linux-kernel - Re: [git pull] iov

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Message-ID: <8d9e4f7c-bcf4-2751-9978-6283cabeda52@kernel.dk>
Date:   Thu, 9 Sep 2021 21:06:58 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Pavel Begunkov <asml.silence@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [git pull] iov_iter fixes

On 9/9/21 8:48 PM, Al Viro wrote:
> On Thu, Sep 09, 2021 at 07:35:13PM -0600, Jens Axboe wrote:
> 
>> Yep ok I follow you now. And yes, if we get a partial one but one that
>> has more consumed than what was returned, that would not work well. I'm
>> guessing that a) we've never seen that, or b) we always end up with
>> either correctly advanced OR fully advanced, and the fully advanced case
>> would then just return 0 next time and we'd just get a short IO back to
>> userspace.
>>
>> The safer way here would likely be to import the iovec again. We're
>> still in the context of the original submission, and the sqe hasn't been
>> consumed in the ring yet, so that can be done safely.
> 
> ... until you end up with something assuming that you've got the same
> iovec from userland the second time around.
> 
> IOW, generally it's a bad idea to do that kind of re-imports.

That's really no different than having one thread do the issue, and
another modify the iovec while it happens. It's only an issue if you
don't validate it, just like you did the first time you imported. No
assumptions need to be made here.

If it's no longer valid, it'll get failed, and it's really on the
application having buggy behavior. The iovec cannot be modified until
we've signaled that it's been consumed, which hasn't happened yet. No
different than if an application modifies it mid readv(2) syscall.

-- 
Jens Axboe