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Rao" , linuxppc-dev@lists.ozlabs.org, "David S. Miller" , Larry Finger Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On 02/17/2020 03:38 PM, Christophe Leroy wrote: > > > Le 17/02/2020 à 11:27, Masami Hiramatsu a écrit : >> On Mon, 17 Feb 2020 10:03:22 +0100 >> Christophe Leroy wrote: >> >>> >>> >>> Le 16/02/2020 à 13:34, Masami Hiramatsu a écrit : >>>> On Sat, 15 Feb 2020 11:28:49 +0100 >>>> Christophe Leroy 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 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 ? >> >> Yes, that is what I meant. >> >>> 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 ? >> >> I'm not sure how the powerpc kernel runs in real mode. >> But clearly, at least kprobe event can not handle that case because >> it tries to access memory by probe_kernel_read(). Unless that function >> correctly handles the address translation, I want to prohibit kprobes >> on such address. >> >> So what I would like to see is, something like below. >> >> diff --git a/arch/powerpc/kernel/kprobes.c >> b/arch/powerpc/kernel/kprobes.c >> index 2d27ec4feee4..4771be152416 100644 >> --- a/arch/powerpc/kernel/kprobes.c >> +++ b/arch/powerpc/kernel/kprobes.c >> @@ -261,7 +261,7 @@ int kprobe_handler(struct pt_regs *regs) >> unsigned int *addr = (unsigned int *)regs->nip; >> struct kprobe_ctlblk *kcb; >> - if (user_mode(regs)) >> + if (user_mode(regs) || !(regs->msr & MSR_IR)) >> return 0; >> /* >> >> > > With this instead change of my patch, I get an Oops everytime a kprobe > event occurs in real-mode. > > This is because kprobe_handler() is now saying 'this trap doesn't belong > to me' for a trap that has been installed by it. > > So the 'program check' exception handler doesn't find the owner of the > trap hence generate an Oops. > > Even if we don't want kprobe() to proceed with the event entirely > (allthough it works at least for simple events), I'd expect it to fail > gracefully. > What about something like that: @@ -264,6 +265,13 @@ int kprobe_handler(struct pt_regs *regs) if (user_mode(regs)) return 0; + if (!(regs->msr & MSR_IR)) { + if (!get_kprobe(phys_to_virt(regs->nip))) + return 0; + regs->nip += 4; + return 1; + } + /* * We don't want to be preempted for the entire * duration of kprobe processing Christophe