Re: [opensuse-packaging] Re: debuginfo

3 Feb 2014

      Am 03.02.2014 13:02, schrieb Andreas Schwab:
...
Wolfgang Rosenauer <wolfgang@rosenauer.org> writes:
...
So the following question:
- what does find-debuginfo.sh do which changes the breakpad ID
How is the breakpad ID computed?
hmm, for details I would need to point you to the google-breakpad
project and source code but a quick and overall explanation I got is:

"the debug id is either the build ID from the ELF headers, or a hash of
the first page of .text"

Just found the relevant code:

// Attempt to locate a .note.gnu.build-id section in an ELF binary
// and copy as many bytes of it as will fit into |identifier|.
static bool FindElfBuildIDNote(const void *elf_mapped_base,
                               uint8_t identifier[kMDGUIDSize]) {
  void* note_section;
  int note_size, elfclass;
  if ((!FindElfSegment(elf_mapped_base, PT_NOTE,
                       (const void**)¬e_section, ¬e_size,
&elfclass) ||
      note_size == 0)  &&
      (!FindElfSection(elf_mapped_base, ".note.gnu.build-id", SHT_NOTE,
                       (const void**)¬e_section, ¬e_size,
&elfclass) ||
      note_size == 0)) {
    return false;
  }

  if (elfclass == ELFCLASS32) {
    return ElfClassBuildIDNoteIdentifier<ElfClass32>(note_section,
note_size,
                                                     identifier);
  } else if (elfclass == ELFCLASS64) {
    return ElfClassBuildIDNoteIdentifier<ElfClass64>(note_section,
note_size,
                                                     identifier);
  }

  return false;
}

// Attempt to locate the .text section of an ELF binary and generate
// a simple hash by XORing the first page worth of bytes into |identifier|.
static bool HashElfTextSection(const void *elf_mapped_base,
                               uint8_t identifier[kMDGUIDSize]) {
  void* text_section;
  int text_size;
  if (!FindElfSection(elf_mapped_base, ".text", SHT_PROGBITS,
                      (const void**)&text_section, &text_size, NULL) ||
      text_size == 0) {
    return false;
  }

  my_memset(identifier, 0, kMDGUIDSize);
  const uint8_t* ptr = reinterpret_cast<const uint8_t*>(text_section);
  const uint8_t* ptr_end = ptr + std::min(text_size, 4096);
  while (ptr < ptr_end) {
    for (unsigned i = 0; i < kMDGUIDSize; i++)
      identifier[i] ^= ptr[i];
    ptr += kMDGUIDSize;
  }
  return true;
}

// static
bool FileID::ElfFileIdentifierFromMappedFile(const void* base,
                                             uint8_t
identifier[kMDGUIDSize]) {
  // Look for a build id note first.
  if (FindElfBuildIDNote(base, identifier))
    return true;

  // Fall back on hashing the first page of the text section.
  return HashElfTextSection(base, identifier);
}

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