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Date: Sun, 20 Mar 2022 22:09:00 +0700
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Subject: Re: [RFC PATCH v1 3/6] tools/nolibc: i386: Implement syscall with 6
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To: Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>
Cc: Willy Tarreau <w@1wt.eu>, "Paul E. McKenney" <paulmck@kernel.org>,
 Nugraha <richiisei@gmail.com>,
 Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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 llvm@lists.linux.dev, David Laight <David.Laight@ACULAB.COM>
References: <20220320093750.159991-1-ammarfaizi2@gnuweeb.org>
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From: Ammar Faizi <ammarfaizi2@gnuweeb.org>
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On 3/20/22 5:42 PM, Alviro Iskandar Setiawan wrote:
> On Sun, Mar 20, 2022 at 5:33 PM Alviro Iskandar Setiawan wrote:
[...]
>> I don't think you can safely use redzone from inline Assembly. The
>> compiler may also use redzone for a leaf function. In case the syscall
>> is done at the same time, your %ebp saving will clobber the redzone
>> that the compiler uses.

It turned out we don't have a redzone for 32-bit.

>>
>> -4(%esp) may be used by the compiler on a leaf call, you can't clobber that.

Yeah, this is still wrong even with a redzone.

> Using xchgl to preserve %ebp in the same place where the arg6 is
> stored in memory is a better solution and doesn't clobber anything.
> 
>      xchgl %ebp, %[arg6]
>      int    $0x80
>      xchgl %ebp, %[arg6]

Addressed your review in my response to David.

Thanks!

-- 
Ammar Faizi