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

From: hjl.tools at gmail dot com
Subject: [Bug ld/22749] Bad alignment and padding of .note.gnu.property
Date: Fri, 26 Jan 2018 15:35:42 +0000


H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
                 CC|                            |hjl.tools at gmail dot com

--- Comment #1 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Mark Wielaard from comment #0)
> 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:
> https://sourceware.org/ml/gnu-gabi/2017-q4/msg00028.html
> 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
> *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;

According to gABI, SHT_NOTE should be aligned to ELFCLASS64.
In any case, you can check note alignment by looking up the
alignment field in its header.  In elf.c, there are

static bfd_boolean
elf_parse_notes (bfd *abfd, char *buf, size_t size, file_ptr offset,
                 size_t align)
  char *p;

  /* NB: CORE PT_NOTE segments may have p_align values of 0 or 1.
     gABI specifies that PT_NOTE alignment should be aligned to 4
     bytes for 32-bit objects and to 8 bytes for 64-bit objects.  If
     align is less than 4, we use 4 byte alignment.   */
  if (align < 4)
    align = 4;

These tools can be updated to follow gABI without breaking backward

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