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SubjectRe: [PATCH] Subject: kprobes-x86: correct post-eip value in post_hander()
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 ?

Yakov


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