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

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Message-ID: <ebea1697-cf33-675b-172c-3e80303a9ee9@codeaurora.org>
Date:   Tue, 18 Sep 2018 12:28:39 +0530
From:   Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
To:     Jiri Slaby <jslaby@...e.cz>, Matthias Kaehlcke <mka@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     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 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?

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