linux-kernel - Re: [PATCH] Subject: kprobes-x86: correct post-eip value in post

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Date:	Mon, 17 Mar 2008 10:49:56 +0530
From:	Ananth N Mavinakayanahalli <ananth@...ibm.com>
To:	"Yakov Lerner" <iler.ml@...il.com>
Cc:	prasanna@...ibm.com, anil.s.keshavamurthy@...el.com,
	davem@...emloft.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Subject: kprobes-x86: correct post-eip value in
	post_hander()

On Sun, Mar 16, 2008 at 03:21:21AM -0500, Yakov Lerner wrote:
> 
> I was trying to get the address of instruction to be executed 
> next after the kprobed instruction.  But regs->eip in post_handler()
> contains value which is useless to the user. It's pre-corrected value.
> This value is difficult to use without access to resume_execution(), which
> is not exported anyway.
> I moved the invocation of post_handler() to *after* resume_execution().
> Now regs->eip contains meaningful value in post_handler().
> 
> I do not think this change breaks any backward-compatibility.
> To make meaning of the old value, post_handler() would need access to
> resume_execution() which is not exported.  I have difficulty to believe
> that previous, uncorrected, regs->eip can be meaningfully used in
> post_handler().

resume_execution() exists not just for the program counter fixups after
out-of-line singlestepping, but is also as an insurance to put the
program counter back to the correct address in case the user's
post_handler() mucks around with it. That isn't possible with this
change :-(

Ananth
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