Re: [PATCH v2] rethook: Use __rcu pointer for rethook::handler

[Google]

On Thu, 30 Nov 2023 13:44:55 -0800
JP Kobryn <inwardvessel@xxxxxxxxx> wrote:

> On Wed, Nov 29, 2023 at 09:24:22PM +0900, Masami Hiramatsu (Google) wrote:
> > From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> > 
> > Since the rethook::handler is an RCU-maganged pointer so that it will
> > notice readers the rethook is stopped (unregistered) or not, it should
> > be an __rcu pointer and use appropriate functions to be accessed. This
> > will use appropriate memory barrier when accessing it. OTOH,
> > rethook::data is never changed, so we don't need to check it in
> > get_kretprobe().
> > 
> > NOTE: To avoid sparse warning, rethook::handler is defined by a raw
> > function pointer type with __rcu instead of rethook_handler_t.
> > 
> > Fixes: 54ecbe6f1ed5 ("rethook: Add a generic return hook")
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > Closes: https://lore.kernel.org/oe-kbuild-all/202311241808.rv9ceuAh-lkp@xxxxxxxxx/
> > Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> > ---
> >  include/linux/kprobes.h |    6 ++----
> >  include/linux/rethook.h |    7 ++++++-
> >  kernel/trace/rethook.c  |   23 ++++++++++++++---------
> >  3 files changed, 22 insertions(+), 14 deletions(-)
> > 
> > diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
> > index 64672bace560..0ff44d6633e3 100644
> > --- a/include/linux/kprobes.h
> > +++ b/include/linux/kprobes.h
> > @@ -197,10 +197,8 @@ extern int arch_trampoline_kprobe(struct kprobe *p);
> >  #ifdef CONFIG_KRETPROBE_ON_RETHOOK
> >  static nokprobe_inline struct kretprobe *get_kretprobe(struct kretprobe_instance *ri)
> >  {
> > -	RCU_LOCKDEP_WARN(!rcu_read_lock_any_held(),
> > -		"Kretprobe is accessed from instance under preemptive context");
> > -
> > -	return (struct kretprobe *)READ_ONCE(ri->node.rethook->data);
> > +	/* rethook::data is non-changed field, so that you can access it freely. */
> > +	return (struct kretprobe *)ri->node.rethook->data;
> >  }
> >  static nokprobe_inline unsigned long get_kretprobe_retaddr(struct kretprobe_instance *ri)
> >  {
> > diff --git a/include/linux/rethook.h b/include/linux/rethook.h
> > index ce69b2b7bc35..ba60962805f6 100644
> > --- a/include/linux/rethook.h
> > +++ b/include/linux/rethook.h
> > @@ -28,7 +28,12 @@ typedef void (*rethook_handler_t) (struct rethook_node *, void *, unsigned long,
> >   */
> >  struct rethook {
> >  	void			*data;
> > -	rethook_handler_t	handler;
> > +	/*
> > +	 * To avoid sparse warnings, this uses a raw function pointer with
> > +	 * __rcu, instead of rethook_handler_t. But this must be same as
> > +	 * rethook_handler_t.
> > +	 */
> > +	void (__rcu *handler) (struct rethook_node *, void *, unsigned long, struct pt_regs *);
> >  	struct objpool_head	pool;
> >  	struct rcu_head		rcu;
> >  };
> > diff --git a/kernel/trace/rethook.c b/kernel/trace/rethook.c
> > index 6fd7d4ecbbc6..fa03094e9e69 100644
> > --- a/kernel/trace/rethook.c
> > +++ b/kernel/trace/rethook.c
> > @@ -48,7 +48,7 @@ static void rethook_free_rcu(struct rcu_head *head)
> >   */
> >  void rethook_stop(struct rethook *rh)
> >  {
> > -	WRITE_ONCE(rh->handler, NULL);
> > +	rcu_assign_pointer(rh->handler, NULL);
> >  }
> >  
> >  /**
> > @@ -63,7 +63,7 @@ void rethook_stop(struct rethook *rh)
> >   */
> >  void rethook_free(struct rethook *rh)
> >  {
> > -	WRITE_ONCE(rh->handler, NULL);
> > +	rethook_stop(rh);
> >  
> >  	call_rcu(&rh->rcu, rethook_free_rcu);
> >  }
> > @@ -82,6 +82,12 @@ static int rethook_fini_pool(struct objpool_head *head, void *context)
> >  	return 0;
> >  }
> >  
> > +static inline rethook_handler_t rethook_get_handler(struct rethook *rh)
> > +{
> > +	return (rethook_handler_t)rcu_dereference_check(rh->handler,
> > +							rcu_read_lock_any_held());
> > +}
> > +
> >  /**
> >   * rethook_alloc() - Allocate struct rethook.
> >   * @data: a data to pass the @handler when hooking the return.
> > @@ -107,7 +113,7 @@ struct rethook *rethook_alloc(void *data, rethook_handler_t handler,
> >  		return ERR_PTR(-ENOMEM);
> >  
> >  	rh->data = data;
> > -	rh->handler = handler;
> > +	rcu_assign_pointer(rh->handler, handler);
> >  
> >  	/* initialize the objpool for rethook nodes */
> >  	if (objpool_init(&rh->pool, num, size, GFP_KERNEL, rh,
> > @@ -135,9 +141,10 @@ static void free_rethook_node_rcu(struct rcu_head *head)
> >   */
> >  void rethook_recycle(struct rethook_node *node)
> >  {
> > -	lockdep_assert_preemption_disabled();
> > +	rethook_handler_t handler;
> >  
> > -	if (likely(READ_ONCE(node->rethook->handler)))
> > +	handler = rethook_get_handler(node->rethook);
> > +	if (likely(handler))
> >  		objpool_push(node, &node->rethook->pool);
> >  	else
> >  		call_rcu(&node->rcu, free_rethook_node_rcu);
> > @@ -153,9 +160,7 @@ NOKPROBE_SYMBOL(rethook_recycle);
> >   */
> >  struct rethook_node *rethook_try_get(struct rethook *rh)
> >  {
> > -	rethook_handler_t handler = READ_ONCE(rh->handler);
> > -
> > -	lockdep_assert_preemption_disabled();
> > +	rethook_handler_t handler = rethook_get_handler(rh);
> >  
> >  	/* Check whether @rh is going to be freed. */
> >  	if (unlikely(!handler))
> > @@ -300,7 +305,7 @@ unsigned long rethook_trampoline_handler(struct pt_regs *regs,
> >  		rhn = container_of(first, struct rethook_node, llist);
> >  		if (WARN_ON_ONCE(rhn->frame != frame))
> >  			break;
> > -		handler = READ_ONCE(rhn->rethook->handler);
> > +		handler = rethook_get_handler(rhn->rethook);
> >  		if (handler)
> >  			handler(rhn, rhn->rethook->data,
> >  				correct_ret_addr, regs);
> > 
> 
> I applied and tested this patch locally on an x86_64 machine and can
> confirm there are no RCU-related sparse warnings. Also, kprobe tests
> within the ftrace selftest suite succeed just the same as before
> applying the patch.
> 
> Tested-by: JP Kobryn <inwardvessel@xxxxxxxxx>

Thank you for testing!
Let me push it with your fix.

-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>