From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Brauner Subject: Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Date: Fri, 6 Sep 2019 09:00:49 +0200 Message-ID: <20190906070048.tmhuemasmsn55spq@wittgenstein> References: <20190904201933.10736-1-cyphar@cyphar.com> <20190904201933.10736-2-cyphar@cyphar.com> <20190905180750.GQ1131@ZenIV.linux.org.uk> <20190905182303.7f6bxpa2enbgcegv@wittgenstein> <20190905182801.GR1131@ZenIV.linux.org.uk> <20190905195618.pwzgvuzadkfpznfz@yavin.dot.cyphar.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Return-path: Content-Disposition: inline In-Reply-To: <20190905195618.pwzgvuzadkfpznfz@yavin.dot.cyphar.com> Sender: linux-kernel-owner@vger.kernel.org To: Aleksa Sarai Cc: Al Viro , Jeff Layton , "J. Bruce Fields" , Arnd Bergmann , David Howells , Shuah Khan , Shuah Khan , Ingo Molnar , Peter Zijlstra , Christian Brauner , Rasmus Villemoes , Eric Biederman , Andy Lutomirski , Andrew Morton , Alexei Starovoitov , Kees Cook , Jann Horn , Tycho Andersen , David Drysdale , Chanho Min , Oleg List-Id: linux-api@vger.kernel.org On Fri, Sep 06, 2019 at 05:56:18AM +1000, Aleksa Sarai wrote: > On 2019-09-05, Al Viro 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 I think we should just do a really dumb, easy to understand minimal thing for now. It could even just be what every caller is doing right now anyway with the get_user() loop. The only way to settle memchr_inv() vs all the other clever ways suggested here is to benchmark it and see if it matters *for the current users* of this helper. If it does, great we can do it. If it doesn't why bother having that argument right now? Once we somehow end up in a possible world where we apparently have decided it's a great idea to copy 2GB argument structures for a syscall into or from the kernel we can start optimizing the hell out of this. Before that and especially with current callers I honestly doubt it matters whether we use memchr_inv() or while() {get_user()} loops. Christian