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

Willy Tarreau
2022-03-22T13:41:10Z

On Tue, Mar 22, 2022 at 01:39:41PM +0000, David Laight wrote:
> 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'.

Got it and you're right indeed, sorry for the noise :-)

Willy