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elf: fix parsing of notes after patchelf mangling #3111

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merged 2 commits into from
May 11, 2020
Merged

elf: fix parsing of notes after patchelf mangling #3111

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b9d0051
elf: fix parsing of notes after patchelf mangling
May 8, 2020
ed0a3b7
get section by name and check for SHT_NOBITS
May 11, 2020
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13 changes: 11 additions & 2 deletions snapcraft/internal/elf.py
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Original file line number Diff line number Diff line change
Expand Up @@ -298,8 +298,10 @@ def __init__(self, *, path: str) -> None:
self.elf_type: str = "ET_NONE"

try:
logger.debug("Extracting ELF attributes:", path)
self._extract_attributes()
except (UnicodeDecodeError, AttributeError, ConstructError) as exception:
logger.debug("Extracting ELF attributes exception: ", str(exception))
raise errors.CorruptedElfFileError(path, exception)

def _extract_attributes(self) -> None: # noqa: C901
Expand Down Expand Up @@ -335,8 +337,15 @@ def _extract_attributes(self) -> None: # noqa: C901
self.execstack_set = True
elif isinstance(segment, elftools.elf.segments.InterpSegment):
self.interp = segment.get_interp_name()
elif isinstance(segment, elftools.elf.segments.NoteSegment):
for note in segment.iter_notes():

# We cannot rely on iterating on the notes via the note segment.
# Patchelf may move note sections into other segments, and worse
# yet, possibly leave the note segment standing with invalid data.
# Here we instead iterate over each section and (more) reliably
# gather the note information.
for section in elf.iter_sections():
if isinstance(section, elftools.elf.sections.NoteSection):
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I was curious about what the best canonical way of determining the build ID is. I think this is the relevant part of the GDB source code:

https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/opncls.c;h=99097a9e39989e377af70e9450a970b009439e70;hb=HEAD#l1797

It seems to use the following process:

  1. Find the section named .note.gnu.build-id
  2. Read the contents of the section (this presumably makes sure its not a SHT_NOBITS section).
  3. Look for a note with name "GNU" and type "NT_GNU_BUILD_ID". The description field of that note is the build ID.

So you are right to switch over to checking sections, but I think we can switch this to section = elf.get_section_by_name('.note.gnu-build-id'), and work from there.

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Good idea, will do! Thanks!

for note in section.iter_notes():
if note.n_name == "GNU" and note.n_type == "NT_GNU_BUILD_ID":
self.build_id = _ensure_str(note.n_desc)

Expand Down