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

[Peter Zijlstra]

On Tue, Jun 18, 2019 at 10:12:50AM +0100, Dave Martin wrote:
> On Mon, Jun 17, 2019 at 02:20:40PM +0200, Thomas Gleixner wrote:
> > On Mon, 17 Jun 2019, Florian Weimer wrote:
> > > * Dave Martin:
> > > > On Tue, Jun 11, 2019 at 12:31:34PM -0700, Yu-cheng Yu wrote:
> > > >> We can probably check PT_GNU_PROPERTY first, and fallback (based on ld-linux
> > > >> version?) to PT_NOTE scanning?
> > > >
> > > > For arm64, we can check for PT_GNU_PROPERTY and then give up
> > > > unconditionally.
> > > >
> > > > For x86, we would fall back to PT_NOTE scanning, but this will add a bit
> > > > of cost to binaries that don't have NT_GNU_PROPERTY_TYPE_0.  The ld.so
> > > > version doesn't tell you what ELF ABI a given executable conforms to.
> > > >
> > > > Since this sounds like it's largely a distro-specific issue, maybe there
> > > > could be a Kconfig option to turn the fallback PT_NOTE scanning on?
> > > 
> > > I'm worried that this causes interop issues similarly to what we see
> > > with VSYSCALL today.  If we need both and a way to disable it, it should
> > > be something like a personality flag which can be configured for each
> > > process tree separately.  Ideally, we'd settle on one correct approach
> > > (i.e., either always process both, or only process PT_GNU_PROPERTY) and
> > > enforce that.
> > 
> > Chose one and only the one which makes technically sense and is not some
> > horrible vehicle.
> > 
> > Everytime we did those 'oh we need to make x fly workarounds' we regretted
> > it sooner than later.
> 
> So I guess that points to keeping PT_NOTE scanning always available as a
> fallback on x86.  This sucks a bit, but if there are binaries already in
> the wild that rely on this, I don't think we have much choice...

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.