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[Bug ld/22749] New: Bad alignment and padding of .note.gnu.property

From: mark at klomp dot org
Subject: [Bug ld/22749] New: Bad alignment and padding of .note.gnu.property
Date: Fri, 26 Jan 2018 14:46:59 +0000


            Bug ID: 22749
           Summary: Bad alignment and padding of .note.gnu.property
           Product: binutils
           Version: 2.31 (HEAD)
            Status: NEW
          Severity: normal
          Priority: P2
         Component: ld
          Assignee: unassigned at sourceware dot org
          Reporter: mark at klomp dot org
  Target Milestone: ---

The .note.gnu.property SHT_NOTE uses wrong alignment and padding on ELFCLASS64.
This breaks note parsers which will see garbage data inside a NOTE and might
flag the ELF file as invalid or stop parsing any note data (since they cannot
know how to skip the note).

Unlike what gabi says, GNU always uses 4 byte alignment and padding, plus 32bit
sizes for namesz and descrsz in notes independent from ELF class.

gabi says to use 8 byte alignment and padding, plus 64bit sizes for ELFCLASS64,
but others (except HPUX it seems) also don't follow that. Nobody uses 8 byte
alignment/padding and 32bit sizes.

This was discussed on the gnu-gabi and gabi mailinglists:

Proposed fix:

diff --git a/bfd/elf-properties.c b/bfd/elf-properties.c
index 2549dd0..647931b 100644
--- a/bfd/elf-properties.c
+++ b/bfd/elf-properties.c
@@ -394,7 +394,8 @@ _bfd_elf_link_setup_gnu_properties (struct bfd_link_info
       unsigned int descsz;
       bfd_byte *contents;
       Elf_External_Note *e_note;
-      unsigned int align_size = bed->s->elfclass == ELFCLASS64 ? 8 : 4;
+      /* Note GNU SHT_NOTEs are always 4 byte aligned.  */
+      unsigned int align_size = 4;

       sec = bfd_get_section_by_name (first_pbfd,
diff --git a/bfd/elfxx-x86.c b/bfd/elfxx-x86.c
index a7db5d9..e6d45f3 100644
--- a/bfd/elfxx-x86.c
+++ b/bfd/elfxx-x86.c
@@ -2424,7 +2424,12 @@ _bfd_x86_elf_link_setup_gnu_properties
          if (sec == NULL)
            info->callbacks->einfo (_("%F%P: failed to create GNU property

-         if (!bfd_set_section_alignment (ebfd, sec, class_align))
+       /* GNU ABI always aligns and pads SHT_NOTES 4 bytes.  Using
+          32bit sizes for name_sz and desc_sz.  Independent from
+          the ELF class.  This is different from gabi, which uses
+          alignement and padding of 8 bytes, plus 64bit sizes for
+          ELFCLASS64.  Changing this breaks existing NOTE parsers.  */
+         if (!bfd_set_section_alignment (ebfd, sec, 2))
              info->callbacks->einfo (_("%F%A: failed to align section\n"),

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