Re: x86-64: Maintain 16-byte stack alignment

[Andy Lutomirski <luto@amacapital.net>]

On Thu, Jan 12, 2017 at 12:15 PM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> On Thu, Jan 12, 2017 at 12:08:07PM -0800, Andy Lutomirski wrote:
>> On Thu, Jan 12, 2017 at 11:51 AM, Linus Torvalds
>> <torvalds@linux-foundation.org> wrote:
>> > On Thu, Jan 12, 2017 at 6:02 AM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>> >>
>> >> Just to clarify, I think you're asking if, for versions of gcc which
>> >> don't support -mpreferred-stack-boundary=3, objtool can analyze all C
>> >> functions to ensure their stacks are 16-byte aligned.
>> >>
>> >> It's certainly possible, but I don't see how that solves the problem.
>> >> The stack will still be misaligned by entry code.  Or am I missing
>> >> something?
>> >
>> > I think the argument is that we *could* try to align things, if we
>> > just had some tool that actually then verified that we aren't missing
>> > anything.
>> >
>> > I'm not entirely happy with checking the generated code, though,
>> > because as Ingo says, you have a 50:50 chance of just getting it right
>> > by mistake. So I'd much rather have some static tool that checks
>> > things at a code level (ie coccinelle or sparse).
>>
>> What I meant was checking the entry code to see if it aligns stack
>> frames, and good luck getting sparse to do that.  Hmm, getting 16-byte
>> alignment for real may actually be entirely a lost cause.  After all,
>> I think we have some inline functions that do asm volatile ("call
>> ..."), and I don't see any credible way of forcing alignment short of
>> generating an entirely new stack frame and aligning that.
>
> Actually we already found all such cases and fixed them by forcing a new
> stack frame, thanks to objtool.  For example, see 55a76b59b5fe.

What I mean is: what guarantees that the stack is properly aligned for
the subroutine call?  gcc promises to set up a stack frame, but does
it promise that rsp will be properly aligned to call a C function?