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

David Laight
2022-03-22T10:57:04Z

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.
>
> 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.

You need to use the 'clang' pattern for gcc.
#pragma optimise is fundamentally broken.
What actually happens here is the 'inline' gets lost
(because of the implied -O0) and you get far worse code
than you might expect.

Since you need the 'clang' version, use it all the time.

David

>
> Cc: x86@kernel.org
> Cc: llvm@lists.linux.dev
> Link: https://lore.kernel.org/lkml/2e335ac54db44f1d8496583d97f9dab0@AcuMS.aculab.com
> Suggested-by: David Laight <David.Laight@ACULAB.COM>
> Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
> ---
>
> @@ Changelog:
>
> Link RFC v1: https://lore.kernel.org/llvm/20220320093750.159991-4-ammarfaizi2@gnuweeb.org
> RFC v1 -> RFC v2:
> - Fix %ebp saving method. Don't use redzone, i386 doesn't have a redzone
> (comment from David and Alviro).
> ---
> tools/include/nolibc/arch-i386.h | 66 ++++++++++++++++++++++++++++++++
> 1 file changed, 66 insertions(+)
>
> 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)
> +{
> + __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),
> + [arg6]"m"(ebp)
> + : "memory", "cc"
> + );
> + return eax;
> +}
> +
> +#else /* #if defined(__clang__) */
> +#pragma GCC push_options
> +#pragma GCC optimize "-fomit-frame-pointer"
> +static long ____do_syscall6(long eax, long ebx, long ecx, long edx, long esi,
> + long edi, long ebp)
> +{
> + register long __ebp __asm__("ebp") = ebp;
> + __asm__ volatile (
> + "int $0x80"
> + : "=a"(eax)
> + : "a"(eax), "b"(ebx), "c"(ecx), "d"(edx), "S"(esi), "D"(edi),
> + "r"(__ebp)
> + : "memory", "cc"
> + );
> + return eax;
> +}
> +#pragma GCC pop_options
> +#endif /* #if defined(__clang__) */
> +
> +#define my_syscall6(num, arg1, arg2, arg3, arg4, arg5, arg6) ( \
> + ____do_syscall6((long)(num), (long)(arg1), \
> + (long)(arg2), (long)(arg3), \
> + (long)(arg4), (long)(arg5), \
> + (long)(arg6)) \
> +)
> +
> +
> /* startup code */
> /*
> * i386 System V ABI mandates:
> --
> Ammar Faizi

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