Messages in this thread Patch in this message | | | From | Ammar Faizi <> | Subject | [RFC PATCH v2 3/8] tools/nolibc: i386: Implement syscall with 6 arguments | Date | Tue, 22 Mar 2022 17:21:10 +0700 |
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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.
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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