[ELF] Place SHT_NOTE sections with the same alignment into one PT_NOTE

Summary:
While the generic ABI requires notes to be 8-byte aligned in ELF64, many
vendor-specific notes (from Linux, NetBSD, Solaris, etc) use 4-byte
alignment.

In a PT_NOTE segment, if 4-byte aligned notes are followed by an 8-byte
aligned note, the possible 4-byte padding may make consumers fail to
parse the 8-byte aligned note. See PR41000 for a recent report about
.note.gnu.property (NT_GNU_PROPERTY_TYPE_0).
(Note, for NT_GNU_PROPERTY_TYPE_0, the consumers should probably migrate
to PT_GNU_PROPERTY, but the alignment issue affects other notes as well.)

To fix the issue, don't mix notes with different alignments in one
PT_NOTE. If compilers emit 4-byte aligned notes before 8-byte aligned
notes, we'll create at most 2 segments.

sh_size%sh_addralign=0 is actually implied by the rule for linking
unrecognized sections (in generic ABI), so we don't have to check that.
Notes that match in name, type and attribute flags are concatenated into
a single output section. The compilers have to ensure
sh_size%sh_addralign=0 to make concatenated notes parsable.

An alternative approach is to create a PT_NOTE for each SHT_NOTE, but
we'll have to incur the sizeof(Elf64_Phdr)=56 overhead every time a new
note section is introduced.

Reviewers: ruiu, jakehehrlich, phosek, jhenderson, pcc, espindola

Subscribers: emaste, arichardson, krytarowski, fedor.sergeev, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61296

llvm-svn: 359853

Author: MaskRay

lld/ELF/Writer.cpp

@@ -1999,11 +1999,12 @@ template <class ELFT> std::vector<PhdrEntry *> Writer<ELFT>::createPhdrs() {
   if (Config->ZWxneeded)
     AddHdr(PT_OPENBSD_WXNEEDED, PF_X);
 
-  // Create one PT_NOTE per a group of contiguous .note sections.
+  // Create one PT_NOTE per a group of contiguous SHT_NOTE sections with the
+  // same alignment.
   PhdrEntry *Note = nullptr;
   for (OutputSection *Sec : OutputSections) {
     if (Sec->Type == SHT_NOTE && (Sec->Flags & SHF_ALLOC)) {
-      if (!Note || Sec->LMAExpr)
+      if (!Note || Sec->LMAExpr || Note->LastSec->Alignment != Sec->Alignment)
         Note = AddHdr(PT_NOTE, PF_R);
       Note->add(Sec);
     } else {

lld/test/ELF/build-id.s

@@ -65,15 +65,15 @@ _start:
 # DEFAULT:      Contents of section .note.test:
 # DEFAULT:      Contents of section .note.gnu.build-id:
 # DEFAULT-NEXT: 04000000 08000000 03000000 474e5500  ............GNU.
-# DEFAULT-NEXT: 894c04e8 fbf5556b
+# DEFAULT-NEXT: 95849665 2621c734
 
 # MD5:      Contents of section .note.gnu.build-id:
 # MD5-NEXT: 04000000 10000000 03000000 474e5500  ............GNU.
-# MD5-NEXT: 6a51bbd7 9e8ee3f9 2e02d213 711cfec9
+# MD5-NEXT: 1882c01f 71698eed 229b3994 eb554c80
 
 # SHA1:      Contents of section .note.gnu.build-id:
 # SHA1-NEXT: 04000000 14000000 03000000 474e5500  ............GNU.
-# SHA1-NEXT: 9a8618b1 d6fd0e5c eda73dd8 76de5596
+# SHA1-NEXT: 96820adf d90d5470 0a0c32ff a88c4017
 
 # UUID:      Contents of section .note.gnu.build-id:
 # UUID-NEXT: 04000000 10000000 03000000 474e5500  ............GNU.

lld/test/ELF/note-alignment.s

@@ -0,0 +1,36 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+# RUN: ld.lld %t.o -o %t
+# RUN: llvm-readelf -l %t | FileCheck %s
+
+# Check that we don't mix 4-byte and 8-byte aligned notes in one PT_LOAD.
+# The possible 4-byte padding before the 8-byte align note may make consumers
+# fail to parse it.
+
+# CHECK: NOTE {{0x[0-9a-f]+}} {{0x[0-9a-f]+}} {{0x[0-9a-f]+}} 0x000004 0x000004 R   0x4
+# CHECK: NOTE {{0x[0-9a-f]+}} {{0x[0-9a-f]+}} {{0x[0-9a-f]+}} 0x000010 0x000010 R   0x8
+# CHECK: NOTE {{0x[0-9a-f]+}} {{0x[0-9a-f]+}} {{0x[0-9a-f]+}} 0x000008 0x000008 R   0x4
+
+# CHECK:      03     .note.a
+# CHECK-NEXT: 04     .note.b .note.c
+# CHECK-NEXT: 05     .note.d .note.e
+
+.section .note.a, "a", @note
+.align 4
+.long 0
+
+.section .note.b, "a", @note
+.align 8
+.quad 0
+
+.section .note.c, "a", @note
+.align 8
+.quad 0
+
+.section .note.d, "a", @note
+.align 4
+.long 0
+
+.section .note.e, "a", @note
+.align 4
+.long 0