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

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Message-ID: <20080317123951.GC7229@in.ibm.com>
Date:	Mon, 17 Mar 2008 18:09:51 +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 Mon, Mar 17, 2008 at 12:59:05PM +0200, Yakov Lerner wrote:
> 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 ?

Of course, there still are other ways to shoot yourself in the foot with
the post_handler(), but, atleast for cases we can control, we need to do
the right thing.

Ananth
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