linux-kernel - Re: [git pull] kgdb-light -v10

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Date:	Tue, 12 Feb 2008 16:36:12 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Jason Wessel <jason.wessel@...driver.com>
Cc:	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, "Frank Ch. Eigler" <fche@...hat.com>,
	Roland McGrath <roland@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [git pull] kgdb-light -v10

> Basically when you reach this chunk of code it is before the hand off
> to the source debugger.  We cannot continue because there was a
> breakpoint in a part of the system kgdb was using while doing its
> normal work.  The reality is that KGDB is not self contained.  It
> relies on some external I/O methods, atomic page fault handling and
> some other pieces.  If you take an exception there, the kgdb integrity
> check absolutely needs to fail.

Don't you just need a simple recursion counter for this? 

I cannot think of a reliable simple way to check for this using the instruction
pointer. The only way would be to use explicit annotations for all
possible code similar to what kprobes does (__kprobes), but that would be 
hugely intrusive all over the tree.

With the recursion counter the only problem would be someone 
setting a break point on the early notifier code itself that contains
a recursion check, but that would be only a few lines of code
so the risk of someone setting a break point exactly there would
be low.

> This check is absolutely required to help prevent silent death via
> dumb breakpoints or stepping around in random places (which is ill
> advised anyway).

I believe you, but I don't believe that your implementation
of this handles all cases.

-Andi
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