[ELF] Do not create multiple NOTE segments

rui314 · Sep 12, 2022 · cda57cf · cda57cf
1 parent 428aac7
commit cda57cf
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Showing 4 changed files with 15 additions and 12 deletions.
diff --git a/elf/output-chunks.cc b/elf/output-chunks.cc
@@ -213,8 +213,7 @@ static std::vector<ElfPhdr<E>> create_phdr(Context<E> &ctx) {
   if (ctx.interp)
     define(PT_INTERP, PF_R, 1, ctx.interp);
 
-  // Create a PT_NOTE for each group of SHF_NOTE sections with the same
-  // alignment requirement.
+  // Create a PT_NOTE for SHF_NOTE sections.
   for (i64 i = 0, end = ctx.chunks.size(); i < end;) {
     Chunk<E> *first = ctx.chunks[i++];
     if (!is_note(first))
@@ -225,8 +224,7 @@ static std::vector<ElfPhdr<E>> create_phdr(Context<E> &ctx) {
     define(PT_NOTE, flags, alignment, first);
 
     while (i < end && is_note(ctx.chunks[i]) &&
-           to_phdr_flags(ctx, ctx.chunks[i]) == flags &&
-           ctx.chunks[i]->shdr.sh_addralign == alignment)
+           to_phdr_flags(ctx, ctx.chunks[i]) == flags)
       append(ctx.chunks[i++]);
   }
 

diff --git a/elf/passes.cc b/elf/passes.cc
@@ -1287,14 +1287,21 @@ void clear_padding(Context<E> &ctx) {
 // sections in a single seek.
 //
 // .note sections are also placed at the beginning so that they are
-// included in a core crash dump even if it's truncated by ulimit.  In
+// included in a core crash dump even if it's truncated by ulimit. In
 // particular, if .note.gnu.build-id is in a truncated core file, you
 // can at least identify which executable has crashed.
 //
+// A PT_NOTE segment will contain multiple .note sections if exists,
+// but there's no way to represent a gap between .note sections.
+// Therefore, we sort .note sections by decreasing alignment
+// requirement. I believe each .note section size is a multiple of its
+// alignment, so by sorting them by alignment, we should be able to
+// avoid a gap between .note sections.
+//
 // .toc is placed right after .got for PPC64. PPC-specific .toc section
 // contains data that may be accessed with a 16-bit offset relative to
-// %r2. %r2 is set to .got + 0x8000. Therefore, .toc needs to be within
-// [.got, .got + 0x10000).
+// %r2. %r2 is set to .got + 32 KiB. Therefore, .toc needs to be within
+// [.got, .got + 64 KiB).
 //
 // Other file layouts are possible, but this layout is chosen to keep
 // the number of segments as few as possible.
@@ -1346,7 +1353,7 @@ void sort_output_sections(Context<E> &ctx) {
 
   auto get_rank2 = [&](Chunk<E> *chunk) -> i64 {
     if (chunk->shdr.sh_type == SHT_NOTE)
-      return chunk->shdr.sh_addralign;
+      return -chunk->shdr.sh_addralign;
 
     if (chunk->name == ".toc")
       return 2;

diff --git a/test/elf/note.sh b/test/elf/note.sh
@@ -45,8 +45,7 @@ grep -Eq '.note.baz\s+NOTE.+000008 00   A  0   0  8' $t/log
 grep -Eq '.note.nonalloc\s+NOTE.+000008 00      0   0  1' $t/log
 
 readelf --segments $t/exe > $t/log
-fgrep -q '01     .note.bar' $t/log
-fgrep -q '02     .note.baz .note.foo' $t/log
+grep -Fq '01     .note.baz .note.foo .note.bar' $t/log
 ! grep -q 'NOTE.*0x0000000000000000 0x0000000000000000' $t/log || false
 
 echo OK
diff --git a/test/elf/note2.sh b/test/elf/note2.sh
@@ -39,7 +39,6 @@ EOF
 ./mold -static -o $t/exe $t/a.o $t/b.o $t/c.o $t/d.o
 
 readelf --segments $t/exe > $t/log
-fgrep -q '01     .note.b' $t/log
-fgrep -q '02     .note.a .note.c' $t/log
+grep -Fq '01     .note.a .note.c .note.b' $t/log
 
 echo OK