Re: [PATCH rcu 07/12] rcu: tiny: Record kvfree_call_rcu() call stack for KASAN

Paul E. McKenney
2022-06-21T19:31:43Z

On Tue, Jun 21, 2022 at 12:01:29PM +0530, Neeraj Upadhyay wrote:
>
>
> On 6/21/2022 3:50 AM, Paul E. McKenney wrote:
> > From: Johannes Berg <johannes.berg@intel.com>
> >
> > When running KASAN with Tiny RCU (e.g. under ARCH=um, where
> > a working KASAN patch is now available), we don't get any
> > information on the original kfree_rcu() (or similar) caller
> > when a problem is reported, as Tiny RCU doesn't record this.
> >
> > Add the recording, which required pulling kvfree_call_rcu()
> > out of line for the KASAN case since the recording function
> > (kasan_record_aux_stack_noalloc) is neither exported, nor
> > can we include kasan.h into rcutiny.h.
> >
> > without KASAN, the patch has no size impact (ARCH=um kernel):
> > text data bss dec hex filename
> > 6151515 4423154 33148520 43723189 29b29b5 linux
> > 6151515 4423154 33148520 43723189 29b29b5 linux + patch
> >
> > with KASAN, the impact on my build was minimal:
> > text data bss dec hex filename
> > 13915539 7388050 33282304 54585893 340ea25 linux
> > 13911266 7392114 33282304 54585684 340e954 linux + patch
> > -4273 +4064 +-0 -209
> >
> > Acked-by: Dmitry Vyukov <dvyukov@google.com>
> > Signed-off-by: Johannes Berg <johannes.berg@intel.com>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
> > include/linux/rcutiny.h | 11 ++++++++++-
> > kernel/rcu/tiny.c | 14 ++++++++++++++
> > 2 files changed, 24 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> > index 5fed476f977f6..d84e13f2c3848 100644
> > --- a/include/linux/rcutiny.h
> > +++ b/include/linux/rcutiny.h
> > @@ -38,7 +38,7 @@ static inline void synchronize_rcu_expedited(void)
> > */
> > extern void kvfree(const void *addr);
> > -static inline void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > +static inline void __kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > {
> > if (head) {
> > call_rcu(head, func);
> > @@ -51,6 +51,15 @@ static inline void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > kvfree((void *) func);
> > }
> > +#ifdef CONFIG_KASAN_GENERIC
> > +void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func);
> > +#else
> > +static inline void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > +{
> > + __kvfree_call_rcu(head, func);
> > +}
> > +#endif
> > +
> > void rcu_qs(void);
> > static inline void rcu_softirq_qs(void)
> > diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> > index 340b3f8b090d4..58ff3721d975c 100644
> > --- a/kernel/rcu/tiny.c
> > +++ b/kernel/rcu/tiny.c
> > @@ -217,6 +217,20 @@ bool poll_state_synchronize_rcu(unsigned long oldstate)
> > }
> > EXPORT_SYMBOL_GPL(poll_state_synchronize_rcu);
> > +#ifdef CONFIG_KASAN_GENERIC
> > +void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> > +{
> > + if (head) {
> > + void *ptr = (void *) head - (unsigned long) func;
> > +
> > + kasan_record_aux_stack_noalloc(ptr);
> > + }
>
> For the !head case; similar to Tree RCU's kvfree_call_rcu() implementation,
> we do not need to record 'ptr' (which will be 'func')?

My understanding is that we do not need to record in that case
because __kvfree_call_rcu() will simply invoke the almost-zero-cost
synchronize_rcu() and then invoke kfree().

Johannes, Dmitry, Marco, anything that I am missing?

Thanx, Paul

> Thanks
> Neeraj
>
> > +
> > + __kvfree_call_rcu(head, func);
> > +}
> > +EXPORT_SYMBOL_GPL(kvfree_call_rcu);
> > +#endif
> > +
> > void __init rcu_init(void)
> > {
> > open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);