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

[Herbert Xu <herbert@gondor.apana.org.au>]

On Sat, Aug 10, 2024 at 09:30:25PM -0400, Waiman Long wrote:
> 
> On 8/10/24 00:05, Herbert Xu wrote:
> > Waiman Long <longman@redhat.com> wrote:
> > > 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().`
> > > +        */
> > > +       if (!ps.chunk_size)
> > > +               ps.chunk_size = 1U;
> > Perhaps change the first ps.chunk_size assignment to use DIV_ROUND_UP
> > instead?
> 
> I think DIV_ROUND_UP() will exactly the same problem that if chunk_size is
> 0, you still got a 0 result. round_up() only if the 2nd argument is a power
> of 2 while with DIV_ROUND_UP(), the second argument can be any number except
> 0.

Unless I'm missing something chunk_size cannot be zero before the
division because that's the first thing we check upon entry into
this function.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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