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Date:   Thu, 31 Aug 2017 17:36:45 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "x86@...nel.org" <x86@...nel.org>,
        LKML <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>,
        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 5:21 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 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.

Yes, as far as I understand, there is just no defined semantics for
this. Passing sp as is when asm block asks to pass in sp looks like a
perfectly reasonable thing to do (also faster code). We could use
something like asm("..." ::: "frame"), but we don't have this in
compilers.

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