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Date:   Tue, 18 Jun 2019 10:12:50 +0100
From:   Dave Martin <Dave.Martin@arm.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Florian Weimer <fweimer@redhat.com>,
        Yu-cheng Yu <yu-cheng.yu@intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-mm@kvack.org, linux-arch@vger.kernel.org,
        linux-api@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@amacapital.net>,
        Balbir Singh <bsingharora@gmail.com>,
        Borislav Petkov <bp@alien8.de>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Eugene Syromiatnikov <esyr@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>
Subject: Re: [PATCH v7 22/27] binfmt_elf: Extract .note.gnu.property from an
 ELF file
Message-ID: <20190618091248.GB2790@e103592.cambridge.arm.com>
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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'd still favour a Kconfig option to allow this support to be suppressed
by arches that don't have a similar legacy to be compatible with.

Cheers
---Dave