linux-kernel - Re: BUG: GCC-4.4.x changes the function frame on some functions

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Message-ID: <84fc9c000911191003t244eb864o3d5b355ab5485f@mail.gmail.com>
Date:	Thu, 19 Nov 2009 19:03:04 +0100
From:	Richard Guenther <richard.guenther@...il.com>
To:	rostedt@...dmis.org
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	feng.tang@...el.com, "Fr??d??ric Weisbecker" <fweisbec@...il.com>,
	Peter Zijlstra <peterz@...radead.org>, jakub@...hat.com,
	gcc@....gnu.org
Subject: Re: BUG: GCC-4.4.x changes the function frame on some functions

On Thu, Nov 19, 2009 at 6:59 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Thu, 2009-11-19 at 09:39 -0800, Linus Torvalds wrote:
>
>> > This modification leads to a hard to solve problem in the kernel
>> > function graph tracer which assumes that the stack looks like:
>> >
>> >        return address
>> >        saved  ebp
>>
>> Umm. But it still does, doesn't it? That
>>
>>       pushl  -0x4(%edi)
>>       push   %ebp
>>
>> should do it - the "-0x4(%edi)" thing seems to be trying to reload the
>> return address. No?
>
> Yes that is what it is doing. The problem we have is that it is putting
> into the frame pointer a "copy" of the return address, and not the
> actual pointer. Which is fine for the function tracer, but breaks the
> function graph tracer (which is a much more powerful tracer).
>
> Technically, this is all that mcount must have. And yes, we are making
> an assumption that the return address in the frame pointer is the one
> that will be used to leave the function. But the reason for making this
> copy just seems to be all messed up.
>
> I don't know if the ABI says anything about the return address in the
> frame pointer must be the actual return address. But it would be nice if
> the gcc folks would let us guarantee that it is.

Note that I only can reproduce the issue with
-mincoming-stack-boundary=2, not with -mpreferred-stack-boundary=2.
And
you didn't provide us with a testcase either ... so please open
a bugzilla and attach preprocessed source of a file that
shows the problem, note the function it happens in and provide
the command-line options you used for building.

Otherwise it's going to be all speculation on our side.

Thanks,
Richard.
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