linux-kernel - Re: [PATCH 0/2] jump label: update for .39

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Date:	Thu, 10 Mar 2011 16:39:54 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	David Daney <ddaney@...iumnetworks.com>,
	Jason Baron <jbaron@...hat.com>, peterz@...radead.org,
	hpa@...or.com, mingo@...e.hu, tglx@...utronix.de,
	andi@...stfloor.org, roland@...hat.com, rth@...hat.com,
	masami.hiramatsu.pt@...achi.com, fweisbec@...il.com,
	avi@...hat.com, davem@...emloft.net, sam@...nborg.org,
	michael@...erman.id.au, linux-kernel@...r.kernel.org,
	Ralf Baechle <ralf@...ux-mips.org>
Subject: Re: [PATCH 0/2] jump label: update for .39

* Steven Rostedt (rostedt@...dmis.org) wrote:
> On Thu, 2011-03-10 at 10:47 -0800, David Daney wrote:
> 
> > The alignment requested by the assembler will have to satisfy *all* the 
> > requested alignments, so manually forcing everything to .align 8 (or 
> > .align 4 for 32-bit) should ensure that the linker doesn't put in any holes.
> 
> I would agree with the assessment although, I don't know that it is
> documented anywhere that this is what happens. As the previous "bug"
> with the trace_events was solved by me adding .align(4) everywhere, I
> would think that .align(sizeof(long)) would work here too.
> 
> It may be a good ideal to force this alignment, and not add wasted
> space. If anything, if this (hypothetical) bug appears, it will most
> likely show up as a crash on boot up. I'm not too concerned about it.

How can you be so sure it will trigger a crash on boot up ?

The sorting phase only compare key values, so NULL pointers will be
thought as valid. Following that, there is the initial no-op'ing phrase
that might crash (on the architectures using it). Then a NULL code
pointer is considered as the discarded "init" section. A NULL key will
just be a non-match, and thus skipped.

So I can very much see scenarios where this bug would silently skip jump
labels without a crash. This is what I am concerned about. Using the
approach that "a crash will happen" as a safety net to tell us that we
missed something seems very risky to me.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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