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

[Sai Prakash Ranjan]

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@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;
> > > > >
> > > > > 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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