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

[David Laight <David.Laight@ACULAB.COM>]

From: Ammar Faizi
> Sent: 22 March 2022 10:21
> On 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.
> 
...
> diff --git a/tools/include/nolibc/arch-i386.h b/tools/include/nolibc/arch-i386.h
> index 125a691fc631..9f4dc36e6ac2 100644
> --- a/tools/include/nolibc/arch-i386.h
> +++ b/tools/include/nolibc/arch-i386.h
> @@ -167,6 +167,72 @@ struct sys_stat_struct {
>  	_ret;                                                                 \
>  })
> 
> +
> +/*
> + * 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) Push the 6-th argument.
> + *   2) Push %ebp.
> + *   3) Load the 6-th argument from 4(%esp) to %ebp.
> + *   4) Do the syscall (int $0x80).
> + *   5) Pop %ebp (restore the old value of %ebp).
> + *   6) Add %esp by 4 (undo the stack pointer).
> + *
> + * For GCC, fortunately it has a #pragma that can force a specific function
> + * to be compiled with -fomit-frame-pointer, so it can use "r"(var) where
> + * var is a variable bound to %ebp.
> + *
> + */
> +#if defined(__clang__)
> +static inline long ____do_syscall6(long eax, long ebx, long ecx, long edx,
> +				   long esi, long edi, long ebp)

That should probably be:
static inline long ____do_syscall6(long nr, long arg1, long arg2, long arg3,
				   long arg4, long arg5, long arg6)
and the input constraints changed to match.

> +{
> +	__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"(eax)
> +		: "a"(eax), "b"(ebx), "c"(ecx), "d"(edx), "S"(esi), "D"(edi),

Does having "=a" for an output constraint and "a" for an input
constraint actually DTRT?
There is a special syntax for tying input and output to
the same register.
Or you could use "+a"(nr_rval) and 'return nr_rval'.

	David

> +		  [arg6]"m"(ebp)
> +		: "memory", "cc"
> +	);
> +	return eax;
> +}

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