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

[Dave Martin]

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?

Cheers
---Dave