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[ObjectYAML][ELF] Take alignment into account when generating notes (#118157)
The [System V ABI](https://www.sco.com/developers/gabi/latest/ch5.pheader.html#note_section) states that the note entries and their descriptor fields must be aligned to 4 or 8 bytes for 32-bit or 64-bit objects respectively. In practice, 64-bit systems can use both alignments, with the actual format being determined by the alignment of the segment. For example, the [Linux gABI extension](https://github.com/hjl-tools/linux-abi/wiki/linux-abi-draft.pdf) contains a special note on this, see 2.1.7 "Alignment of Note Sections". This patch adjusts the format of the generated notes to the specified section alignment. Since `llvm-readobj` was fixed in a similar way in [D150022](https://reviews.llvm.org/D150022), "[Object] Fix handling of Elf_Nhdr with sh_addralign=8", the generated notes can now be parsed successfully by the tool.
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‎llvm/lib/ObjectYAML/ELFEmitter.cpp‎

Lines changed: 17 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -1799,6 +1799,20 @@ void ELFState<ELFT>::writeSectionContent(Elf_Shdr &SHeader,
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if (!Section.Notes)
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return;
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unsigned Align;
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switch (SHeader.sh_addralign) {
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case 0:
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case 4:
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Align = 4;
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break;
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case 8:
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Align = 8;
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break;
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default:
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reportError(Section.Name + ": invalid alignment for a note section: 0x" +
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Twine::utohexstr(SHeader.sh_addralign));
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}
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uint64_t Offset = CBA.tell();
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for (const ELFYAML::NoteEntry &NE : *Section.Notes) {
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// Write name size.
@@ -1820,14 +1834,15 @@ void ELFState<ELFT>::writeSectionContent(Elf_Shdr &SHeader,
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if (!NE.Name.empty()) {
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CBA.write(NE.Name.data(), NE.Name.size());
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CBA.write('\0');
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CBA.padToAlignment(4);
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}
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// Write description and padding.
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if (NE.Desc.binary_size() != 0) {
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CBA.padToAlignment(Align);
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CBA.writeAsBinary(NE.Desc);
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CBA.padToAlignment(4);
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}
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CBA.padToAlignment(Align);
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}
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SHeader.sh_size = CBA.tell() - Offset;

