Messages in this thread | | | Date | Mon, 17 Mar 2008 18:17:21 -0400 | From | Masami Hiramatsu <> | Subject | Re: [PATCH] Subject: kprobes-x86: correct post-eip value in post_hander() |
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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
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