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

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Message-ID: <f36b08ee0803170359w675130bas296b5276da5f2a3d@mail.gmail.com>
Date:	Mon, 17 Mar 2008 12:59:05 +0200
From:	"Yakov Lerner" <iler.ml@...il.com>
To:	ananth@...ibm.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 Mon, Mar 17, 2008 at 7:19 AM, Ananth N Mavinakayanahalli
<ananth@...ibm.com> wrote:
> 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 :-(

I see your point. This can be prevented by saving and restoring regs->ip
around the post_handler() call, no ? Current code is beautiful. Saving and
restoring regs->ip would make this place look ugly.

Otoh, if the post_handler() wants to crash the kernel, it can do it
in thousand ways, not just by trashing regs->ip, no ?

Yakov
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