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Date: Tue, 7 Jul 2015 17:35:19 -0500
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>
Cc: Michal Marek <mmarek@suse.cz>, Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andi Kleen <andi@firstfloor.org>, Pedro Alves <palves@redhat.com>,
        x86@kernel.org, live-patching@vger.kernel.org,
        linux-kernel@vger.kernel.org, Martin Jambor <mjambor@suse.cz>,
        Jiri Kosina <jkosina@suse.cz>,
        Segher Boessenkool <segher@kernel.crashing.org>
Subject: Re: [PATCH v6 0/4] Compile-time stack validation
Message-ID: <20150707223519.GA31294@treble.redhat.com>
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On Tue, Jul 07, 2015 at 09:54:09AM -0500, Josh Poimboeuf wrote:
>    I did some more looking and it turns out that inline assembly doesn't
>    play nicely with frame pointers at all.  If the inline asm is at the
>    beginning of the function, gcc sometimes emits the inline asm code
>    before setting up the frame pointer.  That can break stack traces
>    when the inline asm has a call instruction.
> 
>    That turns out to be a very common problem.  Stackvalidate found 37 C
>    object files which break frame pointer rules, thanks to inline asm.
> 
>    I don't know of a solution to this problem yet.  Basically I think we
>    need a way to ensure that gcc emits the frame pointer setup before
>    inserting any inline asm (particularly when the inline asm has a call
>    instruction).

A solution to this problem was posted by Segher Boessenkool in a related
thread on the gcc mailing list:

  https://gcc.gnu.org/ml/gcc/2015-07/msg00080.html

The suggestion is to use something like:
  
	register void *sp asm("%sp");
	asm volatile("call func" : "+r"(sp));

I can confirm that it seems to fix the issue.  (I had tried something
like this before, but I guess I wasn't able to get the incantation just
right.)

Thanks to Jiri for the pointer to the thread, and Martin for raising the
issue on the gcc list.

-- 
Josh