linux-kernel - Re: [PATCH v2] The value may overflow

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Message-ID: <89dc5f3f-f959-0586-6f3c-1481c5d3efc4@efficios.com>
Date:   Tue, 5 Sep 2023 08:26:51 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     David Laight <David.Laight@...LAB.COM>,
        Denis Arefev <arefev@...mel.ru>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "lvc-project@...uxtesting.org" <lvc-project@...uxtesting.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "trufanov@...mel.ru" <trufanov@...mel.ru>,
        "vfh@...mel.ru" <vfh@...mel.ru>
Subject: Re: [PATCH v2] The value may overflow

On 9/5/23 05:31, David Laight wrote:
> From: Mathieu Desnoyers
>> Sent: 04 September 2023 11:24
>>
>> On 9/4/23 05:42, Denis Arefev wrote:
>>> The value of an arithmetic expression 1 << (cpu - sdp->mynode->grplo)
>>> is subject to overflow due to a failure to cast operands to a larger
>>> data type before performing arithmetic
>>
>> The patch title should identify more precisely its context, e.g.:
>>
>> "srcu: Fix srcu_struct node grpmask overflow on 64-bit systems"
>>
>> Also, as I stated in my reply to the previous version, the patch commit
>> message should describe the impact of the bug it fixes.
> 
> And is the analysis complete?
> Is 1UL right for 32bit archs??
> Is 64 bits even enough??

I understand from include/linux/rcu_node_tree.h and kernel/rcu/Kconfig 
RCU_FANOUT and RCU_FANOUT_LEAF ranges that a 32-bit integer is 
sufficient to hold the mask on 32-bit architectures, and a 64-bit 
integer is enough on 64-bit architectures given the current implementation.

At least this appears to be the intent. I did not do a thorough analysis 
of the various parameter limits.

Thanks,

Mathieu

> 
> 	David
> 
>>
>> Thanks,
>>
>> Mathieu
>>
>>
>>>
>>> Found by Linux Verification Center (linuxtesting.org) with SVACE.
>>>
>>> Signed-off-by: Denis Arefev <arefev@...mel.ru>
>>> ---
>>> v2: Added fixes to the srcu_schedule_cbs_snp function as suggested by
>>> Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
>>>    kernel/rcu/srcutree.c | 4 ++--
>>>    1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
>>> index 20d7a238d675..6c18e6005ae1 100644
>>> --- a/kernel/rcu/srcutree.c
>>> +++ b/kernel/rcu/srcutree.c
>>> @@ -223,7 +223,7 @@ static bool init_srcu_struct_nodes(struct srcu_struct *ssp, gfp_t gfp_flags)
>>>    				snp->grplo = cpu;
>>>    			snp->grphi = cpu;
>>>    		}
>>> -		sdp->grpmask = 1 << (cpu - sdp->mynode->grplo);
>>> +		sdp->grpmask = 1UL << (cpu - sdp->mynode->grplo);
>>>    	}
>>>    	smp_store_release(&ssp->srcu_sup->srcu_size_state, SRCU_SIZE_WAIT_BARRIER);
>>>    	return true;
>>> @@ -833,7 +833,7 @@ static void srcu_schedule_cbs_snp(struct srcu_struct *ssp, struct srcu_node *snp
>>>    	int cpu;
>>>
>>>    	for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
>>> -		if (!(mask & (1 << (cpu - snp->grplo))))
>>> +		if (!(mask & (1UL << (cpu - snp->grplo))))
>>>    			continue;
>>>    		srcu_schedule_cbs_sdp(per_cpu_ptr(ssp->sda, cpu), delay);
>>>    	}
>>
>> --
>> Mathieu Desnoyers
>> EfficiOS Inc.
>> https://www.efficios.com
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com