[PATCH] tty/sysrq: Make local variable 'killer' in sysrq_handle_crash() global

[Matthias Kaehlcke]

sysrq_handle_crash() dereferences a NULL pointer on purpose to force
an exception, the local variable 'killer' is assigned to NULL and
dereferenced later. Clang detects the NULL pointer dereference at compile
time and emits a BRK instruction (on arm64) instead of the expected NULL
pointer exception. Change 'killer' to a global variable (and rename it
to 'sysrq_killer' to avoid possible clashes) to prevent Clang from
detecting the condition. By default global variables are initialized
with zero/NULL in C, therefore an explicit initialization is not needed.

Reported-by: Sai Prakash Ranjan <saiprakash.ranjan@xxxxxxxxxxxxxx>
Suggested-by: Evan Green <evgreen@xxxxxxxxxxxx>
Signed-off-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
---
 drivers/tty/sysrq.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index 06ed20dd01ba..49fa8e758690 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -132,10 +132,10 @@ static struct sysrq_key_op sysrq_unraw_op = {
 #define sysrq_unraw_op (*(struct sysrq_key_op *)NULL)
 #endif /* CONFIG_VT */
 
+char *sysrq_killer;
+
 static void sysrq_handle_crash(int key)
 {
-	char *killer = NULL;
-
 	/* we need to release the RCU read lock here,
 	 * otherwise we get an annoying
 	 * 'BUG: sleeping function called from invalid context'
@@ -144,7 +144,7 @@ static void sysrq_handle_crash(int key)
 	rcu_read_unlock();
 	panic_on_oops = 1;	/* force panic */
 	wmb();
-	*killer = 1;
+	*sysrq_killer = 1;
 }
 static struct sysrq_key_op sysrq_crash_op = {
 	.handler	= sysrq_handle_crash,
-- 
2.19.0.397.gdd90340f6a-goog