linux-kernel - RE: [RFC PATCH v2 3/8] tools/nolibc: i386: Implement syscall with 6 arguments

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Date:   Tue, 22 Mar 2022 13:39:41 +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 v2 3/8] tools/nolibc: i386: Implement syscall with 6
 arguments

From: Ammar Faizi
> Sent: 22 March 2022 13:37
> 
> On 3/22/22 8:34 PM, Willy Tarreau wrote:
> >> I turned out GCC refuses to use "rm" if we compile without -fomit-frame-pointer
> >> (e.g. without optimization / -O0). So I will still use "m" here.
> >
> > OK that's fine. then you can probably simplify it like this:
> >
> >        long _arg6 = (long)(arg6); /* Might be in memory */    \
> >                                                               \
> >        asm volatile (                                         \
> >            "pushl  %%ebp\n\t"                                 \
> >            "movl   %[_arg6], %%ebp\n\t"                       \
> >            "int    $0x80\n\t"                                 \
> >            "popl   %%ebp\n\t"                                 \
> >            : "=a"(_ret)                                       \
> >            : "r"(_num), "r"(_arg1), "r"(_arg2), "r"(_arg3),   \
> >              "r"(_arg4),"r"(_arg5), [_arg6]"m"(_arg6)        \
> >            : "memory", "cc"                                   \
> >        );                                                     \
> >
> > See ? no more push, no more addl, direct load from memory.
> 
> Uggh... I crafted the same code like you suggested before, but then
> I realized it's buggy, it's buggy because %[_arg6] may live in N(%esp).
> 
> When you pushl %ebp, the %esp changes, N(%esp) no longer points to the
> 6-th argument.

Yep - that is why I wrote the 'push arg6'.

	David

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