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

Ammar Faizi
2022-03-22T12:07:41Z

On 3/22/22 7:02 PM, Ammar Faizi wrote:
> Well, I agree with your previous email. Now since we no longer use a #pragma
> optimize with -fomit-frame-pointer, the function is not needed. I propose the
> following macro (this is not so much different with other my_syscall macro),
> expect the 6th argument can be in reg or mem.
>
> The "rm" constraint here gives the opportunity for the compiler to use %ebp
> instead of memory if -fomit-frame-pointer is turned on.
>
> #define my_syscall6(num, arg1, arg2, arg3, arg4, arg5, arg6) \
> ({                                                         \
>     long _ret;                                             \
>     register long _num asm("eax") = (num);                 \
>     register long _arg1 asm("ebx") = (long)(arg1);         \
>     register long _arg2 asm("ecx") = (long)(arg2);         \
>     register long _arg3 asm("edx") = (long)(arg3);         \
>     register long _arg4 asm("esi") = (long)(arg4);         \
>     register long _arg5 asm("edi") = (long)(arg5);         \
>     long _arg6 = (long)(arg6); /* Might be in memory */    \
>                                                            \
>     asm volatile (                                         \
>         "pushl  %[_arg6]\n\t"                              \
>         "pushl  %%ebp\n\t"                                 \
>         "movl   4(%%esp), %%ebp\n\t"                       \
>         "int    $0x80\n\t"                                 \
>         "popl   %%ebp\n\t"                                 \
>         "addl   $4,%%esp\n\t"                              \
>         : "=a"(_ret)                                       \
>         : "r"(_num), "r"(_arg1), "r"(_arg2), "r"(_arg3),   \
>           "r"(_arg4),"r"(_arg5), [_arg6]"rm"(_arg6)        \
>         : "memory", "cc"                                   \
>     );                                                     \
>     _ret;                                                  \
> })
>
> What do you think?
>

For the following code:

int main()
{
mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_ANONYMOUS|MAP_PRIVATE, -1, 0);
return 0;
}

GCC generates this:

00001000 :
1000: push %ebp
1001: mov $0xc0,%eax
1006: mov $0x1000,%ecx
100b: mov $0x3,%edx
1010: push %edi
1011: xor %ebp,%ebp
1013: mov $0xffffffff,%edi
1018: push %esi
1019: mov $0x22,%esi
101e: push %ebx
101f: xor %ebx,%ebx
1021: push %ebp <--- arg6 here
1022: push %ebp
1023: mov 0x4(%esp),%ebp
1027: int $0x80
1029: pop %ebp
102a: add $0x4,%esp
102d: xor %eax,%eax
102f: pop %ebx
1030: pop %esi
1031: pop %edi
1032: pop %ebp
1033: ret

--
Ammar Faizi