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

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Date:	Fri, 25 Apr 2008 12:19:43 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	Andi Kleen <andi@...stfloor.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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, pageexec@...email.hu,
	Jeremy Fitzhardinge <jeremy@...p.org>
Subject: Re: [PATCH 1/1] x86: fix text_poke

Mathieu Desnoyers wrote:
> 
>>> b) there might be a jump into the middle of this instruction sequence?
>>>
>> If we change that, as discussed above, so the liveliness of ZF and of
>> the %al register is still insured by leaving the mov and test
>> instructions in place, we end up only modifying a single instruction and
>> the problem fades away. We would end up changing a jne for a jmp.
> 
> So, if we do is I propose here, we have to take into account this
> question too. Any jump that jumps in the middle of this instruction
> sequence would have to insure correct liveliness of %al and ZF. However,
> since we just limited the scope of their liveliness, there are no other
> code paths which can jump in the middle of our instruction sequence and
> insure correct ZF and %al liveliness.
> 

I wanted to point out that this, in particular, is utter nonsense. 
Consider a sequence that looks something like this:

if (foo ? bar : imv_cond(var)) {
	blah();
}

An entirely sane transformation of this (as far as gcc is concerned), is 
something like:

	cmpl $0,foo
	je 1f
	cmpl $0,bar
	jmp 2f
1:
#APP
	movb var,%al	/* This is your imv */
#NO_APP
	testb %al,%al
2:
	je 3f
	call blah
3:

Your code would take the movb-testb-je sequence and combine them, then 
we jump into the middle of the new instruction when jumping at 2!

There are only two ways to deal with this - extensive analysis of the 
entire flow of control, or telling the compiler exactly what is 
*actually* going on.  The latter is the preferred way, obviously.

	-hpa

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