linux-kernel - Re: [RFC PATCH 4/4] x86/asm: Use ASM_CALL() macro for inline asm statements with call instructions

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Message-ID: <20170831152153.dre6gjdqcucmi5oc@treble>
Date:   Thu, 31 Aug 2017 10:21:53 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [RFC PATCH 4/4] x86/asm: Use ASM_CALL() macro for inline asm
 statements with call instructions

On Thu, Aug 31, 2017 at 04:50:41PM +0200, Peter Zijlstra wrote:
> On Thu, Aug 31, 2017 at 09:11:20AM -0500, Josh Poimboeuf wrote:
> > Inline asm statements which have call instructions can be problematic.
> > GCC doesn't know about the call instructions, so in some cases it can
> > insert the asm before setting up the frame pointer.  This can result in
> > bad stack traces when unwinding from the called function.
> > 
> > Previously we worked around this issue by listing the stack pointer as
> > an input/output constraint for the inline asm.  That works for GCC, but
> > unfortunately it doesn't work for Clang.  In fact, it causes Clang to
> > corrupt the stack pointer.
> 
> Sounds like it ought to get fixed regardless and then it might as well
> do the right thing ;-)

There was some disagreement about what the "right thing" is because it's
an undocumented and unintuitive interface.

And I use the term "interface" loosely.  It was apparently a side effect
which was mentioned to me on the GCC mailing list.

-- 
Josh