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Date: Mon, 17 Feb 2020 10:03:22 +0100 From: Christophe Leroy <christophe.leroy@....fr> To: Masami Hiramatsu <mhiramat@...nel.org> Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>, Paul Mackerras <paulus@...ba.org>, Michael Ellerman <mpe@...erman.id.au>, Larry Finger <Larry.Finger@...inger.net>, "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>, linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, stable@...nel.vger.org, Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>, "David S. Miller" <davem@...emloft.net> Subject: Re: [PATCH] powerpc/kprobes: Fix trap address when trap happened in real mode Le 16/02/2020 à 13:34, Masami Hiramatsu a écrit : > On Sat, 15 Feb 2020 11:28:49 +0100 > Christophe Leroy <christophe.leroy@....fr> wrote: > >> Hi, >> >> Le 14/02/2020 à 14:54, Masami Hiramatsu a écrit : >>> Hi, >>> >>> On Fri, 14 Feb 2020 12:47:49 +0000 (UTC) >>> Christophe Leroy <christophe.leroy@....fr> wrote: >>> >>>> When a program check exception happens while MMU translation is >>>> disabled, following Oops happens in kprobe_handler() in the following >>>> test: >>>> >>>> } else if (*addr != BREAKPOINT_INSTRUCTION) { >>> >>> Thanks for the report and patch. I'm not so sure about powerpc implementation >>> but at where the MMU translation is disabled, can the handler work correctly? >>> (And where did you put the probe on?) >>> >>> Your fix may fix this Oops, but if the handler needs special care, it is an >>> option to blacklist such place (if possible). >> >> I guess that's another story. Here we are not talking about a place >> where kprobe has been illegitimately activated, but a place where there >> is a valid trap, which generated a valid 'program check exception'. And >> kprobe was off at that time. > > Ah, I got it. It is not a kprobe breakpoint, but to check that correctly, > it has to know the address where the breakpoint happens. OK. > >> >> As any 'program check exception' due to a trap (ie a BUG_ON, a WARN_ON, >> a debugger breakpoint, a perf breakpoint, etc...) calls >> kprobe_handler(), kprobe_handler() must be prepared to handle the case >> where the MMU translation is disabled, even if probes are not supposed >> to be set for functions running with MMU translation disabled. > > Can't we check the MMU is disabled there (as same as checking the exception > happened in user space or not)? > What do you mean by 'there' ? At the entry of kprobe_handler() ? That's what my patch does, it checks whether MMU is disabled or not. If it is, it converts the address to a virtual address. Do you mean kprobe_handler() should bail out early as it does when the trap happens in user mode ? Of course we can do that, I don't know enough about kprobe to know if kprobe_handler() should manage events that happened in real-mode or just ignore them. But I tested adding an event on a function that runs in real-mode, and it (now) works. So, what should we do really ? Christophe
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