linux-kernel - Re: [PATCH] Fix copy_user on x86

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Message-ID: <alpine.LFD.1.10.0806260837290.10755@hp.linux-foundation.org>
Date:	Thu, 26 Jun 2008 08:37:49 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Anton Arapov <aarapov@...hat.com>
cc:	Vitaly Mayatskikh <v.mayatskih@...il.com>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] Fix copy_user on x86_64

On Thu, 26 Jun 2008, Anton Arapov wrote:
> 
> This is the patch patch for copy_user routine, you've discussed recently.

I don't think it works right.

Isn't this same routine also used for copy_in_user()? For that case both 
source _and_ destination can fault, but your fixup routines assume that 
onle one of them does (ie the fixup for a load-fault does a store for the 
previously loaded valies, and assumes that it doesn't trap)

Also, I'd realy rather do this all by handling the "taul" case in C. We 
already effectively have _half_ that support: the "clear end" flag ends up 
calling our specialized memset() routine, but it would be much nicer if 
we:

 - extended the "clear end" flag to be not just "clear end", but also 
   which direction things are going.
 - always call a (fixed) fixup-routine that is written in C (because 
   performance on a cycle basis no longer matters) that gets the remaining 
   length and the source and destination as arguments, along with the 
   "clear and direction flag".
 - make that fixup routine do the byte-exact tests and any necessary 
   clearing (and return the possibly-fixed-up remaining length).

Notice how this way we still have _optimal_ performance for the case where 
no fault happens, and we don't need any complex fixups in assembly code at 
all - the only thing the asm routines need to do is to get the right 
length (we already have this) and fix up the source/dest pointers (we 
don't generally have this, although the zero-at-end fixes up the 
destination pointer in order to zero it, of course).

Hmm?

			Linus
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