linux-kernel - Re: v4.10: kernel stack frame pointer .. has bad value (null)

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Date:   Thu, 9 Mar 2017 09:14:47 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Pavel Machek <pavel@....cz>,
        kernel list <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Peter Anvin <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: v4.10: kernel stack frame pointer .. has bad value (null)

On Wed, 8 Mar 2017 15:29:59 -0600
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> [adding Steven Rostedt to CC as an FYI]
> 
> On Wed, Mar 08, 2017 at 10:25:01AM -0800, Linus Torvalds wrote:
> > On Wed, Mar 8, 2017 at 9:37 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:  
> > > - CONFIG_FUNCTION_GRAPH_TRACER sets it on x86-32 because of a gcc bug
> > >   where the stack gets aligned before the mcount call.  This issue
> > >   should be mostly obsolete as most modern compilers now have -mfentry.
> > >   We could make it dependent on CC_USING_FENTRY.  
> > 
> > Yeah. At some point we might even upgrade the compiler requirements to
> > no longer accept the mcount model.  
> 
> The plot slightly thickens...
> 
> So I was mistaken about this problem not existing with newer versions of
> gcc, because the x86-32 ftrace code doesn't use -mfentry.  It still
> relies on mcount.  So CONFIG_FUNCTION_GRAPH_TRACER will still need
> -maccumulate-outgoing-args for *all* versions of gcc on x86-32.

OK, I admit, I was lazy here. I thought, who cares about x86-32
anymore ;-)

> 
> (Of course, that situation would improve if ftrace on x86-32 were ported
> to use -mfentry.)

That can easily be done.

> 
> Also, since -Os tells gcc to ignore -maccumulate-outgoing-args, this
> means that CONFIG_FUNCTION_GRAPH_TRACER with mcount needs a dependency
> on CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE.
> 
> I suspect these issues also affect x86-64 with gcc 4.4.x and 4.5.x,
> which corresponds to the window after the funky DRAP prologue was
> introduced but before -mfentry was introduced.
> 
> In summary, here are the changes I'm looking at:
> 
> - set -maccumulate-outgoing-args if CONFIG_FUNCTION_GRAPH_TRACER &&
>   !CC_USING_ENTRY
>   (for both 32- and 64-bit)
> 
> - somehow make CONFIG_FUNCTION_GRAPH_TRACER depend on either
>   CC_USING_FENTRY or CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
>   (for both 32- and 64-bit)
> 
>   (not sure how to do that -- maybe just fail the build in the
>   graph tracer + mcount + '-Os' case)

Could just place something like this in the x86 code:

#if defined(CONFIG_FUNCTION_GRAPH_TRACER) && \
    !defined(CC_USING_FENTRY) && \
    !defined(CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE)
# error Your compiler doesn't support function graph tracing
#endif

-- Steve

> 
> - set -maccumulate-outgoing-args if CONFIG_JUMP_LABEL && gcc < 4.5.2
>   (for both 32-bit and 64-bit)
>