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

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Message-ID: <58cb5455-d065-b508-b328-20b57c3a67a7@gnuweeb.org>
Date:   Tue, 22 Mar 2022 20:37:03 +0700
From:   Ammar Faizi <ammarfaizi2@...weeb.org>
To:     Willy Tarreau <w@....eu>
Cc:     David Laight <David.Laight@...LAB.COM>,
        "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

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.

-- 
Ammar Faizi