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 ?

lld/trunk/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) {
   write32(Buf, 4);                      // Name size
   write32(Buf + 4, HashSize);           // Content size
   write32(Buf + 8, NT_GNU_BUILD_ID);    // Type
   memcpy(Buf + 12, "GNU", 4);           // Name string
   HashBuf = Buf + 16;

lld/trunk/test/ELF/build-id.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 
+# RUN: ld.lld --build-id %t -o %t2
+# RUN: llvm-readobj -s %t2 | FileCheck -check-prefix=ALIGN %s
+
 # RUN: ld.lld --build-id %t -o %t2 -threads
 # RUN: llvm-objdump -s %t2 | FileCheck -check-prefix=DEFAULT %s
 # RUN: ld.lld --build-id %t -o %t2 -no-threads
 # RUN: llvm-objdump -s %t2 | FileCheck -check-prefix=DEFAULT %s
 
 # RUN: ld.lld --build-id=md5 %t -o %t2 -threads
 # RUN: llvm-objdump -s %t2 | FileCheck -check-prefix=MD5 %s
 # RUN: ld.lld --build-id=md5 %t -o %t2 -no-threads

 
 .globl _start
 _start:
   nop
 
 .section .note.test, "a", @note
    .quad 42
 
+# ALIGN:      Name: .note.gnu.build-id
+# ALIGN-NEXT: Type: SHT_NOTE
+# ALIGN-NEXT: Flags [
+# ALIGN-NEXT:   SHF_ALLOC
+# ALIGN-NEXT: ]
+# ALIGN-NEXT: Address:
+# ALIGN-NEXT: Offset: [[_:0x[0-9A-Z]*(0|4|8|C)$]]
+# ALIGN-NEXT: Size:
+# ALIGN-NEXT: Link:
+# ALIGN-NEXT: Info:
+# ALIGN-NEXT: AddressAlignment: 4
+
 # DEFAULT:      Contents of section .note.test:
 # DEFAULT:      Contents of section .note.gnu.build-id:
 # DEFAULT-NEXT: 04000000 08000000 03000000 474e5500  ............GNU.
-# DEFAULT-NEXT: bc208f98 7f8e4770
+# DEFAULT-NEXT: 1950bcad 6ffba153
 
 # MD5:      Contents of section .note.gnu.build-id:
 # MD5-NEXT: 04000000 10000000 03000000 474e5500  ............GNU.
-# MD5-NEXT: 8e13f077 f9c94068 9d63c715 6c952cd3
+# MD5-NEXT: 6560b957 58afb40a 70f61d5a 7d76104e
 
 # SHA1:      Contents of section .note.gnu.build-id:
 # SHA1-NEXT: 04000000 14000000 03000000 474e5500  ............GNU.
-# SHA1-NEXT: 754ac1ea 0a2e5144 0f4642b9 3f096bbb
+# SHA1-NEXT: c41b2962 9fd0d863 0a35299a 746a626f
 
 # UUID:      Contents of section .note.gnu.build-id:
 # UUID-NEXT: 04000000 10000000 03000000 474e5500  ............GNU.
 
 # HEX:      Contents of section .note.gnu.build-id:
 # HEX-NEXT: 04000000 04000000 03000000 474e5500  ............GNU.
 # HEX-NEXT: 12345678
 
 # NONE-NOT: Contents of section .note.gnu.build-id: