linux-kernel - Re: [PATCH] padata: Fix possible divide-by-0 panic in padata_mt

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Message-ID: <878qwpoa3i.fsf@kamlesh.i-did-not-set--mail-host-address--so-tickle-me>
Date: Thu, 22 Aug 2024 02:40:57 +0530
From: Kamlesh Gurudasani <kamlesh@...com>
To: Daniel Jordan <daniel.m.jordan@...cle.com>,
        Herbert Xu
	<herbert@...dor.apana.org.au>
CC: Waiman Long <longman@...hat.com>, <steffen.klassert@...unet.com>,
        <akpm@...ux-foundation.org>, <linux-crypto@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] padata: Fix possible divide-by-0 panic in
 padata_mt_helper()

Kamlesh Gurudasani <kamlesh@...com> writes:

> Daniel Jordan <daniel.m.jordan@...cle.com> writes:
>
>> On Tue, Aug 20, 2024 at 12:06:47PM GMT, Herbert Xu wrote:
>>> On Mon, Aug 19, 2024 at 06:29:52PM -0400, Daniel Jordan wrote:
>>> >
>>> > The DIV_ROUND_UP approach reads a bit nicer to me, but I can imagine
>>> > oddball cases where rounding up is undesirable (say, near-zero values
>>> > for size, min_chunk, and align; padata_work_alloc_mt returns many fewer
>>> > works than requested; and a single unit of work is very expensive) so
>>> > that rounding up makes a bigger difference.  So, the way it now is seems
>>> > ok.
>>> 
>>> In that case let's do the max ahead of the align check:
>>> 
>>> 	ps.chunk_size = max(ps.chunk_size, 1ul);
>>> 	ps.chunk_size = roundup(ps.chunk_size, job->align);
>>> 
>>> If we do it after then it may come out unaligned (e.g., job->align = 8
>>> and ps.chunk_size = 1).
>>
>> Sure, I think Kamlesh was the first to suggest max, so maybe Kamlesh
>> would like to make the change.  I'll send a patch otherwise.
> Thanks for consideration, Daniel. I'll send a patch.
Sent.

Just curious about one thing on line 495,

nworks = max(job->size / max(job->min_chunk, job->align), 1ul);

what happens if both min_chunk and align are 0.

cheers,
Kamlesh