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

From: David Laight <David.Laight@ACULAB.COM>
To: 'Ammar Faizi' <ammarfaizi2@gnuweeb.org>, Willy Tarreau <w@1wt.eu>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
	Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>,
	Nugraha <richiisei@gmail.com>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	GNU/Weeb Mailing List <gwml@vger.gnuweeb.org>,
	"x86@kernel.org" <x86@kernel.org>,
	"llvm@lists.linux.dev" <llvm@lists.linux.dev>
Subject: RE: [RFC PATCH v1 3/6] tools/nolibc: i386: Implement syscall with 6 arguments
Date: Sun, 20 Mar 2022 13:10:33 +0000	[thread overview]
Message-ID: <2e335ac54db44f1d8496583d97f9dab0@AcuMS.aculab.com> (raw)
In-Reply-To: <20220320093750.159991-4-ammarfaizi2@gnuweeb.org>

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)

>   2) Load the 6-th argument from memory to %ebp.
>   3) Subtract the %esp by 4.
>   4) Do the syscall (int $0x80).
>   5) Pop %ebp.
> 
> For GCC, fortunately it has a #pragma that can force a specific function
> to be compiled with -fomit-frame-pointer, so it can always use "r"(var)
> where `var` is a variable bound to %ebp.

How is that going to work for an inlined functon?

And using xchg is slow - it is always locked.

One possibility might be to do:
	push arg6
	push %ebp
	mov  %ebp, 4(%sp)
	int  0x80
	pop  %ebp
	add  %esp,4

Although I'm not sure you really want to allocate 4k pages
for every malloc() call.

Probably better to write a mini 'libc' that uses sbrk()
and a best fit scan of a linear free list.

	David

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