linux-kernel - Re: kernel/rcutree.c:2850:13: warning: array subscript is above array bounds

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Date:	Thu, 29 Nov 2012 19:22:54 +0100
From:	Markus Trippelsdorf <markus@...ppelsdorf.de>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: kernel/rcutree.c:2850:13: warning: array subscript is above
 array bounds

On 2012.11.29 at 10:10 -0800, Paul E. McKenney wrote:
> On Thu, Nov 29, 2012 at 06:43:58PM +0100, Markus Trippelsdorf wrote:
> > On 2012.11.29 at 09:02 -0800, Paul E. McKenney wrote:
> > > On Thu, Nov 29, 2012 at 02:47:52PM +0100, Markus Trippelsdorf wrote:
> > > > With gcc-4.8 I get:
> > > > 
> > > >   CC      kernel/rcutree.o
> > > >   kernel/rcutree.c: In function ‘rcu_init_one’:
> > > >   kernel/rcutree.c:2850:13: warning: array subscript is above array bounds [-Warray-bounds]
> > > >      rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
> > > >                   ^
> > > > 2849    for (i = 1; i < rcu_num_lvls; i++)
> > > > 2850           rsp->level[i] = rsp->level[i - 1] + rsp->levelcnt[i - 1];
> > > > 
> > > > At first I thought that the warning was bogus, but rcu_num_lvls isn't static
> > > > and gets modified prior to the for loop.
> > > 
> > > You are quite correct that rcu_num_lvls does get modified, but there
> > > are checks in rcu_init_geometry() to ensure that it does not increase:
> > > 
> > > 	/*
> > > 	 * The boot-time rcu_fanout_leaf parameter is only permitted
> > > 	 * to increase the leaf-level fanout, not decrease it.  Of course,
> > > 	 * the leaf-level fanout cannot exceed the number of bits in
> > > 	 * the rcu_node masks.  Finally, the tree must be able to accommodate
> > > 	 * the configured number of CPUs.  Complain and fall back to the
> > > 	 * compile-time values if these limits are exceeded.
> > > 	 */
> > > 	if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF ||
> > > 	    rcu_fanout_leaf > sizeof(unsigned long) * 8 ||
> > > 	    n > rcu_capacity[MAX_RCU_LVLS]) {
> > > 		WARN_ON(1);
> > > 		return;
> > > 	}
> > > 
> > > The value of rcu_num_lvls starts out at RCU_NUM_LVLS, the same as
> > > the dimension of the ->level[] array.  The loop goes only to one less
> > > than rcu_num_lvls, as needed, and rcu_num_lvls is never greater than
> > > RCU_NUM_LVLS, so this should be safe.
> > > 
> > > So what am I missing here?
> > 
> > rcu_num_lvls does get modified in rcu_init_geometry:
> > 
> > 2942         /* Calculate the number of rcu_nodes at each level of the tree. */
> > 2943         for (i = 1; i <= MAX_RCU_LVLS; i++)
> > 2944                 if (n <= rcu_capacity[i]) {
> > 2945                         for (j = 0; j <= i; j++)
> > 2946                                 num_rcu_lvl[j] =
> > 2947                                         DIV_ROUND_UP(n, rcu_capacity[i - j]);
> > 2948                         rcu_num_lvls = i;
> > 
> > And rcu_init_geometry gets called before rcu_init_one, so the compiler assumes
> > the worst and issues a warning.
> > So, in your opinion, what would be the best way to silence this warning?
> 
> Good question.  Are you saying that if the compiler cannot prove that
> the index is in bounds, it is going to throw a warning?  

Yes, it does seem to be the case. See also my gcc bug report (closed as
invalid): http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55529

> If that is the case, perhaps telling the compiler to cool it via the
> command line would be best.
> Or is this really one of a very few places in the kernel where the
> compiler is complaining?

Yes. With my (admittedly minimal) config this is only place. 

-- 
Markus
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