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

[Andy Lutomirski]

On Thu, Jan 12, 2017 at 7:11 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> On Thu, Jan 12, 2017 at 05:46:55PM -0800, Andy Lutomirski wrote:
>> On Thu, Jan 12, 2017 at 12:15 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxx> wrote:
>> >> > On Thu, Jan 12, 2017 at 6:02 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> 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?
>
> Yes, I did an experiment and you're right.  I had naively assumed that
> all stack frames would be aligned.

Just to check: did you do your experiment with -mpreferred-stack-boundary=4?

--Andy