selftests/x86/fsgsbase: Add a missing memory constraint

The manual call to set_thread_area() via int $0x80 was missing any
indication that the descriptor was a pointer, causing gcc to
occasionally generate wrong code.  Add the missing constraint.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/432968af67259ca92d68b774a731aff468eae610.1593192140.git.luto@kernel.org

Author: amluto

tools/testing/selftests/x86/fsgsbase.c

@@ -285,7 +285,8 @@ static unsigned short load_gs(void)
 		/* 32-bit set_thread_area */
 		long ret;
 		asm volatile ("int $0x80"
-			      : "=a" (ret) : "a" (243), "b" (low_desc)
+			      : "=a" (ret), "+m" (*low_desc)
+			      : "a" (243), "b" (low_desc)
 			      : "r8", "r9", "r10", "r11");
 		memcpy(&desc, low_desc, sizeof(desc));
 		munmap(low_desc, sizeof(desc));