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Date: Tue, 22 Mar 2022 14:41:00 +0100
From: Willy Tarreau <w@1wt.eu>
To: David Laight <David.Laight@aculab.com>
Cc: "'Ammar Faizi'" <ammarfaizi2@gnuweeb.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
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Subject: Re: [RFC PATCH v2 3/8] tools/nolibc: i386: Implement syscall with 6
 arguments
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On Tue, Mar 22, 2022 at 01:39:41PM +0000, David Laight wrote:
> 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'.

Got it and you're right indeed, sorry for the noise :-)

Willy