linux-kernel - Re: [PATCHSET 0/2] Fix splice from random/urandom

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Date:   Thu, 19 May 2022 14:49:13 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     tytso@....edu, hch@....de, linux-kernel@...r.kernel.org
Subject: Re: [PATCHSET 0/2] Fix splice from random/urandom

On 5/19/22 2:05 PM, Jason A. Donenfeld wrote:
> Hi Jens,
> 
> On Thu, May 19, 2022 at 01:31:31PM -0600, Jens Axboe wrote:
>> Hi,
>>
>> We recently had a failure on a kernel upgrade because splice no longer
>> works on random/urandom. This is due to:
>>
>> 6e2c7421f02 ("fs: don't allow splice read/write without explicit ops")
> 
> Thanks for this. I'd noticed this a few months ago and assumed it has
> just always been that way, and hadn't gotten to looking at what was up.
> 
> I'll take a look at these patches in detail when I'm home in a few
> hours, but one thing maybe you can answer more easily than my digging
> is:

Sounds good, thanks!

> There's a lot of attention in random.c devoted to not leaving any output
> around on the stack or in stray buffers. The explicit use of
> copy_to_user() makes it clear that the output isn't being copied
> anywhere other than what's the user's responsibility to cleanup. I'm
> wondering if the switch to copy_to_iter() introduces any buffering or
> gotchas that you might be aware of.

No, it's just a wrapper around copying to the user memory pointed to by
the iov_iter. No extra buffering or anything like that. So I think it
should be fine in that respect, and it actually cleans up the code a bit
imho since the copy_to_iter() since the return value of "bytes copied"
is easier to work with than the "bytes not copied".

> Also you may need to rebase this on the random.git tree at
> https://git.kernel.org/pub/scm/linux/kernel/git/crng/random.git

OK, I will rebase it on that branch, not a problem.

-- 
Jens Axboe