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

David Laight
2022-03-22T13:39:44Z

From: Ammar Faizi
> Sent: 22 March 2022 13:37
>
> 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.

Yep - that is why I wrote the 'push arg6'.

David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)