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

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Message-ID: <20121129202522.GB2474@linux.vnet.ibm.com>
Date:	Thu, 29 Nov 2012 12:25:22 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Markus Trippelsdorf <markus@...ppelsdorf.de>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: kernel/rcutree.c:2850:13: warning: array subscript is above
 array bounds

On Thu, Nov 29, 2012 at 09:03:21PM +0100, Markus Trippelsdorf wrote:
> 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.)

OK, I am applying this change with your Reported-by.  But please
understand that if something like this shows up in a fastpath, the
compiler will likely need to be told to keep its array-bounds opinions
to itself.

							Thanx, Paul

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