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

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Message-ID: <20161214144244.GL9728@tardis.cn.ibm.com>
Date:   Wed, 14 Dec 2016 22:42:44 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     Colin Ian King <colin.king@...onical.com>
Cc:     "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

On Tue, Dec 13, 2016 at 11:33:19AM +0000, Colin Ian King wrote:
> On 13/12/16 11:21, Boqun Feng wrote:
> > 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.
> >>
> > 
> > Right, you are, and the tool is ;-)
> > 
> > If @bit is greater than 32, we even got an undefined behavior in C ;-(
> > This is my careless mistake, thank you for finding it out and fix it!
> > 
> >> 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>
> > 
> > I think Paul only queued that for running tests and I have almost
> > finished a v2. I will fold your fix in my patch and add your SoB along
> > with mine, does that work for you?
> 
> Sure, that's good with me.
> 

Colin, as I'm going to take Mark's suggestion and use
leaf_node_cpu_bit() instead. So I'm going to drop your SoB but keep a
commit message saying you spotted the problem at the first place. Hope
that works with you ;-)

Regards,
Boqun

> > 
> > TBH, this situation is kinda new to me, so if anyone has any suggestion,
> > please let me know ;-)
> > 
> > Regards,
> > Boqun
> > 
> >> ---
> >>  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;
> >>  
> >>  		if (mask != 0) {
> >>  			/* Idle/offline CPUs, report (releases rnp->lock. */
> >> -- 
> >> 2.10.2
> >>
> 
> 




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