[tip: x86/fsgsbase] selftests/x86/fsgsbase: Add a missing memory constraint

[tip-bot2 for Andy Lutomirski]

The following commit has been merged into the x86/fsgsbase branch of tip:

Commit-ID:     8e259031c67a5ea0666428edb64c89e8c6ebd18e
Gitweb:        https://git.kernel.org/tip/8e259031c67a5ea0666428edb64c89e8c6ebd18e
Author:        Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate:    Fri, 26 Jun 2020 10:24:28 -07:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Wed, 01 Jul 2020 15:27:20 +02:00

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

---
 tools/testing/selftests/x86/fsgsbase.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/x86/fsgsbase.c b/tools/testing/selftests/x86/fsgsbase.c
index f47495d..9983195 100644
--- 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));