Re: What integer type should ELF note header have?

[Mark Wielaard <mark@klomp.org>]

On Sun, 2017-12-10 at 22:22 -0700, Ali Bahrami wrote:
> To my reading, the words of the gABI support what HP-UX did, and the
> comment in the Solaris code makes me think our definition of
> ELF64_Nhdr
> was a mistake. And given that the rest of us don't have any installed
> base (yet) of 8 byte alignment notes, it seems that the most
> compatible
> thing that we could do would be to also use 8-byte words for these
> fields.
> triggered by an sh_addralign, or p_align, of 8. That seems to match
> the
> gABI words, and preserves interoperability with HP-UX.
> 
> An open question to you and Mark, and any others in the GNU world:
> Would you consider making that change? If so, I think we (Solaris)
> would have no problem in doing likewise.

I am afraid changing ELF64_Nhdr/GElf_Nhdr now from 32bit Words, or
changing the alignment of the namesz or descsz from 4 byte alignment is
 not going to work. It is too hardcoded in various GNU/Linux code bases
now to change. On GNU/Linux ELF notes have been used a lot already, if
only to parse the build-id embedded in every executable. So you will
find a lot of code that will simply do:

/*
 * Align offset to 4 bytes as needed for note name and descriptor data.
 */
#define NOTE_ALIGN(n) (((n) + 3) & -4U)

                GElf_Nhdr *nhdr = ptr;
                size_t namesz = NOTE_ALIGN(nhdr->n_namesz),
                       descsz = NOTE_ALIGN(nhdr->n_descsz);
                const char *name;

                ptr += sizeof(*nhdr);
                name = ptr;
                ptr += namesz;
[...]
                ptr += descsz;

The above is from the linux kernel perf tool (tools/perf/util/symbol-
elf.c), but I can easily find more examples of hard coded parsing of
ELF notes that simply assumes they are 32 bit Words, 4 byte aligned (it
is basically what elfutils libelf also does).

You could play with the SHT_NOTE sh_align and PT_NOTE p_align values,
but I haven't actually found any code that pays attention to it. So a
lot of code would have to be rewritten.

Cheers,

Mark