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

From: Colin Ian King <colin.king@canonical.com>
To: Boqun Feng <boqun.feng@gmail.com>
Cc: "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] rcu: shift by 1UL rather than 1 to fix sign extension error
Date: Wed, 14 Dec 2016 14:54:34 +0000	[thread overview]
Message-ID: <21d2e114-26a9-2eff-893b-9ed2295248ce@canonical.com> (raw)
In-Reply-To: <20161214144244.GL9728@tardis.cn.ibm.com>

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On 14/12/16 14:42, Boqun Feng wrote:
> 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@canonical.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@canonical.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 ;-)

Yep, that's totally fine. Thanks.

Colin

> 
> 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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