Re: copying from/to user question

[Christian Brauner <brauner@kernel.org>]

[Forgot to add Thomas and Arnd as Mike pointed out]

On Mon, Sep 09, 2024 at 10:50:10AM GMT, Christian Brauner wrote:
> Hey,
> 
> This is another round of Christian's asking sus questions about kernel
> apis. I asked them a few people and generally the answers I got was
> "Good question, I don't know." or the reasoning varied a lot. So I take
> it I'm not the only one with that question.
> 
> I was looking at a potential epoll() bug and it got me thinking about
> dos & don'ts for put_user()/copy_from_user() and related helpers as
> epoll does acquire the epoll mutex and then goes on to loop over a list
> of ready items and calls __put_user() for each item. Granted, it only
> puts a __u64 and an integer but still that seems adventurous to me and I
> wondered why.
> 
> Generally, new vfs apis always try hard to call helpers that copy to or
> from userspace without any locks held as my understanding has been that
> this is best practice as to avoid risking taking page faults while
> holding a mutex or semaphore even though that's supposedly safe.
> 
> Is this understanding correct? And aside from best practice is it in
> principle safe to copy to or from userspace with sleeping locks held?