[PATCH v6 1/3] x86_64: Set the syscall register right before doing the syscall.

[Joe Simmons-Talbott <josimmon@redhat.com>]

To make identifying syscalls easier during call tree analysis load the
syscall number just before performing the syscall.

Compiler optimizations can place quite a few instructions between the
setting of the syscall number and the syscall instruction.  During call
tree analysis the number of instructions between the two can lead to
more difficulty for both tools and humans in properly identifying the
syscall number.  Having the syscall number set in the prior instruction
to the syscall instruction makes this task easier and less error prone.
Being able to reliably identify syscalls made by a given API will make
it easier to understand and verify the safety and security of glibc.
---
 sysdeps/unix/sysv/linux/x86_64/sysdep.h | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/sysdeps/unix/sysv/linux/x86_64/sysdep.h b/sysdeps/unix/sysv/linux/x86_64/sysdep.h
index cfb51be8c5..0db8660531 100644
--- a/sysdeps/unix/sysv/linux/x86_64/sysdep.h
+++ b/sysdeps/unix/sysv/linux/x86_64/sysdep.h
@@ -257,9 +257,9 @@
     TYPEFY (arg1, __arg1) = ARGIFY (arg1);			 	\
     register TYPEFY (arg1, _a1) asm ("rdi") = __arg1;			\
     asm volatile (							\
-    "syscall\n\t"							\
+    "movl %1, %k0\n\tsyscall\n\t"					\
     : "=a" (resultvar)							\
-    : "0" (number), "r" (_a1)						\
+    : "g" (number), "r" (_a1)						\
     : "memory", REGISTERS_CLOBBERED_BY_SYSCALL);			\
     (long int) resultvar;						\
 })
@@ -273,9 +273,9 @@
     register TYPEFY (arg2, _a2) asm ("rsi") = __arg2;			\
     register TYPEFY (arg1, _a1) asm ("rdi") = __arg1;			\
     asm volatile (							\
-    "syscall\n\t"							\
+    "movl %1, %k0\n\tsyscall\n\t"					\
     : "=a" (resultvar)							\
-    : "0" (number), "r" (_a1), "r" (_a2)				\
+    : "g" (number), "r" (_a1), "r" (_a2)				\
     : "memory", REGISTERS_CLOBBERED_BY_SYSCALL);			\
     (long int) resultvar;						\
 })
@@ -291,9 +291,9 @@
     register TYPEFY (arg2, _a2) asm ("rsi") = __arg2;			\
     register TYPEFY (arg1, _a1) asm ("rdi") = __arg1;			\
     asm volatile (							\
-    "syscall\n\t"							\
+    "movl %1, %k0\n\tsyscall\n\t"					\
     : "=a" (resultvar)							\
-    : "0" (number), "r" (_a1), "r" (_a2), "r" (_a3)			\
+    : "g" (number), "r" (_a1), "r" (_a2), "r" (_a3)			\
     : "memory", REGISTERS_CLOBBERED_BY_SYSCALL);			\
     (long int) resultvar;						\
 })
@@ -311,9 +311,9 @@
     register TYPEFY (arg2, _a2) asm ("rsi") = __arg2;			\
     register TYPEFY (arg1, _a1) asm ("rdi") = __arg1;			\
     asm volatile (							\
-    "syscall\n\t"							\
+    "movl %1, %k0\n\tsyscall\n\t"					\
     : "=a" (resultvar)							\
-    : "0" (number), "r" (_a1), "r" (_a2), "r" (_a3), "r" (_a4)		\
+    : "g" (number), "r" (_a1), "r" (_a2), "r" (_a3), "r" (_a4)		\
     : "memory", REGISTERS_CLOBBERED_BY_SYSCALL);			\
     (long int) resultvar;						\
 })
@@ -333,9 +333,9 @@
     register TYPEFY (arg2, _a2) asm ("rsi") = __arg2;			\
     register TYPEFY (arg1, _a1) asm ("rdi") = __arg1;			\
     asm volatile (							\
-    "syscall\n\t"							\
+    "movl %1, %k0\n\tsyscall\n\t"					\
     : "=a" (resultvar)							\
-    : "0" (number), "r" (_a1), "r" (_a2), "r" (_a3), "r" (_a4),		\
+    : "g" (number), "r" (_a1), "r" (_a2), "r" (_a3), "r" (_a4),		\
       "r" (_a5)								\
     : "memory", REGISTERS_CLOBBERED_BY_SYSCALL);			\
     (long int) resultvar;						\
@@ -358,9 +358,9 @@
     register TYPEFY (arg2, _a2) asm ("rsi") = __arg2;			\
     register TYPEFY (arg1, _a1) asm ("rdi") = __arg1;			\
     asm volatile (							\
-    "syscall\n\t"							\
+    "movl %1, %k0\n\tsyscall\n\t"					\
     : "=a" (resultvar)							\
-    : "0" (number), "r" (_a1), "r" (_a2), "r" (_a3), "r" (_a4),		\
+    : "g" (number), "r" (_a1), "r" (_a2), "r" (_a3), "r" (_a4),		\
       "r" (_a5), "r" (_a6)						\
     : "memory", REGISTERS_CLOBBERED_BY_SYSCALL);			\
     (long int) resultvar;						\
-- 
2.39.2