Date: Thu, 30 Mar 2017 20:10:09 +0100
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Russell King - ARM Linux <linux@armlinux.org.uk>,
        "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Richard Henderson <rth@twiddle.net>, Will Deacon <will.deacon@arm.com>,
        Haavard Skinnemoen <hskinnemoen@gmail.com>,
        Vineet Gupta <vgupta@synopsys.com>, Steven Miao <realmz6@gmail.com>,
        Jesper Nilsson <jesper.nilsson@axis.com>,
        Mark Salter <msalter@redhat.com>,
        Yoshinori Sato <ysato@users.sourceforge.jp>,
        Richard Kuo <rkuo@codeaurora.org>, Tony Luck <tony.luck@intel.com>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        James Hogan <james.hogan@imgtec.com>, Michal Simek <monstr@monstr.eu>,
        David Howells <dhowells@redhat.com>, Ley Foon Tan <lftan@altera.com>,
        Jonas Bonn <jonas@southpole.se>, Helge Deller <deller@gmx.de>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Ralf Baechle <ralf@linux-mips.org>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Chen Liqin <liqin.linux@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Chris Metcalf <cmetcalf@mellanox.com>,
        Richard Weinberger <richard@nod.at>, Guan Xuetao <gxt@mprc.pku.edu.cn>,
        Thomas Gleixner <tglx@linutronix.de>, Chris Zankel <chris@zankel.net>
Subject: Re: [RFC][CFT][PATCHSET v1] uaccess unification
Message-ID: <20170330191009.GU29622@ZenIV.linux.org.uk>
References: <20170329055706.GH29622@ZenIV.linux.org.uk>
 <20170330162241.GG7909@n2100.armlinux.org.uk>
 <20170330164342.GR29622@ZenIV.linux.org.uk>
 <CA+55aFzsGSKzbQjWAPxFvc=HeXyr6stk1KsB-1xGCKALgazYuQ@mail.gmail.com>
 <20170330184824.GS29622@ZenIV.linux.org.uk>
 <20170330185427.GT29622@ZenIV.linux.org.uk>
 <CA+55aFx2u6iUzJV1ZH4w_o6tTZesSGoOxMrOtJB8aXScou6zPg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CA+55aFx2u6iUzJV1ZH4w_o6tTZesSGoOxMrOtJB8aXScou6zPg@mail.gmail.com>
User-Agent: Mutt/1.7.1 (2016-10-04)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 734
Lines: 16

On Thu, Mar 30, 2017 at 11:59:16AM -0700, Linus Torvalds wrote:

> But regardless of that, I think you're being silly to even look at the
> iovec code. That code simply *isn't* critical enough that one or two
> extra instructions matter.
> 
> Show me profiles to the contrary. I dare you.
> 
> Those things shouldn't be using *anything* odd at all. They should be
> using "copy_from_user()". Nothing else.

That they very definitely should not.  And not because of access_ok() or
might_fault() - this is one place where zero-padding is absolutely wrong.
So unless you are going to take it out of copy_from_user() and pray
that random shit ioctls in random shit drivers check the return value
properly, copy_from_user() is no-go here.