linux-ext4 - Re: [PATCH 1/1] x86: fix text

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Message-ID: <20080425155444.GB9503@Krystal>
Date:	Fri, 25 Apr 2008 11:54:44 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Jiri Slaby <jirislaby@...il.com>,
	David Miller <davem@...emloft.net>, zdenek.kabelac@...il.com,
	rjw@...k.pl, paulmck@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	linux-ext4@...r.kernel.org, herbert@...dor.apana.org.au,
	penberg@...helsinki.fi, clameter@....com,
	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>,
	pageexec@...email.hu, "H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [PATCH 1/1] x86: fix text_poke

* Linus Torvalds (torvalds@...ux-foundation.org) wrote:
> 
> 
> On Fri, 25 Apr 2008, Ingo Molnar wrote:
> > 
> > something like the patch below? (untested)
> 
> No. That whole code sequence is total and utter crap. It needs to be 
> rewritten.
> 
> It first does a BUG_ON() if it's not naturally aligned (because that 
> wouldn't be atomic), and then it has code for page crossing! What a TOTAL 
> PIECE OF SH*T!
> 
> Hint:
>  - if it's naturally aligned, it couldn't be page crossing ANYWAY
>  - and if it was a page-crosser, it sure as hell couldn't be atomic!
> 
> The code is just crap, crap, crap. It needs to be rewritten from scratch. 
> I'll have a patch soonish.
> 
> 		Linus


Woooow, just a sec here. I removed the atomicity test _because_ there
happen to be a case where it's safe to do non-atomic instruction
modification. If we do :

1) replace the instruction first byte by a breakpoint, execute an
  instruction bypass (see the immediate values patches for detail)
2) modify the instruction non-atomically
3) put back the original instruction first byte.

That's why I removed the BUG_ONs at the beginning of the function.
That's also why it's required to deal with page crossing.

Mathieu


-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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