tools/nolibc: i386: Implement syscall with 6 arguments

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>
Acked-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/include/nolibc/arch-i386.h

@@ -167,6 +167,29 @@ 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: