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

[Ananth N Mavinakayanahalli <ananth@in.ibm.com>]

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