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

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Date:   Sun, 20 Mar 2022 13:10:33 +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 v1 3/6] tools/nolibc: i386: Implement syscall with 6
 arguments

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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