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Date: Thu, 5 Sep 2019 13:29:59 +0200
From: Christian Brauner <christian.brauner@ubuntu.com>
To: Rasmus Villemoes <linux@rasmusvillemoes.dk>
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>, 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>,
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 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>, 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-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 01:17:38PM +0200, Rasmus Villemoes wrote:
> On 05/09/2019 13.05, Christian Brauner wrote:
> > On Thu, Sep 05, 2019 at 06:19:22AM +1000, Aleksa Sarai wrote:
> 
> >> +	if (unlikely(!access_ok(dst, usize)))
> >> +		return -EFAULT;
> >> +
> >> +	/* Deal with trailing bytes. */
> >> +	if (usize < ksize) {
> >> +		if (memchr_inv(src + size, 0, rest))
> >> +			return -EFBIG;
> >> +	} else if (usize > ksize) {
> >> +		if (__memzero_user(dst + size, rest))
> >> +			return -EFAULT;
> > 
> > Is zeroing that memory really our job? Seems to me we should just check
> > it is zeroed.
> 
> Of course it is, otherwise you'd require userspace to clear the output
> buffer it gives us, which in the majority of cases is wasted work. It's
> much easier to reason about if we just say "the kernel populates [uaddr,
> uaddr + usize)".

I don't really mind either way so sure. :)