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 15:23:51 +0530 |
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On 9/18/2018 2:47 PM, Greg Kroah-Hartman wrote: > On Tue, Sep 18, 2018 at 02:35:02PM +0530, Sai Prakash Ranjan wrote: >> On 9/18/2018 12:50 PM, Greg Kroah-Hartman wrote: >>> On Tue, Sep 18, 2018 at 12:28:39PM +0530, Sai Prakash Ranjan wrote: >>>> 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? >>> >>> Ick, why is this needed at all? Why are we trying to "roll our own >>> panic" in this code? >>> >> >> Hi Greg, do you mean like why there is a killer var at all or why this >> change is required? > > I understand you are using a compiler that thinks it wants to protect > yourself from your code and tries to "fix" it for you. That's fine, and > is up to the compiler writers (personally that seems not a good idea.) > > My question is why we just don't call panic() here instead of trying to > duplicate the logic of that function here. Why is that happening? >
It seems fine to call panic() here. Dont no why they chose to have a null pointer dereference.
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