linux-kernel - Re: [RFC][CFT][PATCHSET v1] uaccess unification

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Date:   Thu, 30 Mar 2017 20:10:09 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Russell King - ARM Linux <linux@...linux.org.uk>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Richard Henderson <rth@...ddle.net>,
        Will Deacon <will.deacon@....com>,
        Haavard Skinnemoen <hskinnemoen@...il.com>,
        Vineet Gupta <vgupta@...opsys.com>,
        Steven Miao <realmz6@...il.com>,
        Jesper Nilsson <jesper.nilsson@...s.com>,
        Mark Salter <msalter@...hat.com>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Richard Kuo <rkuo@...eaurora.org>,
        Tony Luck <tony.luck@...el.com>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        James Hogan <james.hogan@...tec.com>,
        Michal Simek <monstr@...str.eu>,
        David Howells <dhowells@...hat.com>,
        Ley Foon Tan <lftan@...era.com>,
        Jonas Bonn <jonas@...thpole.se>, Helge Deller <deller@....de>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Ralf Baechle <ralf@...ux-mips.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Chen Liqin <liqin.linux@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Richard Weinberger <richard@....at>,
        Guan Xuetao <gxt@...c.pku.edu.cn>,
        Thomas Gleixner <tglx@...utronix.de>,
        Chris Zankel <chris@...kel.net>
Subject: Re: [RFC][CFT][PATCHSET v1] uaccess unification

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.