x86: um: vdso: Add '%rcx' and '%r11' to the syscall clobber list

[ Upstream commit 5541992 ]

The 'syscall' instruction clobbers '%rcx' and '%r11', but they are not
listed in the inline Assembly that performs the syscall instruction.

No real bug is found. It wasn't buggy by luck because '%rcx' and '%r11'
are caller-saved registers, and not used in the functions, and the
functions are never inlined.

Add them to the clobber list for code correctness.

Fixes: f1c2bb8 ("um: implement a x86_64 vDSO")
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Sasha Levin <sashal@kernel.org>

arch/x86/um/vdso/um_vdso.c

@@ -17,8 +17,10 @@ int __vdso_clock_gettime(clockid_t clock, struct __kernel_old_timespec *ts)
 {
 	long ret;
 
-	asm("syscall" : "=a" (ret) :
-		"0" (__NR_clock_gettime), "D" (clock), "S" (ts) : "memory");
+	asm("syscall"
+		: "=a" (ret)
+		: "0" (__NR_clock_gettime), "D" (clock), "S" (ts)
+		: "rcx", "r11", "memory");
 
 	return ret;
 }
@@ -29,8 +31,10 @@ int __vdso_gettimeofday(struct __kernel_old_timeval *tv, struct timezone *tz)
 {
 	long ret;
 
-	asm("syscall" : "=a" (ret) :
-		"0" (__NR_gettimeofday), "D" (tv), "S" (tz) : "memory");
+	asm("syscall"
+		: "=a" (ret)
+		: "0" (__NR_gettimeofday), "D" (tv), "S" (tz)
+		: "rcx", "r11", "memory");
 
 	return ret;
 }