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Date: Tue, 7 Jul 2015 17:35:19 -0500
From: Josh Poimboeuf <jpoimboe@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
"H. Peter Anvin" <hpa@...or.com>
Cc: Michal Marek <mmarek@...e.cz>,
Peter Zijlstra <peterz@...radead.org>,
Andy Lutomirski <luto@...nel.org>,
Borislav Petkov <bp@...en8.de>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andi Kleen <andi@...stfloor.org>,
Pedro Alves <palves@...hat.com>, x86@...nel.org,
live-patching@...r.kernel.org, linux-kernel@...r.kernel.org,
Martin Jambor <mjambor@...e.cz>, Jiri Kosina <jkosina@...e.cz>,
Segher Boessenkool <segher@...nel.crashing.org>
Subject: Re: [PATCH v6 0/4] Compile-time stack validation
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
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