Re: [PATCH 2/3] tools/nolibc: i386: fix initial stack alignment

Willy Tarreau
2021-10-25T08:07:12Z

On Mon, Oct 25, 2021 at 07:46:11AM +0000, David Laight wrote:
> From: Willy Tarreau
> > Sent: 24 October 2021 18:28
> >
> > After re-checking in the spec and comparing stack offsets with glibc,
> > The last pushed argument must be 16-byte aligned (i.e. aligned before the
> > call) so that in the callee esp+4 is multiple of 16, so the principle is
> > the 32-bit equivalent to what Ammar fixed for x86_64. It's possible that
> > 32-bit code using SSE2 or MMX could have been affected. In addition the
> > frame pointer ought to be zero at the deepest level.
> >
> ...
> > /* startup code */
> > +/*
> > + * i386 System V ABI mandates:
> > + * 1) last pushed argument must be 16-byte aligned.
> > + * 2) The deepest stack frame should be set to zero
>
> I'm pretty sure that the historic SYSV i386 ABI only every required
> 4-byte alignment for the stack.
>
> At some point it got 'randomly' changed to 16-byte.
> I don't think this happened until after compiler support for SSE2
> intrinsics was added.

It's very possible because I've done a number of tests and noticed
that in some cases the called functions' stack doesn't seem to be
more than 4-aligned. However the deepest function in the stack starts
with an aligned stack so I prefer to follow this same rule.

Willy