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

[Kamlesh Gurudasani]

Waiman Long <longman@xxxxxxxxxx> 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@xxxxxxxxxx>
> ---
>  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.

Kamlesh

>  	list_for_each_entry(pw, &works, pw_list)
>  		if (job->numa_aware) {
>  			int old_node = atomic_read(&last_used_nid);
> -- 
> 2.43.5