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From: Christian Brauner <christian.brauner@ubuntu.com>
To: Aleksa Sarai <cyphar@cyphar.com>
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, linux-m68k@lists.linux-m68k.org,
	Al Viro <viro@zeniv.linux.org.uk>,
	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-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>,
	containers@lists.linux-foundation.org
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> (raw)
In-Reply-To: <20190905195618.pwzgvuzadkfpznfz@yavin.dot.cyphar.com>

On Fri, Sep 06, 2019 at 05:56:18AM +1000, Aleksa Sarai wrote:
> 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

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

  parent reply index

Thread overview: 65+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-04 20:19 [PATCH v12 00/12] namei: openat2(2) path resolution restrictions Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Aleksa Sarai
2019-09-04 20:48   ` [PATCH v12 01/12] lib: introduce copy_struct_{to, from}_user helpers Linus Torvalds
2019-09-04 21:00   ` [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Randy Dunlap
2019-09-05  7:32   ` Peter Zijlstra
2019-09-05  9:26     ` Aleksa Sarai
2019-09-05  9:43       ` Peter Zijlstra
2019-09-05 10:57         ` Peter Zijlstra
2019-09-11 10:37           ` Aleksa Sarai
2019-09-05 13:35         ` Aleksa Sarai
2019-09-05 17:01         ` Aleksa Sarai
2019-09-05  8:43   ` Rasmus Villemoes
2019-09-05  9:50     ` Aleksa Sarai
2019-09-05 10:45       ` Christian Brauner
2019-09-05  9:09   ` [PATCH v12 01/12] lib: introduce copy_struct_{to, from}_user helpers Andreas Schwab
2019-09-05 10:13     ` Gabriel Paubert
2019-09-05 11:05   ` [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Christian Brauner
2019-09-05 11:17     ` Rasmus Villemoes
2019-09-05 11:29       ` Christian Brauner
2019-09-05 13:40     ` Aleksa Sarai
2019-09-05 11:09   ` Christian Brauner
2019-09-05 11:27     ` Aleksa Sarai
2019-09-05 11:40       ` Christian Brauner
2019-09-05 18:07   ` Al Viro
2019-09-05 18:23     ` Christian Brauner
2019-09-05 18:28       ` Al Viro
2019-09-05 18:35         ` Christian Brauner
2019-09-05 19:56         ` Aleksa Sarai
2019-09-05 22:31           ` Al Viro
2019-09-06  7:00           ` Christian Brauner [this message]
2019-09-05 23:00     ` Aleksa Sarai
2019-09-05 23:49       ` Al Viro
2019-09-06  0:09         ` Aleksa Sarai
2019-09-06  0:14         ` Al Viro
2019-09-04 20:19 ` [PATCH v12 02/12] clone3: switch to copy_struct_from_user() Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 03/12] sched_setattr: switch to copy_struct_{to, from}_user() Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 04/12] perf_event_open: switch to copy_struct_from_user() Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 05/12] namei: obey trailing magic-link DAC permissions Aleksa Sarai
2019-09-17 21:30   ` Jann Horn
2019-09-18 13:51     ` Aleksa Sarai
2019-09-18 15:46       ` Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 06/12] procfs: switch magic-link modes to be more sane Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 07/12] open: O_EMPTYPATH: procfs-less file descriptor re-opening Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 08/12] namei: O_BENEATH-style path resolution flags Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 09/12] namei: LOOKUP_IN_ROOT: chroot-like path resolution Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 10/12] namei: aggressively check for nd->root escape on ".." resolution Aleksa Sarai
2019-09-04 21:09   ` Linus Torvalds
2019-09-04 21:35     ` Linus Torvalds
2019-09-04 21:36       ` Linus Torvalds
2019-09-04 21:48     ` Aleksa Sarai
2019-09-04 22:16       ` Linus Torvalds
2019-09-04 22:31       ` David Howells
2019-09-04 22:38         ` Linus Torvalds
2019-09-04 23:29           ` Al Viro
2019-09-04 23:44             ` Linus Torvalds
2019-09-04 20:19 ` [PATCH v12 11/12] open: openat2(2) syscall Aleksa Sarai
2019-09-04 21:00   ` Randy Dunlap
2019-09-07 12:40   ` Jeff Layton
2019-09-07 16:58     ` Linus Torvalds
2019-09-07 17:42       ` Andy Lutomirski
2019-09-07 17:45         ` Linus Torvalds
2019-09-07 18:15           ` Andy Lutomirski
2019-09-10  6:35           ` Ingo Molnar
2019-09-08 16:24     ` Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 12/12] selftests: add openat2(2) selftests Aleksa Sarai

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