From: Aleksa Sarai <cyphar@cyphar.com>
To: Al Viro <viro@zeniv.linux.org.uk>
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,
Jiri Olsa <jolsa@redhat.com>,
linux-arch@vger.kernel.org, linux-s390@vger.kernel.org,
Tycho Andersen <tycho@tycho.ws>, Aleksa Sarai <asarai@suse.de>,
Shuah Khan <shuah@kernel.org>,
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,
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 10:09:08 +1000 [thread overview]
Message-ID: <20190906000908.xpvkuhun7v6onp6w@yavin.dot.cyphar.com> (raw)
In-Reply-To: <20190905234944.GT1131@ZenIV.linux.org.uk>
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On 2019-09-06, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Fri, Sep 06, 2019 at 09:00:03AM +1000, Aleksa Sarai wrote:
> > > > + return -EFAULT;
> > > > + }
> > > > + /* Copy the interoperable parts of the struct. */
> > > > + if (__copy_to_user(dst, src, size))
> > > > + return -EFAULT;
> > >
> > > Why not simply clear_user() and copy_to_user()?
> >
> > I'm not sure I understand what you mean -- are you asking why we need to
> > do memchr_inv(src + size, 0, rest) earlier?
>
> I'm asking why bother with __ and separate access_ok().
Ah right, it was a dumb "optimisation" (since we need to do access_ok()
anyway since we should early -EFAULT in that case). I've dropped the __
usages in my working copy.
> > > if ((unsigned long)addr & 1) {
> > > u8 v;
> > > if (get_user(v, (__u8 __user *)addr))
> > > return -EFAULT;
> > > if (v)
> > > return -E2BIG;
> > > addr++;
> > > }
> > > if ((unsigned long)addr & 2) {
> > > u16 v;
> > > if (get_user(v, (__u16 __user *)addr))
> > > return -EFAULT;
> > > if (v)
> > > return -E2BIG;
> > > addr +=2;
> > > }
> > > if ((unsigned long)addr & 4) {
> > > u32 v;
> > > if (get_user(v, (__u32 __user *)addr))
> > > return -EFAULT;
> > > if (v)
> > > return -E2BIG;
> > > }
> > > <read the rest like you currently do>
>
> Actually, this is a dumb way to do it - page size on anything
> is going to be a multiple of 8, so you could just as well
> read 8 bytes from an address aligned down. Then mask the
> bytes you don't want to check out and see if there's anything
> left.
>
> You can have readability boundaries inside a page - it's either
> the entire page (let alone a single word) being readable, or
> it's EFAULT for all parts.
>
> > > would be saner, and things like x86 could trivially add an
> > > asm variant - it's not hard. Incidentally, memchr_inv() is
> > > an overkill in this case...
> >
> > Why is memchr_inv() overkill?
>
> Look at its implementation; you only care if there are
> non-zeroes, you don't give a damn where in the buffer
> the first one would be. All you need is the same logics
> as in "from userland" case
> if (!count)
> return true;
> offset = (unsigned long)from & 7
> p = (u64 *)(from - offset);
> v = *p++;
> if (offset) { // unaligned
> count += offset;
> v &= ~aligned_byte_mask(offset); // see strnlen_user.c
> }
> while (count > 8) {
> if (v)
> return false;
> v = *p++;
> count -= 8;
> }
> if (count != 8)
> v &= aligned_byte_mask(count);
> return v == 0;
>
> All there is to it...
Alright, will do (for some reason I hadn't made the connection that
memchr_inv() is doing effectively the same word-by-word comparison but
also detecting where the first byte is).
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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next prev parent reply other threads:[~2019-09-06 0:12 UTC|newest]
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
2019-09-05 23:00 ` Aleksa Sarai
2019-09-05 23:49 ` Al Viro
2019-09-06 0:09 ` Aleksa Sarai [this message]
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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