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

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Date:   Thu, 19 May 2022 22:05:29 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     tytso@....edu, hch@....de, linux-kernel@...r.kernel.org
Subject: Re: [PATCHSET 0/2] Fix splice from random/urandom

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:

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.

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

Regards,
Jason