linux-kernel - Re: [PATCH] rcu: shift by 1UL rather than 1 to fix sign extension error

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Date:   Tue, 13 Dec 2016 17:16:32 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Colin King <colin.king@...onical.com>
Cc:     Boqun Feng <boqun.feng@...il.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: shift by 1UL rather than 1 to fix sign extension
 error

Hi,

On Tue, Dec 13, 2016 at 10:56:46AM +0000, Colin King wrote:
> From: Colin Ian King <colin.king@...onical.com>
> 
> mask and bit are unsigned longs, so if bit is 31 we end up sign
> extending the 1 and mask ends up as 0xffffffff80000000. Fix this
> by explicitly adding integer suffix UL ensure 1 is a unsigned long
> rather than an signed int.
> 
> Issue found with static analysis with CoverityScan, CID 1388564
> 
> Fixes: 8965c3ce4718754db ("rcu: Use leaf_node_for_each_mask_possible_cpu() in force_qs_rnp()")
> Signed-off-by: Colin Ian King <colin.king@...onical.com>
> ---
>  kernel/rcu/tree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 10162ac..6ecedd8 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3051,7 +3051,7 @@ static void force_qs_rnp(struct rcu_state *rsp,
>  
>  		leaf_node_for_each_mask_possible_cpu(rnp, rnp->qsmask, bit, cpu)
>  			if (f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
> -				mask |= 1 << bit;
> +				mask |= 1UL << bit;

So as to match the rest of the code altered in commit bc75e99983df1efd
("rcu: Correctly handle sparse possible cpus"), and regardless of
naming, I think it'd be nicer to use leaf_node_cpu_bit(), e.g.

	leaf_node_for_each_mask_possible_cpu(rnp, rnp->qsmask, bit, cpu)
		if (f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
			mask |= leaf_node_cpu_bit(rnp, cpu);

IMO, it would be nice to hide the iterator bit somehow, to match
for_each_leaf_node_possible_cpu(), which this largely looks similar to
otherwise.

Thanks,
Mark.