[peterz-queue:locking/jump_label 19/30] kernel/tracepoint.c:296 tracepoint_remove_func() error: we previously assumed 'tp_funcs' could be null (see line 286)

* [peterz-queue:locking/jump_label 19/30] kernel/tracepoint.c:296 tracepoint_remove_func() error: we previously assumed 'tp_funcs' could be null (see line 286)
@ 2019-10-09 19:14 ` Dan Carpenter
  0 siblings, 0 replies; 2+ messages in thread
From: Dan Carpenter @ 2019-10-09 19:14 UTC (permalink / raw)
  To: kbuild

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tree:   https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git locking/jump_label
head:   ef36041eb3c323ddc4050649298755a82b367782
commit: 0e669b8d46545cae6903cb0dd6fbfb4058728ce1 [19/30] tracepoints: Use static_call

If you fix the issue, kindly add following tag
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>

smatch warnings:
kernel/tracepoint.c:296 tracepoint_remove_func() error: we previously assumed 'tp_funcs' could be null (see line 286)

# https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?id=0e669b8d46545cae6903cb0dd6fbfb4058728ce1
git remote add peterz-queue https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git
git remote update peterz-queue
git checkout 0e669b8d46545cae6903cb0dd6fbfb4058728ce1
vim +/tp_funcs +296 kernel/tracepoint.c

de7b2973903c6c Mathieu Desnoyers       2014-04-08  272  static int tracepoint_remove_func(struct tracepoint *tp,
de7b2973903c6c Mathieu Desnoyers       2014-04-08  273  		struct tracepoint_func *func)
97e1c18e8d17bd Mathieu Desnoyers       2008-07-18  274  {
de7b2973903c6c Mathieu Desnoyers       2014-04-08  275  	struct tracepoint_func *old, *tp_funcs;
0e669b8d46545c Steven Rostedt (VMware  2018-10-05  276) 	int probes_left = 0;
97e1c18e8d17bd Mathieu Desnoyers       2008-07-18  277  
b725dfea24b89d Mathieu Desnoyers       2014-04-09  278  	tp_funcs = rcu_dereference_protected(tp->funcs,
b725dfea24b89d Mathieu Desnoyers       2014-04-09  279  			lockdep_is_held(&tracepoints_mutex));
0e669b8d46545c Steven Rostedt (VMware  2018-10-05  280) 	old = func_remove(&tp_funcs, func, &probes_left);

I'm looking at FETCH_HEAD and func_remove() doesn't touch/use
probes_left.

de7b2973903c6c Mathieu Desnoyers       2014-04-08  281  	if (IS_ERR(old)) {
d66a270be3310d Mathieu Desnoyers       2018-03-15  282  		WARN_ON_ONCE(PTR_ERR(old) != -ENOMEM);
de7b2973903c6c Mathieu Desnoyers       2014-04-08  283  		return PTR_ERR(old);
b75ef8b44b1cb9 Mathieu Desnoyers       2011-08-10  284  	}
b75ef8b44b1cb9 Mathieu Desnoyers       2011-08-10  285  
de7b2973903c6c Mathieu Desnoyers       2014-04-08 @286  	if (!tp_funcs) {
de7b2973903c6c Mathieu Desnoyers       2014-04-08  287  		/* Removed last function */
de7b2973903c6c Mathieu Desnoyers       2014-04-08  288  		if (tp->unregfunc && static_key_enabled(&tp->key))
de7b2973903c6c Mathieu Desnoyers       2014-04-08  289  			tp->unregfunc();
b75ef8b44b1cb9 Mathieu Desnoyers       2011-08-10  290  
de7b2973903c6c Mathieu Desnoyers       2014-04-08  291  		if (static_key_enabled(&tp->key))
de7b2973903c6c Mathieu Desnoyers       2014-04-08  292  			static_key_slow_dec(&tp->key);
97e1c18e8d17bd Mathieu Desnoyers       2008-07-18  293  	}
0e669b8d46545c Steven Rostedt (VMware  2018-10-05  294) 
0e669b8d46545c Steven Rostedt (VMware  2018-10-05  295) 	__static_call_update(tp->static_call_key, tp->static_call_tramp,
0e669b8d46545c Steven Rostedt (VMware  2018-10-05 @296) 			     probes_left == 1 ? tp_funcs->func : tp->iterator);

So Smatch says that "The 'probes_left == 1' condition is impossible, so
probably that just means I'm confused so I'm going to ignore it." so
then it complains that it's not clear if "tp_funcs" is NULL or not.

I could silence this false positive if I added an if statement:

	if (is_impossible_path())
		return;

I do that for some warnings but not for this one.

0e669b8d46545c Steven Rostedt (VMware  2018-10-05  297) 
de7b2973903c6c Mathieu Desnoyers       2014-04-08  298  	rcu_assign_pointer(tp->funcs, tp_funcs);
8058bd0faad860 Mathieu Desnoyers       2014-05-08  299  	release_probes(old);
de7b2973903c6c Mathieu Desnoyers       2014-04-08  300  	return 0;
127cafbb276266 Lai Jiangshan           2008-10-28  301  }
127cafbb276266 Lai Jiangshan           2008-10-28  302  

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