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

Ammar Faizi
2022-03-22T10:21:45Z

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