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

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Message-ID: <3607072d-2418-df43-fd9a-2708a95f97da@canonical.com>
Date:   Tue, 13 Dec 2016 11:33:19 +0000
From:   Colin Ian King <colin.king@...onical.com>
To:     Boqun Feng <boqun.feng@...il.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 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.

> 
> 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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