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

[Florian Weimer]

* 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?

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.

Thanks,
Florian