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

[Andy Lutomirski]

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.  Ick.  This
whole situation stinks, and I wish that the gcc developers had been
less daft here in the first place or that we'd noticed and gotten it
fixed much longer ago.

Can we come up with a macro like STACK_ALIGN_16 that turns into
__aligned__(32) on bad gcc versions and combine that with your sparse
patch?