Give .note.gnu.build-id section alignment 4

[mcgrathr]

Summary

All SHT_NOTE sections should have minimum alignment 4.

[ruiu]

Is there a documentation about this? (I understand there's little hope, but just asking.)

I can see that why we want to align it to 4 bytes. Since the section does not contain 64-bit values even on 64-bit, 32-bit should suffice.

[mcgrathr]

http://www.sco.com/developers/gabi/latest/ch5.pheader.html#note_section describes alignment of the contents of notes. Maintaining that alignment is obviously impossible if the section is not so aligned.

[ruiu]

Ah, thanks. It is a good surprise. So, the document says that "In 64-bit objects (files with e_ident[EI_CLASS] equal to ELFCLASS64), each entry is an array of 8-byte words in the format of the target processor." Maybe you should align it to Config->Wordsize instead of 4?

[hjl.tools]

See:

https://groups.google.com/forum/#!topic/generic-abi/vT-_QVcckXo

[grimar]

Probably needs testcase ?

ELF/SyntheticSections.cpp

static size_t getHashSize() {
   case BuildIdKind::Hexstring:
     return Config->BuildIdVector.size();
   default:
     llvm_unreachable("unknown BuildIdKind");
   }
 }
 
 BuildIdSection::BuildIdSection()
-    : SyntheticSection(SHF_ALLOC, SHT_NOTE, 1, ".note.gnu.build-id"),
+    : SyntheticSection(SHF_ALLOC, SHT_NOTE, 4, ".note.gnu.build-id"),
       HashSize(getHashSize()) {}
 
 void BuildIdSection::writeTo(uint8_t *Buf) {
   endianness E = Config->Endianness;
   write32(Buf, 4, E);                   // Name size
   write32(Buf + 4, HashSize, E);        // Content size
   write32(Buf + 8, NT_GNU_BUILD_ID, E); // Type
   memcpy(Buf + 12, "GNU", 4);           // Name string