linux-kernel - [PATCH] lkdtm: Prevent the compiler from optimising lkdtm_CORRUPT

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Message-Id: <1479193352-486-1-git-send-email-mpe@ellerman.id.au>
Date:   Tue, 15 Nov 2016 18:02:32 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     keescook@...omium.org
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH] lkdtm: Prevent the compiler from optimising lkdtm_CORRUPT_STACK()

At least on powerpc with GCC 6, the compiler is smart enough to optimise
lkdtm_CORRUPT_STACK() into an empty function that just returns.

If we print the buffer after we've written to it that prevents the
compiler from optimising away data and the memset().

Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
---
 drivers/misc/lkdtm_bugs.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/misc/lkdtm_bugs.c b/drivers/misc/lkdtm_bugs.c
index 182ae1894b32..30e62dd7e7ca 100644
--- a/drivers/misc/lkdtm_bugs.c
+++ b/drivers/misc/lkdtm_bugs.c
@@ -80,7 +80,8 @@ noinline void lkdtm_CORRUPT_STACK(void)
 	/* Use default char array length that triggers stack protection. */
 	char data[8];
 
-	memset((void *)data, 0, 64);
+	memset((void *)data, 'a', 64);
+	pr_info("Corrupted stack with '%16s'...\n", data);
 }
 
 void lkdtm_UNALIGNED_LOAD_STORE_WRITE(void)
-- 
2.7.4