linux-kernel - [PATCH] lkdtm: hide stack overflow warning for corrupt-stack test

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Message-Id: <20170111145711.810229-1-arnd@arndb.de>
Date:   Wed, 11 Jan 2017 15:56:44 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Kees Cook <keescook@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Michael Ellerman <mpe@...erman.id.au>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] lkdtm: hide stack overflow warning for corrupt-stack test

After the latest change to make sure the compiler actually does a memset,
it is now smart enough to flag the stack overflow at compile time,
at least with gcc-7.0:

drivers/misc/lkdtm_bugs.c: In function 'lkdtm_CORRUPT_STACK':
drivers/misc/lkdtm_bugs.c:88:144: warning: 'memset' writing 64 bytes into a region of size 8 overflows the destination [-Wstringop-overflow=]

To outsmart the compiler again, this moves the memset into a noinline
function where (for now) it doesn't see that we intentionally write
broken code here.

Fixes: c55d240003ae ("lkdtm: Prevent the compiler from optimising lkdtm_CORRUPT_STACK()")
Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
 drivers/misc/lkdtm_bugs.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/misc/lkdtm_bugs.c b/drivers/misc/lkdtm_bugs.c
index 91edd0b55e5c..bb3bb8ef5f44 100644
--- a/drivers/misc/lkdtm_bugs.c
+++ b/drivers/misc/lkdtm_bugs.c
@@ -80,12 +80,17 @@ void lkdtm_OVERFLOW(void)
 	(void) recursive_loop(recur_count);
 }
 
+static noinline void __lkdtm_CORRUPT_STACK(void *stack)
+{
+	memset(stack, 'a', 64);
+}
+
 noinline void lkdtm_CORRUPT_STACK(void)
 {
 	/* Use default char array length that triggers stack protection. */
 	char data[8];
+	__lkdtm_CORRUPT_STACK(&data);
 
-	memset((void *)data, 'a', 64);
 	pr_info("Corrupted stack with '%16s'...\n", data);
 }
 
-- 
2.9.0