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SubjectRe: [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@redhat.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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