linux-kernel - RE: [RFC PATCH v1 3/6] tools/nolibc: i386: Implement syscall with 6 arguments

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Date:   Sun, 20 Mar 2022 18:22:35 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Ammar Faizi' <ammarfaizi2@...weeb.org>, Willy Tarreau <w@....eu>
CC:     "Paul E. McKenney" <paulmck@...nel.org>,
        Alviro Iskandar Setiawan <alviro.iskandar@...weeb.org>,
        Nugraha <richiisei@...il.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        GNU/Weeb Mailing List <gwml@...r.gnuweeb.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>
Subject: RE: [RFC PATCH v1 3/6] tools/nolibc: i386: Implement syscall with 6
 arguments

From: Ammar Faizi
> Sent: 20 March 2022 15:04
> On 3/20/22 8:10 PM, David Laight wrote:
> > From: Ammar Faizi
> >> Sent: 20 March 2022 09:38
> >>
> >> In i386, the 6th argument of syscall goes in %ebp. However, both Clang
> >> and GCC cannot use %ebp in the clobber list and in the "r" constraint
> >> without using -fomit-frame-pointer. To make it always available for any
> >> kind of compilation, the below workaround is implemented.
> >>
> >> For clang (the Assembly statement can't clobber %ebp):
> >>    1) Save the %ebp value to the redzone area -4(%esp).
> >
> > i386 doesn't have a redzone.
> > If you get a signal it will trash -4(%sp)
> 
> OK, I missed that one. Thanks for reviewing this.
> 
...
> >
> > One possibility might be to do:
> > 	push arg6
> > 	push %ebp
> > 	mov  %ebp, 4(%sp)
> 
> Did you mean `mov 4(%esp), %ebp`?
> 
> > 	int  0x80
> > 	pop  %ebp
> > 	add  %esp,4
> 
> I think your solution is better than the xchg approach (with the 3rd line
> fixed). Will take this in for the next version.

It has to be said that although I've been writing x86 asm
for 40 years (and others for longer) I can never actually
remember the exact syntax or order of the operands!
Probably because it is randomly different between assemblers.
You want the 'memory read' instruction: 8b /r.

	David

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