44_u32.to_le_bytes());
+        }
+        put(&mut image, 0x200, &4_u32.to_le_bytes());
+        put(&mut image, 0x204, &16_u32.to_le_bytes());
+        put(&mut image, 0x208, &1_u32.to_le_bytes());
+        put(&mut image, 0x20C, b"GNU\0");
+        put(&mut image, 0x220, &4_u32.to_le_bytes());
+        put(&mut image, 0x224, &20_u32.to_le_bytes());
+        put(&mut image, 0x228, &3_u32.to_le_bytes());
+        put(&mut image, 0x22C, b"GNU\0");
+        put(&mut image, 0x230, &BUILD_ID);
+        image
+    }
+
+    #[test]
+    fn reads_the_build_id_from_a_mapped_image() {
+        for wide in [true, false] {
+            with_process(&[(BASE, &image(wide))], |process| {
+                let build_id = build_id(process, BASE.into()).unwrap();
+                assert_eq!(build_id.as_bytes(), BUILD_ID);
+            });
+        }
+    }
+
+    #[test]
+    fn renders_the_id_as_hex() {
+        with_process(&[(BASE, &image(true))], |process| {
+            let build_id = build_id(process, BASE.into()).unwrap();
+            assert_eq!(
+                format!("{build_id:?}"),
+                "e6aa000a9a5201635743c6a1b2781e057d651dad",
+            );
+        });
+    }
+
+    #[test]
+    fn answers_nothing_without_a_note_segment() {
+        let mut image = image(true);
+        put(&mut image, 0x78, &0_u32.to_le_bytes());
+        with_process(&[(BASE, &image)], |process| {
+            assert!(build_id(process, BASE.into()).is_none());
+        });
+    }
+}