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Date: Thu, 5 Sep 2019 19:28:01 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Christian Brauner <christian.brauner@ubuntu.com>
Subject: Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user
 helpers
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Cc: linux-ia64@vger.kernel.org, linux-sh@vger.kernel.org,
 Peter Zijlstra <peterz@infradead.org>,
 Rasmus Villemoes <linux@rasmusvillemoes.dk>,
 Alexei Starovoitov <ast@kernel.org>, linux-kernel@vger.kernel.org,
 David Howells <dhowells@redhat.com>, linux-kselftest@vger.kernel.org,
 sparclinux@vger.kernel.org, Shuah Khan <shuah@kernel.org>,
 linux-arch@vger.kernel.org, linux-s390@vger.kernel.org,
 Tycho Andersen <tycho@tycho.ws>, Aleksa Sarai <asarai@suse.de>,
 Jiri Olsa <jolsa@redhat.com>,
 Alexander Shishkin <alexander.shishkin@linux.intel.com>,
 Ingo Molnar <mingo@redhat.com>, linux-arm-kernel@lists.infradead.org,
 linux-mips@vger.kernel.org, linux-xtensa@linux-xtensa.org,
 Kees Cook <keescook@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
 Jann Horn <jannh@google.com>, linuxppc-dev@lists.ozlabs.org,
 Aleksa Sarai <cyphar@cyphar.com>, Andy Lutomirski <luto@kernel.org>,
 Shuah Khan <skhan@linuxfoundation.org>, Namhyung Kim <namhyung@kernel.org>,
 David Drysdale <drysdale@google.com>, Christian Brauner <christian@brauner.io>,
 "J. Bruce Fields" <bfields@fieldses.org>, linux-parisc@vger.kernel.org,
 linux-m68k@lists.linux-m68k.org, linux-api@vger.kernel.org,
 Chanho Min <chanho.min@lge.com>, Jeff Layton <jlayton@kernel.org>,
 Oleg Nesterov <oleg@redhat.com>, Eric Biederman <ebiederm@xmission.com>,
 linux-alpha@vger.kernel.org, linux-fsdevel@vger.kernel.org,
 Andrew Morton <akpm@linux-foundation.org>,
 Linus Torvalds <torvalds@linux-foundation.org>,
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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?