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Subject[PATCH v1 06/11] tools/nolibc: i386: Implement syscall with 6 arguments
Date
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.

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).

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 v2: https://lore.kernel.org/lkml/20220322102115.186179-4-ammarfaizi2@gnuweeb.org
RFC v2 -> v1:
- Use the same pattern for syscall6, regardless using GCC or Clang
(comment from David).
- Use appropriate constraints for syscall6 instead of always using
register variables (comment from David).

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 | 23 +++++++++++++++++++++++
1 file changed, 23 insertions(+)

diff --git a/tools/include/nolibc/arch-i386.h b/tools/include/nolibc/arch-i386.h
index 6eb96ee8c4f7..cdffd99664ae 100644
--- a/tools/include/nolibc/arch-i386.h
+++ b/tools/include/nolibc/arch-i386.h
@@ -145,6 +145,29 @@ struct sys_stat_struct {
_ret; \
})

+#define my_syscall6(num, arg1, arg2, arg3, arg4, arg5, arg6) \
+({ \
+ long _eax = (long)(num); \
+ long _arg6 = (long)(arg6); /* Always in memory */ \
+ __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) /* %eax */ \
+ : "b"(arg1), /* %ebx */ \
+ "c"(arg2), /* %ecx */ \
+ "d"(arg3), /* %edx */ \
+ "S"(arg4), /* %esi */ \
+ "D"(arg5), /* %edi */ \
+ [_arg6]"m"(_arg6) /* memory */ \
+ : "memory", "cc" \
+ ); \
+ _eax; \
+})
+
/* startup code */
/*
* i386 System V ABI mandates:
--
Ammar Faizi
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