linux-kernel - Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}

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Date:   Thu, 5 Sep 2019 13:29:59 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Shuah Khan <skhan@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Christian Brauner <christian@...uner.io>,
        Eric Biederman <ebiederm@...ssion.com>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Tycho Andersen <tycho@...ho.ws>,
        David Drysdale <drysdale@...gle.com>,
        Chanho Min <chanho.min@....com>,
        Oleg Nesterov <oleg@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Aleksa Sarai <asarai@...e.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        containers@...ts.linux-foundation.org, linux-alpha@...r.kernel.org,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        linux-xtensa@...ux-xtensa.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user
 helpers

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. :)