linux-kernel - Re: [PATCH] tty/sysrq: Make local variable 'killer' in sysrq_handle

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Message-ID: <2e5ca973-ea52-5d6f-2f78-294c7be577bd@codeaurora.org>
Date:   Tue, 18 Sep 2018 15:23:51 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jslaby@...e.cz>, Matthias Kaehlcke <mka@...omium.org>,
        Douglas Anderson <dianders@...omium.org>,
        Evan Green <evgreen@...omium.org>,
        Manoj Gupta <manojgupta@...omium.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty/sysrq: Make local variable 'killer' in
 sysrq_handle_crash() global

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@...eaurora.org>
>>>>>> Suggested-by: Evan Green <evgreen@...omium.org>
>>>>>> Signed-off-by: Matthias Kaehlcke <mka@...omium.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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