Re: OSABI on Linux Distros

[Carlos O'Donell <carlos@redhat.com>]

On 06/11/2016 02:44 AM, Suprateeka R Hegde wrote:
> On 11-Jun-2016 02:52 AM, Roland McGrath wrote:
>>> What is the reason why we emit OSABI value as "UNIX - System V"
>>> (ELFOSABI_SYSV) even on GNU/Linux systems? Shouldn't it be just "GNU"
>>> (ELFOSABI_GNU)?
>>
>> In GNU systems the use of this field is specifically to indicate that
>> certain GNU extensions to ELF are used in the particular object.  In
>> particular, if any .dynsym entries use STT_GNU_IFUNC o STB_GNU_UNIQUE,
>> then e_ident[EI_OSABI] must be ELFOSABI_GNU.
> 
> OK. I understood this part. But then I did not know this set is
> partial. I thought use of *any* GNU specific extension should mark it
> as GNU OSABI.
> 
> For instance, GNU_RELRO, GNU_EH_FRAME, GNU_STACK, etc. Even with all
> these, it is still marked SYSV ABI.

I'm not as authoritative on this as Roland is, but in my opinion I expect
that because all three of these entries are optional they do not constitute
a change in the EI_OSABI.

You can ignore GNU_RELRO without any problem, you just won't have a
read-only segment after relocation.

You can ignore GNU_STACK without any problems, it's information
for the dynamic loader to set default stack permissions. When unset the
glibc dynamic loader will use sensible per-machine defaults, likewise
other implementations should also.

The GNU_EH_FRAME can be ignored if you're not using the GNU-based
unwinder which uses this PT_* entry to find .eh_frame_hdr.

Other dynamic linkers should ignore program header segments they don't
understand, and thus the above optionally processed entries should not
change the EI_OSABI.

> 1. Do we have a documented list of GNU extensions that are necessary
> to mark an ELF as GNU ABI?

Not that I am aware of. H.J. might better answer this question.

> 2. Why the list is partial? Why not all GNU extensions?

Only the required GNU extensions should mark EI_OSABI as ELFOSABI_GNU.

If you don't implement STT_GNU_IFUNC or STB_GNU_UNIQUE then things
actually fail.

-- 
Cheers,
Carlos.