linux-kernel - Re: [PATCH][RFC] jump_labels/x86: Use either 5 byte or 2 byte jumps

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Message-ID: <4E8F772D.20408@zytor.com>
Date:	Fri, 07 Oct 2011 15:03:25 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Jason Baron <jbaron@...hat.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Richard Henderson <rth@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	David Daney <david.daney@...ium.com>,
	Michael Ellerman <michael@...erman.id.au>,
	Jan Glauber <jang@...ux.vnet.ibm.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	peterz@...radead.org
Subject: Re: [PATCH][RFC] jump_labels/x86: Use either 5 byte or 2 byte jumps

On 10/07/2011 03:00 PM, Steven Rostedt wrote:
>
> Ouch, quite shaken by k.org? I guess I should have substituted go blind
> with being hacked.
>

Well, yes, but I would have said exactly the same thing before.

> The thing is, it may be as simple as an out of tree module screwing up
> the jump table. Or worse, gcc not doing things that we did not expect.
> If this is the case, jump labels can be disabled from modifying code.
>
> But if we just want to do the BUG() case, this will be a big hammer to
> the code and we just prevent any further progress until the issue is
> addressed. Which may be tell people to disable jump labels in their
> code, or use a different compiler.

That is EXACTLY what should happen.  Something is wrong to the point of 
the kernel is *known* to be executing the wrong code.  That is an 
extremely serious condition and should be treated as such.

If you want, you could have a debug option to demote this to WARN, but I 
really don't want to see it by default.

	-hpa
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