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

[Josh Poimboeuf]

On Wed, Jan 11, 2017 at 10:21:07PM -0800, Andy Lutomirski wrote:
> On Tue, Jan 10, 2017 at 10:01 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > On Tue, Jan 10, 2017 at 8:35 PM, Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> wrote:
> >> On Tue, Jan 10, 2017 at 08:17:17PM -0800, Linus Torvalds wrote:
> >>>
> >>> That said, I do think that the "don't assume stack alignment, do it by
> >>> hand" may be the safer thing. Because who knows what the random rules
> >>> will be on other architectures.
> >>
> >> Sure we can ban the use of attribute aligned on stacks.  But
> >> what about indirect uses through structures?  For example, if
> >> someone does
> >>
> >> struct foo {
> >> } __attribute__ ((__aligned__(16)));
> >>
> >> int bar(...)
> >> {
> >>         struct foo f;
> >>
> >>         return baz(&f);
> >> }
> >>
> >> then baz will end up with an unaligned argument.  The worst part
> >> is that it is not at all obvious to the person writing the function
> >> bar.
> >
> > Linus, I'm starting to lean toward agreeing with Herbert here, except
> > that we should consider making it conditional on having a silly GCC
> > version.  After all, the silly GCC versions are wasting space and time
> > with alignment instructions no matter what we do, so this would just
> > mean tweaking the asm and adding some kind of check_stack_alignment()
> > helper to throw out a WARN_ONCE() if we miss one.  The problem with
> > making it conditional is that making pt_regs effectively live at a
> > variable offset from %rsp is just nasty.
> 
> So actually doing this is gross because we have calls from asm to C
> all over the place.  But... maybe we can automate all the testing.
> Josh, how hard would it be to teach objtool to (if requested by an
> option) check that stack frames with statically known size preserve
> 16-byte stack alignment?
> 
> I find it rather annoying that gcc before 4.8 malfunctions when it
> sees __aligned__(16) on x86_64 kernels.  Sigh.

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?

-- 
Josh