linux-kernel - Re: [PATCH v4 1/3] aio_ring_remap: kill the bogus ctx->dead check

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Message-ID: <x49h9q3fsdt.fsf@segfault.boston.devel.redhat.com>
Date:	Fri, 19 Jun 2015 14:46:38 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Benjamin LaHaise <bcrl@...ck.org>, linux-aio@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/3] aio_ring_remap: kill the bogus ctx->dead check

Oleg Nesterov <oleg@...hat.com> writes:

> On 06/18, Jeff Moyer wrote:
>>
>> Oleg Nesterov <oleg@...hat.com> writes:
>>
>> > kill_ioctx() sets ctx->dead and removes ctx from ->ioctx_table
>> > "atomically" under mm->ioctx_lock, so aio_ring_remap() can never
>> > see a dead ctx.
>> >
>> > And even -EINVAL doesn't look necessary. Yes, if mremap() races
>> > with kill_ioctx() vm_munmap(ctx->mmap_base, ctx->mmap_size) can
>> > unmap the wrong region. In this case the buggy application should
>> > blame itself. And there are other reasons why that vm_munmap() can
>> > be wrong. Say, an application can mremap() the part of aio region
>> > and then do io_destroy(). We could change aio_ring_remap() to
>> > verify vma->that vma_end - vma->vma_start == ctx->mmap_size but
>> > this won't help if the application does munmap() instead.
>>
>> I don't think this paragraph really fits with the patch.  It's
>> interesting commentary,
>
> Well, this time I disagree. It would be better to add a comment, but the
> changelog can help too to understand the code and potential problems if
> it races with kill_ioctx().
>
>> but if you feel strongly enough about it, send a
>> patch that undoes b2edffdd912b.  ;-)
>
> No, I think that commit makes sense. Just it was wrong and we need to
> fix it. And, in particular, its changelog was wrong (at least confusing),
> it looks as if there are strong reasons to prevent this race. So the
> changelog above can unconfuse the git-log reader.
>
>
> But. I never argue with maintainers about non-technical issues ;)

Well, then we can have Ben weigh in.  I'm not trying to make a huge deal
out of it.

Cheers,
Jeff
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