[PATCH] D150022: [Object] Fix handling of Elf_Nhdr with sh_addralign=8

[Fangrui Song via Phabricator via llvm-commits]

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The generic ABI says:

> Padding is present, if necessary, to ensure 8 or 4-byte alignment for the next note entry (depending on whether the file is a 64-bit or 32-bit object). Such padding is not included in descsz.

Our parsing code currently aligns n_namesz. Fix the bug by aligning the start
offset of the descriptor instead. This issue has been benign because the primary
uses of sh_addralign=8 notes are `.note.gnu.property`, where
`sizeof(Elf_Nhdr) + sizeof("GNU") = 16` (already aligned by 8).

In practice, many 64-bit systems incorrectly use sh_addralign=4 notes.
We can use sh_addralign (= p_align) to decide the descriptor padding.
Treat an alignment of 0 and 1 as 4. This approach matches modern GNU readelf
(since 2018).

We have a few tests incorrectly using sh_addralign=0. We may make our behavior
stricter after fixing these tests.

Linux kernel dumped core files use `p_align=0` notes, so we need to support the
case for compatibility.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D150022

Files:
  lld/ELF/InputFiles.cpp
  llvm/include/llvm/Object/ELF.h
  llvm/include/llvm/Object/ELFTypes.h
  llvm/lib/Object/BuildID.cpp
  llvm/test/tools/llvm-readobj/ELF/AArch64/aarch64-note-gnu-property.s
  llvm/test/tools/llvm-readobj/ELF/note-alignment-invalid.test
  llvm/test/tools/llvm-readobj/ELF/note-gnu-property2.s
  llvm/test/tools/llvm-readobj/ELF/note-unknown.s
  llvm/tools/llvm-readobj/ELFDumper.cpp
  llvm/tools/obj2yaml/elf2yaml.cpp
  llvm/unittests/Object/ELFTypesTest.cpp

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