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

diff --git a/tools/testing/selftests/x86/fsgsbase.c b/tools/testing/selftests/x86/fsgsbase.c
index f47495d2f070dd3479224eee7a29ab480f62ffee..9983195535237214f5085cc212f38592f109cc6a 100644 (file)
--- a/tools/testing/selftests/x86/fsgsbase.c
+++ b/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));