Re: [PATCH v7 22/27] binfmt_elf: Extract .note.gnu.property from an ELF file

Dave Martin
2019-06-18T13:32:34Z

On Tue, Jun 18, 2019 at 02:55:12PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 18, 2019 at 02:47:00PM +0200, Florian Weimer wrote:
> > * Peter Zijlstra:
> >
> > > I'm not sure I read Thomas' comment like that. In my reading keeping the
> > > PT_NOTE fallback is exactly one of those 'fly workarounds'. By not
> > > supporting PT_NOTE only the 'fine' people already shit^Hpping this out
> > > of tree are affected, and we don't have to care about them at all.
> >
> > Just to be clear here: There was an ABI document that required PT_NOTE
> > parsing.
>
> URGH.
>
> > The Linux kernel does *not* define the x86-64 ABI, it only
> > implements it. The authoritative source should be the ABI document.
> >
> > In this particularly case, so far anyone implementing this ABI extension
> > tried to provide value by changing it, sometimes successfully. Which
> > makes me wonder why we even bother to mainatain ABI documentation. The
> > kernel is just very late to the party.
>
> How can the kernel be late to the party if all of this is spinning
> wheels without kernel support?

PT_GNU_PROPERTY is mentioned and allocated a p_type value in hjl's
spec [1], but otherwise seems underspecified.

In particular, it's not clear whether a PT_GNU_PROPERTY phdr _must_ be
emitted for NT_GNU_PROPERTY_TYPE_0. While it seems a no-brainer to emit
it, RHEL's linker already doesn't IIUC, and there are binaries in the
wild.

Maybe this phdr type is a late addition -- I haven't attempted to dig
through the history.

For arm64 we don't have this out-of-tree legacy to support, so we can
avoid exhausitvely searching for the note: no PT_GNU_PROPERTY ->
no note.

So, can we do the same for x86, forcing RHEL to carry some code out of
tree to support their legacy binaries? Or do we accept that there is
already a de facto ABI and try to be compatible with it?

From my side, I want to avoid duplication between x86 and arm64, and
keep unneeded complexity out of the ELF loader where possible.

Cheers
---Dave

[1] https://github.com/hjl-tools/linux-abi/wiki/Linux-Extensions-to-gABI