‎llvm/test/tools/yaml2obj/ELF/note-section.yaml‎

Lines changed: 129 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -333,3 +333,132 @@ Sections:
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- Name: ABC
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Desc: '123456'
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Type: NT_VERSION
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## Check that an incorrect alignment is reported
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# RUN: not yaml2obj --docnum=16 %s 2>&1 | FileCheck %s --check-prefix=ERR_ALIGN1
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# ERR_ALIGN1: error: .note.foo: invalid alignment for a note section: 0x1
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--- !ELF
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FileHeader:
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Class: ELFCLASS64
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Data: ELFDATA2LSB
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Type: ET_EXEC
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Sections:
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- Name: .note.foo
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Type: SHT_NOTE
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AddressAlign: 1
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Notes:
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- Type: 0x1
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## Check that note entries and their `Desc` fields are aligned according to the
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## specified section alignment
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# RUN: yaml2obj --docnum=17 %s -o - | \
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# RUN: llvm-readobj --sections --section-data --notes - | \
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# RUN: FileCheck %s --check-prefix=TEST17
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# TEST17: Name: .note.foo4
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# TEST17: SectionData (
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# TEST17-NEXT: 0000: 05000000 02000000 01000000 41424344 |............ABCD|
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# TEST17-NEXT: 0010: 00000000 01020000 00000000 03000000 |................|
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# TEST17-NEXT: 0020: 02000000 03040500 04000000 00000000 |................|
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# TEST17-NEXT: 0030: 03000000 474E5500 |....GNU.|
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# TEST17-NEXT: )
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# TEST17: Name: .note.foo8
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# TEST17: SectionData (
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# TEST17-NEXT: 0000: 05000000 02000000 01000000 41424344 |............ABCD|
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# TEST17-NEXT: 0010: 00000000 00000000 01020000 00000000 |................|
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# TEST17-NEXT: 0020: 00000000 03000000 02000000 00000000 |................|
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# TEST17-NEXT: 0030: 03040500 00000000 04000000 00000000 |................|
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# TEST17-NEXT: 0040: 03000000 474E5500 |....GNU.|
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# TEST17-NEXT: )
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# TEST17: NoteSections [
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# TEST17-NEXT: NoteSection {
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# TEST17-NEXT: Name: .note.foo4
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# TEST17-NEXT: Offset:
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# TEST17-NEXT: Size:
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# TEST17-NEXT: Notes [
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# TEST17-NEXT: {
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# TEST17-NEXT: Owner: ABCD
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# TEST17-NEXT: Data size: 0x2
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# TEST17-NEXT: Type: NT_VERSION (version)
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# TEST17-NEXT: Description data (
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# TEST17-NEXT: 0000: 0102 |..|
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# TEST17-NEXT: )
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# TEST17-NEXT: }
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# TEST17-NEXT: {
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# TEST17-NEXT: Owner:
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# TEST17-NEXT: Data size: 0x3
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# TEST17-NEXT: Type: NT_ARCH (architecture)
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# TEST17-NEXT: Description data (
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# TEST17-NEXT: 0000: 030405 |...|
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# TEST17-NEXT: )
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# TEST17-NEXT: }
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# TEST17-NEXT: {
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# TEST17-NEXT: Owner: GNU
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# TEST17-NEXT: Data size: 0x0
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# TEST17-NEXT: Type: NT_GNU_BUILD_ID (unique build ID bitstring)
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# TEST17-NEXT: Build ID:
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# TEST17-NEXT: }
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# TEST17-NEXT: ]
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# TEST17-NEXT: }
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# TEST17-NEXT: NoteSection {
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# TEST17-NEXT: Name: .note.foo8
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# TEST17-NEXT: Offset:
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# TEST17-NEXT: Size:
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# TEST17-NEXT: Notes [
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# TEST17-NEXT: {
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# TEST17-NEXT: Owner: ABCD
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# TEST17-NEXT: Data size: 0x2
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# TEST17-NEXT: Type: NT_VERSION (version)
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# TEST17-NEXT: Description data (
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# TEST17-NEXT: 0000: 0102 |..|
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# TEST17-NEXT: )
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# TEST17-NEXT: }
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# TEST17-NEXT: {
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# TEST17-NEXT: Owner:
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# TEST17-NEXT: Data size: 0x3
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# TEST17-NEXT: Type: NT_ARCH (architecture)
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# TEST17-NEXT: Description data (
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# TEST17-NEXT: 0000: 030405 |...|
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# TEST17-NEXT: )
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# TEST17-NEXT: }
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# TEST17-NEXT: {
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# TEST17-NEXT: Owner: GNU
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# TEST17-NEXT: Data size: 0x0
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# TEST17-NEXT: Type: NT_GNU_BUILD_ID (unique build ID bitstring)
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# TEST17-NEXT: Build ID:
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# TEST17-NEXT: }
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# TEST17-NEXT: ]
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# TEST17-NEXT: }
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# TEST17-NEXT: ]
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--- !ELF
438+
FileHeader:
439+
Class: ELFCLASS64
440+
Data: ELFDATA2LSB
441+
Type: ET_EXEC
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Sections:
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- Name: .note.foo4
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Type: SHT_NOTE
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AddressAlign: 4
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Notes:
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- Name: ABCD
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Type: NT_VERSION
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Desc: 0102
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- Type: NT_ARCH
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Desc: 030405
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- Name: GNU
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Type: NT_GNU_BUILD_ID
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- Name: .note.foo8
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Type: SHT_NOTE
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AddressAlign: 8
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Notes:
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- Name: ABCD
459+
Type: NT_VERSION
460+
Desc: 0102
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- Type: NT_ARCH
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Desc: 030405
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- Name: GNU
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Type: NT_GNU_BUILD_ID

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