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SubjectRe: [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@in.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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