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

David Laight
2022-03-22T11:39:09Z

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;
> +}

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