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

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Message-ID: <20080425161136.GD9503@Krystal>
Date:	Fri, 25 Apr 2008 12:11:36 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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

* Ingo Molnar (mingo@...e.hu) wrote:
> 
> * Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca> wrote:
> 
> > 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.
> 
> but the code as-is is nonsensical. It checks for:
> 
>         BUG_ON(len > sizeof(long));
> 
> but then deals with page crossing...
> 

That was in the initial version, before my patch, yes. I dealt with page
crossing at first, then added a more restrictive test to "play safe" (I
should have removed the page-crossing code at that point), but later on
noticed that there was a single case where it's valid to do non-atomic
updates, and it's when the execution flow is bypassed by a breakpoint
(as the immediate values are doing), so the last patch you have removes
the restrictive test and lets the page-crossing code in place.

> it should also rename text_poke_early() to text_poke_core(), and call 
> _that_ from text_poke() if core_kernel_text(). From that alone the whole 
> poke_text() function would look a whole lot cleaner.
> 

hrm, I am not convinced it's safe to call vmap() very early at boot
time. In the immediate values implementation, I do call text_poke very
very early at boot to populate the initial values. Or maybe are you
proposing something different from what I currently understand ?

Mathieu

> 	Ingo

-- 
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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