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

Yu-cheng Yu
2019-06-18T15:06:50Z

On Tue, 2019-06-18 at 14:32 +0100, Dave Martin wrote:
> 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.

Hi Florian,

The kernel looks at only ld-linux. Other applications are loaded by ld-linux.
So the issues are limited to three versions of ld-linux's. Can we somehow
update those??

Thanks,
Yu-cheng