Re: [PATCH] padata: Fix possible divide-by-0 panic in padata_mt_helper()

[Waiman Long <longman@redhat.com>]

On 8/10/24 13:44, Kamlesh Gurudasani wrote:
> Waiman Long <longman@redhat.com> writes:
>
>> We are hit with a not easily reproducible divide-by-0 panic in padata.c
>> at bootup time.
>>
>>    [   10.017908] Oops: divide error: 0000 1 PREEMPT SMP NOPTI
>>    [   10.017908] CPU: 26 PID: 2627 Comm: kworker/u1666:1 Not tainted 6.10.0-15.el10.x86_64 #1
>>    [   10.017908] Hardware name: Lenovo ThinkSystem SR950 [7X12CTO1WW]/[7X12CTO1WW], BIOS [PSE140J-2.30] 07/20/2021
>>    [   10.017908] Workqueue: events_unbound padata_mt_helper
>>    [   10.017908] RIP: 0010:padata_mt_helper+0x39/0xb0
>>      :
>>    [   10.017963] Call Trace:
>>    [   10.017968]  <TASK>
>>    [   10.018004]  ? padata_mt_helper+0x39/0xb0
>>    [   10.018084]  process_one_work+0x174/0x330
>>    [   10.018093]  worker_thread+0x266/0x3a0
>>    [   10.018111]  kthread+0xcf/0x100
>>    [   10.018124]  ret_from_fork+0x31/0x50
>>    [   10.018138]  ret_from_fork_asm+0x1a/0x30
>>    [   10.018147]  </TASK>
>>
>> Looking at the padata_mt_helper() function, the only way a divide-by-0
>> panic can happen is when ps->chunk_size is 0. The way that chunk_size is
>> initialized in padata_do_multithreaded(), chunk_size can be 0 when the
>> min_chunk in the passed-in padata_mt_job structure is 0.
>>
>> Fix this divide-by-0 panic by making sure that chunk_size will be at
>> least 1 no matter what the input parameters are.
>>
>> Fixes: 004ed42638f4 ("padata: add basic support for multithreaded jobs")
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   kernel/padata.c | 7 +++++++
>>   1 file changed, 7 insertions(+)
>>
>> diff --git a/kernel/padata.c b/kernel/padata.c
>> index 53f4bc912712..0fa6c2895460 100644
>> --- a/kernel/padata.c
>> +++ b/kernel/padata.c
>> @@ -517,6 +517,13 @@ void __init padata_do_multithreaded(struct padata_mt_job *job)
>>   	ps.chunk_size = max(ps.chunk_size, job->min_chunk);
>>   	ps.chunk_size = roundup(ps.chunk_size, job->align);
>>   
>> +	/*
>> +	 * chunk_size can be 0 if the caller sets min_chunk to 0. So force it
>> +	 * to at least 1 to prevent divide-by-0 panic in padata_mt_helper().`
>> +	 */
> Thanks for the patch and detailed comment.
>> +	if (!ps.chunk_size)
>> +		ps.chunk_size = 1U;
>> +
> could it be
>          ps.chunk_size = max(ps.chunk_size, 1U);
>          
> or can be merged with earlier max()
>    	ps.chunk_size = max(ps.chunk_size, max(job->min_chunk, 1U));
>    	ps.chunk_size = roundup(ps.chunk_size, job->align);
>
> sits well with how entire file is written and compiler is optimizing
> them to same level.

I had actually thought about doing that as an alternative. I used the 
current patch to avoid putting too many max() calls there. I can go this 
route if you guys prefer this.

Cheers,
Longman