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

[Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>]

On Sun, Mar 20, 2022 at 4:37 PM Ammar Faizi wrote:
> 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).
>   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.

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.

> 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.
>
> Cc: x86@kernel.org
> Cc: llvm@lists.linux.dev
> Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
[...]
> +#if defined(__clang__)
> +static inline long ____do_syscall6(long eax, long ebx, long ecx, long edx,
> +                                  long esi, long edi, long ebp)
> +{
> +       __asm__ volatile (
> +               "movl   %%ebp, -4(%%esp)\n\t"
> +               "movl   %[arg6], %%ebp\n\t"
> +               "subl   $4, %%esp\n\t"
> +               "int    $0x80\n\t"
> +               "popl   %%ebp\n\t"
> +               : "=a"(eax)
> +               : "a"(eax), "b"(ebx), "c"(ecx), "d"(edx), "S"(esi), "D"(edi),
> +                 [arg6]"m"(ebp)
> +               : "memory", "cc"
> +       );
> +       return eax;
> +}
> +

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

-- Viro