From mboxrd@z Thu Jan  1 00:00:00 1970
From: Aleksa Sarai <cyphar@cyphar.com>
Subject: Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user
 helpers
Date: Fri, 6 Sep 2019 05:56:18 +1000
Message-ID: <20190905195618.pwzgvuzadkfpznfz@yavin.dot.cyphar.com>
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To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <christian.brauner@ubuntu.com>, Jeff Layton <jlayton@kernel.org>, "J. Bruce Fields" <bfields@fieldses.org>, Arnd Bergmann <arnd@arndb.de>, David Howells <dhowells@redhat.com>, Shuah Khan <shuah@kernel.org>, Shuah Khan <skhan@linuxfoundation.org>, Ingo Molnar <mingo@redhat.com>, Peter Zijlstra <peterz@infradead.org>, Christian Brauner <christian@brauner.io>, Rasmus Villemoes <linux@rasmusvillemoes.dk>, Eric Biederman <ebiederm@xmission.com>, Andy Lutomirski <luto@kernel.org>, Andrew Morton <akpm@linux-foundation.org>, Alexei Starovoitov <ast@kernel.org>, Kees Cook <keescook@chromium.org>, Jann Horn <jannh@google.com>, Tycho Andersen <tycho@tycho.ws>, David Drysdale <drysdale@google.com>, Chanho Min <chanho.min@lge.com>
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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:
>=20
> > 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.
>=20
> 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?
>=20
> 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

--=20
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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