Re: [PATCH] kref: warn on uninitialized kref

[Mikulas Patocka]

On Sat, 17 May 2014, Mateusz Guzik wrote:

> On Sat, May 17, 2014 at 06:53:17AM -0400, Mikulas Patocka wrote:
> > I found a memory leak in iSCSI target that was caused by kref initialized
> > to zero (the memory object was allocated with kzalloc, kref_init was not
> > called and kref_put_spinlock_irqsave was called which changed "0" to "-1"
> > and didn't free the object).
> > 
> > Similar bugs may exist in other kernel areas, so I submit this patch that
> > adds a check to kref.h. If the value is zero or negative, we can assume
> > that it is uninitialized and we warn about it.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> > 
> > ---
> >  include/linux/kref.h |    4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > Index: linux-3.15-rc5/include/linux/kref.h
> > ===================================================================
> > --- linux-3.15-rc5.orig/include/linux/kref.h	2013-07-02 22:23:38.000000000 +0200
> > +++ linux-3.15-rc5/include/linux/kref.h	2014-05-16 18:56:18.000000000 +0200
> > @@ -69,7 +69,7 @@ static inline int kref_sub(struct kref *
> >  	     void (*release)(struct kref *kref))
> >  {
> >  	WARN_ON(release == NULL);
> > -
> > +	WARN_ON_ONCE(atomic_read(&kref->refcount) <= 0);
> >  	if (atomic_sub_and_test((int) count, &kref->refcount)) {
> >  		release(kref);
> >  		return 1;
> > @@ -119,6 +119,7 @@ static inline int kref_put_spinlock_irqs
> >  	unsigned long flags;
> >  
> >  	WARN_ON(release == NULL);
> > +	WARN_ON_ONCE(atomic_read(&kref->refcount) <= 0);
> >  	if (atomic_add_unless(&kref->refcount, -1, 1))
> >  		return 0;
> >  	spin_lock_irqsave(lock, flags);
> > @@ -136,6 +137,7 @@ static inline int kref_put_mutex(struct 
> >  				 struct mutex *lock)
> >  {
> >  	WARN_ON(release == NULL);
> > +	WARN_ON_ONCE(atomic_read(&kref->refcount) <= 0);
> >  	if (unlikely(!atomic_add_unless(&kref->refcount, -1, 1))) {
> >  		mutex_lock(lock);
> >  		if (unlikely(!atomic_dec_and_test(&kref->refcount))) {
> 
> This has a side effect of detecting some overputs, which is nice.
> 
> However, could be made better if kref_sub checked that refs you want to
> take don't put the count below 0.
> 
> i.e.
> WARN_ON(count > atomic_read(&kref->refcount));
> 
> this also detects your original problem.
> 
> -- 
> Mateusz Guzik

Good point. I'm sending the updated patch.

Mikulas
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