Messages in this thread | | | Date | Mon, 17 Mar 2008 18:09:51 +0530 | From | Ananth N Mavinakayanahalli <> | Subject | Re: [PATCH] Subject: kprobes-x86: correct post-eip value in post_hander() |
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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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