linux-kernel - RE: [PATCH 2/3] tools/nolibc: i386: fix initial stack alignment

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Message-ID: <7e5ed287476042388779ca3c84483a92@AcuMS.aculab.com>
Date:   Mon, 25 Oct 2021 07:46:11 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Willy Tarreau' <w@....eu>,
        "Paul E . McKenney" <paulmck@...nel.org>
CC:     Bedirhan KURT <windowz414@...weeb.org>,
        Louvian Lyndal <louvianlyndal@...il.com>,
        Ammar Faizi <ammar.faizi@...dents.amikom.ac.id>,
        Peter Cordes <peter@...des.ca>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH 2/3] tools/nolibc: i386: fix initial stack alignment

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.
ISTR the NetBSD found that it was gcc that moved the goalposts.

	David

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