Bug 1505719 - Try harder when computing breakpad IDs for Android syst…

…em libraries. r=ted

Differential Revision: https://phabricator.services.mozilla.com/D12836

--HG--
extra : moz-landing-system : lando

tools/profiler/rust-helper/Cargo.toml

@@ -12,9 +12,17 @@ optional = true
 default-features = false
 features = ["std"]
 
+[dependencies.goblin]
+optional = true
+# The version and features of goblin need to match what's in object's Cargo.toml,
+# because we really want object's goblin and not another instance of goblin.
+version = "0.0.17"
+features = ["endian_fd", "elf32", "elf64", "mach32", "mach64", "pe32", "pe64", "archive"]
+default-features = false
+
 [dependencies.thin-vec]
 version = "0.1.0"
 features = ["gecko-ffi"]
 
 [features]
-parse_elf = ["object"]
+parse_elf = ["object", "goblin"]

tools/profiler/rust-helper/src/elf.rs

@@ -1,6 +1,11 @@
 use compact_symbol_table::CompactSymbolTable;
 use object::{ElfFile, Object, SymbolKind, Uuid};
 use std::collections::HashMap;
+use std::cmp;
+use goblin::elf;
+
+const UUID_SIZE: usize = 16;
+const PAGE_SIZE: usize = 4096;
 
 fn get_symbol_map<'a, 'b, T>(object_file: &'b T) -> HashMap<u32, &'a str>
 where
@@ -16,9 +21,73 @@ where
 
 pub fn get_compact_symbol_table(buffer: &[u8], breakpad_id: &str) -> Option<CompactSymbolTable> {
   let elf_file = ElfFile::parse(buffer).ok()?;
-  let elf_id = Uuid::from_bytes(elf_file.build_id()?).ok()?;
+  let elf_id = get_elf_id(&elf_file, buffer)?;
   if format!("{:X}0", elf_id.simple()) != breakpad_id {
     return None;
   }
   return Some(CompactSymbolTable::from_map(get_symbol_map(&elf_file)));
 }
+
+fn create_elf_id(identifier: &[u8], little_endian: bool) -> Option<Uuid> {
+  // Make sure that we have exactly UUID_SIZE bytes available
+  let mut data = [0 as u8; UUID_SIZE];
+  let len = cmp::min(identifier.len(), UUID_SIZE);
+  data[0..len].copy_from_slice(&identifier[0..len]);
+
+  if little_endian {
+    // The file ELF file targets a little endian architecture. Convert to
+    // network byte order (big endian) to match the Breakpad processor's
+    // expectations. For big endian object files, this is not needed.
+    data[0..4].reverse(); // uuid field 1
+    data[4..6].reverse(); // uuid field 2
+    data[6..8].reverse(); // uuid field 3
+  }
+
+  Uuid::from_bytes(&data).ok()
+}
+
+/// Tries to obtain the object identifier of an ELF object.
+///
+/// As opposed to Mach-O, ELF does not specify a unique ID for object files in
+/// its header. Compilers and linkers usually add either `SHT_NOTE` sections or
+/// `PT_NOTE` program header elements for this purpose. If one of these notes
+/// is present, ElfFile's build_id() method will find it.
+///
+/// If neither of the above are present, this function will hash the first page
+/// of the `.text` section (program code). This matches what the Breakpad
+/// processor does.
+///
+/// If all of the above fails, this function will return `None`.
+pub fn get_elf_id(elf_file: &ElfFile, data: &[u8]) -> Option<Uuid> {
+  if let Some(identifier) = elf_file.build_id() {
+    return create_elf_id(identifier, elf_file.elf().little_endian);
+  }
+
+  // We were not able to locate the build ID, so fall back to hashing the
+  // first page of the ".text" (program code) section. This algorithm XORs
+  // 16-byte chunks directly into a UUID buffer.
+  if let Some(section_data) = find_text_section(elf_file.elf(), data) {
+    let mut hash = [0; UUID_SIZE];
+    for i in 0..cmp::min(section_data.len(), PAGE_SIZE) {
+      hash[i % UUID_SIZE] ^= section_data[i];
+    }
+
+    return create_elf_id(&hash, elf_file.elf().little_endian);
+  }
+
+  None
+}
+
+/// Returns a reference to the data of the the .text section in an ELF binary.
+fn find_text_section<'elf, 'data>(
+  elf: &'elf elf::Elf,
+  data: &'data [u8],
+) -> Option<&'data[u8]> {
+  elf.section_headers.iter().find_map(|header| {
+    match (header.sh_type, elf.shdr_strtab.get(header.sh_name)) {
+      (elf::section_header::SHT_PROGBITS, Some(Ok(".text"))) =>
+        Some(&data[header.sh_offset as usize..][..header.sh_size as usize]),
+      _ => None
+    }
+  })
+}

tools/profiler/rust-helper/src/lib.rs

@@ -3,6 +3,8 @@ extern crate thin_vec;
 
 #[cfg(feature = "parse_elf")]
 extern crate object;
+#[cfg(feature = "parse_elf")]
+extern crate goblin;
 
 mod compact_symbol_table;