Messages in this thread | | | Subject | Re: [PATCH] tty/sysrq: Make local variable 'killer' in sysrq_handle_crash() global | From | Sai Prakash Ranjan <> | Date | Tue, 18 Sep 2018 12:28:39 +0530 |
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On 9/18/2018 11:41 AM, Jiri Slaby wrote: > On 09/17/2018, 11:33 PM, Matthias Kaehlcke wrote: >> 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@codeaurora.org> >> Suggested-by: Evan Green <evgreen@chromium.org> >> Signed-off-by: Matthias Kaehlcke <mka@chromium.org> >> --- >> 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; > > Just because a static analyzer is wrong? Oh wait, even compiler is > wrong. At least make it a static global. Or what about OPTIMIZER_HIDE_VAR? >
static global does not work, clang still inserts brk. As for OPTIMIZE_HIDE_VAR, it seems to work. But, I dont think it is defined for clang in which case it defaults to using barrier(). There is already one wmb(), so will it be right?
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