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

On Wed, Jan 11, 2017 at 10:21:07PM -0800, Andy Lutomirski wrote:

On Tue, Jan 10, 2017 at 10:01 PM, Andy Lutomirski [off-list ref] wrote:

quoted

On Tue, Jan 10, 2017 at 8:35 PM, Herbert Xu [off-list ref] wrote:

quoted

On Tue, Jan 10, 2017 at 08:17:17PM -0800, Linus Torvalds wrote:

quoted

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