From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christian Brauner <christian.brauner@ubuntu.com>
Subject: Re: [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user
 helpers
Date: Thu, 5 Sep 2019 13:09:15 +0200
Message-ID: <20190905110915.4vvhicg4ldmpi5u6@wittgenstein>
References: <20190904201933.10736-1-cyphar@cyphar.com>
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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
List-Id: linux-api@vger.kernel.org

On Thu, Sep 05, 2019 at 06:19:22AM +1000, Aleksa Sarai wrote:
> A common pattern for syscall extensions is increasing the size of a
> struct passed from userspace, such that the zero-value of the new fields
> result in the old kernel behaviour (allowing for a mix of userspace and
> kernel vintages to operate on one another in most cases). This is done
> in both directions -- hence two helpers -- though it's more common to
> have to copy user space structs into kernel space.
> 
> Previously there was no common lib/ function that implemented
> the necessary extension-checking semantics (and different syscalls
> implemented them slightly differently or incompletely[1]). A future
> patch replaces all of the common uses of this pattern to use the new
> copy_struct_{to,from}_user() helpers.
> 
> [1]: For instance {sched_setattr,perf_event_open,clone3}(2) all do do
>      similar checks to copy_struct_from_user() while rt_sigprocmask(2)
>      always rejects differently-sized struct arguments.
> 
> Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

I would probably split this out into a separate patchset. It can very
well go in before openat2(). Thoughts?

Christian