Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers

Aleksa Sarai
2019-09-05T19:56:18Z

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On 2019-09-05, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Thu, Sep 05, 2019 at 08:23:03PM +0200, Christian Brauner wrote:
>
> > Because every caller of that function right now has that limit set
> > anyway iirc. So we can either remove it from here and place it back for
> > the individual callers or leave it in the helper.
> > Also, I'm really asking, why not? Is it unreasonable to have an upper
> > bound on the size (for a long time probably) or are you disagreeing with
> > PAGE_SIZE being used? PAGE_SIZE limit is currently used by sched, perf,
> > bpf, and clone3 and in a few other places.
>
> For a primitive that can be safely used with any size (OK, any within
> the usual 2Gb limit)? Why push the random policy into the place where
> it doesn't belong?
>
> Seriously, what's the point? If they want to have a large chunk of
> userland memory zeroed or checked for non-zeroes - why would that
> be a problem?

Thinking about it some more, there isn't really any r/w amplification --
so there isn't much to gain by passing giant structs. Though, if we are
going to permit 2GB buffers, isn't that also an argument to use
memchr_inv()? :P

--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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