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

[Masami Hiramatsu <mhiramat@redhat.com>]

Ananth N Mavinakayanahalli wrote:
> 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, I think we can not prevent it even if resume_execution() is called
after post_handler, because resume_execution() refers reg->ip...:-(

And Yakov, I think you might need to make a patchset against all arch which
support kprobes, because this patch modifies expected behavior of kprobes
only on x86.

IMHO, Yakov's suggestion will be also good for resume_execution(), because
it only has to clean up after expectable-single-stepping. (user code is
unexpectable... we can not control all of that)


Thanks,

-- 
Masami Hiramatsu

Software Engineer
Hitachi Computer Products (America) Inc.
Software Solutions Division

e-mail: mhiramat@redhat.com