linux-kernel - Re: [PATCH] ARM: move body of head-common.S back to text section

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Date:	Wed, 3 Jul 2013 11:30:12 -0400
From:	Paul Gortmaker <paul.gortmaker@...driver.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Stephen Warren <swarren@...dotorg.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Will Deacon <will.deacon@....com>,
	<linux-kernel@...r.kernel.org>, Joseph Lo <josephl@...dia.com>,
	<linux-tegra@...r.kernel.org>, Stephen Warren <swarren@...dia.com>,
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: move body of head-common.S back to text section

[Re: [PATCH] ARM: move body of head-common.S back to text section] On 03/07/2013 (Wed 11:00) Russell King - ARM Linux wrote:

> On Wed, Jul 03, 2013 at 01:19:07AM -0400, Paul Gortmaker wrote:
> > As an aside, I'm now thinking any __INIT that implicitly rely on EOF for
> > closure are nasty traps waiting to happen and it might be worthwhile to
> > audit and explicitly __FINIT them before someone appends to the file...
> 
> That hides a different kind of bug though - I hate __FINIT for exactly
> that reason.  Consider this:

Agreed - perhaps masking that it is a ".previous" just hides the fact
that it is more like a pop operation vs. an on/off operation, or per
function as we have in C.

> 
> 	.text
> 	blah blah blah
> 	__INIT
> 	lots of init stuff
> 	__FINIT
> 	more .text stuff
> 
> Now, someone comes along and modifies this to be:
> 
> 	.text
> 	blah blah blah
> 	.data
> 	something else

Yeah, that would be kind of careless; not putting .data above the .text,
or at least closing with a .previous, but sure it could sneak past
review.

> 	__INIT
> 	lots of init stuff
> 	__FINIT

The presence of the above 3 lines of init block (i.e. here or not)
doesn't really change the fact that the .data guy broke the below .text
code by grandfathering it into .data -- But you could argue that him
seeing the 1st __INIT and that influenced him to decide to not read any
further down into the file -- which probably does happen, though.... :(

> 	more .text stuff
> 
> Now, what is the effect of that __FINIT now?  You get the following .text
> emitted into the .data section instead.  This is basically the same problem
> you've just encounted.
> 
> Maybe:
> 
> 	__FINIT
> 	.text
> 
> is the safest solution - and __FINIT becomes just a no-op marker to avoid
> anyone relying on its properties.

That seems reasonable to me.  I can't think of any self auditing that is
reasonably simple to implement.  One downside of __FINIT as a no-op vs.
what it is today, is that a dangling __FINIT in a file with no other
previous sections will emit a warning.  But that is a small low value
corner case I think.
--
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