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

[Ammar Faizi]

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