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

From: Dave Martin <Dave.Martin@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Florian Weimer <fweimer@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	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>,
	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
Date: Tue, 18 Jun 2019 14:32:25 +0100	[thread overview]
Message-ID: <20190618133223.GD2790@e103592.cambridge.arm.com> (raw)
In-Reply-To: <20190618125512.GJ3419@hirez.programming.kicks-ass.net>

On Tue, Jun 18, 2019 at 02:55:12PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 18, 2019 at 02:47:00PM +0200, Florian Weimer wrote:
> > * Peter Zijlstra:
> > 
> > > 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.
> > 
> > Just to be clear here: There was an ABI document that required PT_NOTE
> > parsing.
> 
> URGH.
> 
> > The Linux kernel does *not* define the x86-64 ABI, it only
> > implements it.  The authoritative source should be the ABI document.
> >
> > In this particularly case, so far anyone implementing this ABI extension
> > tried to provide value by changing it, sometimes successfully.  Which
> > makes me wonder why we even bother to mainatain ABI documentation.  The
> > kernel is just very late to the party.
> 
> How can the kernel be late to the party if all of this is spinning
> wheels without kernel support?

PT_GNU_PROPERTY is mentioned and allocated a p_type value in hjl's
spec [1], but otherwise seems underspecified.

In particular, it's not clear whether a PT_GNU_PROPERTY phdr _must_ be
emitted for NT_GNU_PROPERTY_TYPE_0.  While it seems a no-brainer to emit
it, RHEL's linker already doesn't IIUC, and there are binaries in the
wild.

Maybe this phdr type is a late addition -- I haven't attempted to dig
through the history.

For arm64 we don't have this out-of-tree legacy to support, so we can
avoid exhausitvely searching for the note: no PT_GNU_PROPERTY ->
no note.

So, can we do the same for x86, forcing RHEL to carry some code out of
tree to support their legacy binaries?  Or do we accept that there is
already a de facto ABI and try to be compatible with it?

From my side, I want to avoid duplication between x86 and arm64, and
keep unneeded complexity out of the ELF loader where possible.

Cheers
---Dave

[1] https://github.com/hjl-tools/linux-abi/wiki/Linux-Extensions-to-gABI