linux-kernel - [PATCH fsgsbase v2 2/4] selftests/x86/fsgsbase: Add a missing memory constraint

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Message-Id: <432968af67259ca92d68b774a731aff468eae610.1593192140.git.luto@kernel.org>
Date:   Fri, 26 Jun 2020 10:24:28 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     Sasha Levin <sashal@...nel.org>, linux-kernel@...r.kernel.org,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH fsgsbase v2 2/4] 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@...nel.org>
---
 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 f47495d2f070..998319553523 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));
-- 
2.25.4