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

[Markus Trippelsdorf <markus@trippelsdorf.de>]

On 2012.11.29 at 11:19 -0800, Paul E. McKenney wrote:
> On Thu, Nov 29, 2012 at 07:22:54PM +0100, Markus Trippelsdorf wrote:
> > 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.
> > > > > 
> > > > 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. 
> 
> Hmmmm...  In that case...
> 
> Given that this is initialization code that is far from any fastpath,
> could you try putting something like this at the beginning of
> rcu_init_one()?
> 
> 	if (rcu_num_lvls > RCU_NUM_LVLS)
> 		panic("rcu_num_lvls overflow");
> 
> If the compiler doesn't know that panic() never returns (despite the
> __noreturn), you could add a "return" after the panic().
> 
> Does that help?

Yes. This fixes the issue. Many thanks.

(Perhaps not surprisingly the warning never occurred for
CONFIG_NR_CPUS>64.

I've also built a allyesconfig config and there was no -Warray-bounds
warning at all.)

-- 
Markus