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

[Dave Martin <Dave.Martin@arm.com>]

On Wed, Jun 12, 2019 at 12:04:01PM -0700, Yu-cheng Yu wrote:
> On Wed, 2019-06-12 at 10:32 +0100, Dave Martin wrote:
> > On Tue, Jun 11, 2019 at 12:31:34PM -0700, Yu-cheng Yu wrote:
> > > On Tue, 2019-06-11 at 12:41 +0100, Dave Martin wrote:
> > > > On Mon, Jun 10, 2019 at 07:24:43PM +0200, Florian Weimer wrote:
> > > > > * Yu-cheng Yu:
> > > > > 
> > > > > > To me, looking at PT_GNU_PROPERTY and not trying to support anything
> > > > > > is a
> > > > > > logical choice.  And it breaks only a limited set of toolchains.
> > > > > > 
> > > > > > I will simplify the parser and leave this patch as-is for anyone who
> > > > > > wants
> > > > > > to
> > > > > > back-port.  Are there any objections or concerns?
> > > > > 
> > > > > Red Hat Enterprise Linux 8 does not use PT_GNU_PROPERTY and is probably
> > > > > the largest collection of CET-enabled binaries that exists today.
> > > > 
> > > > For clarity, RHEL is actively parsing these properties today?
> > > > 
> > > > > My hope was that we would backport the upstream kernel patches for CET,
> > > > > port the glibc dynamic loader to the new kernel interface, and be ready
> > > > > to run with CET enabled in principle (except that porting userspace
> > > > > libraries such as OpenSSL has not really started upstream, so many
> > > > > processes where CET is particularly desirable will still run without
> > > > > it).
> > > > > 
> > > > > I'm not sure if it is a good idea to port the legacy support if it's not
> > > > > part of the mainline kernel because it comes awfully close to creating
> > > > > our own private ABI.
> > > > 
> > > > I guess we can aim to factor things so that PT_NOTE scanning is
> > > > available as a fallback on arches for which the absence of
> > > > PT_GNU_PROPERTY is not authoritative.
> > > 
> > > 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?
> 
> Yes, I will make it a Kconfig option.

OK, that works for me.  This would also help keep the PT_NOTE scanning
separate from the rest of the code.

For arm64 we could then unconditionally select/deselect that option,
where x86 could leave it configurable either way.

Cheers
---Dave