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?
Thanx, Paul
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