Messages in this thread | | | Date | Tue, 3 Oct 2017 10:10:12 +0900 | From | Masami Hiramatsu <> | Subject | Re: [kprobes/x86] a19b2e3d78: WARNING:at_kernel/locking/lockdep.c:#trace_hardirqs_off_caller |
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On Mon, 2 Oct 2017 18:05:43 +0200 Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, Oct 03, 2017 at 12:46:05AM +0900, Masami Hiramatsu wrote: > > On Mon, 2 Oct 2017 09:33:16 +0200 > > Peter Zijlstra <peterz@infradead.org> wrote: > > > > > [ 87.018115] Call Trace: > > > > [ 87.025046] trace_hardirqs_off+0xd/0xf > > > > [ 87.034185] setjmp_pre_handler+0x6c/0x95 > > > > [ 87.043738] kprobe_ftrace_handler+0xc3/0xf4 > > > > > > > > > So setjmp_pre_handler() does: > > > > > > regs->flags &= ~X86_EFLAGS_IF; > > > trace_hardirqs_off(); > > > regs->ip = (unsigned long)(jp->entry); > > > > > > Which clears IF on the regs, but those will only take effect after an > > > IRET, not instantly. This messes up he IRQ state tracing, which you're > > > telling it will instantly disable IRQs. > > > > Thanks for analyzing! > > And right, since IRQ should be off while jump handler, it changes > > regs->flags. (but ...why?) > > Otherwise the IRET could re-enable interrupts?
Ah, I meant why IRQ should be disabled... It doesn't guarantee to avoid nested kprobes (since another kprobe can be hit in jprobe handler).
> > > A possible 'fix' would be to do local_irq_disable() in front of that, > > > but I got pretty lost in that stuff so I can't say for sure if that > > > makes sense or not. > > > > I'm not sure how lockdep traces irq-disabling state, but it seems > > that "enabling" irq state(trace_hardirqs_on()) is already missing > > from kprobes. > > If you could point me at where that is supposed to happen I can have a > look at how that tracing works. I got lost in the code this morning.
The right place to decrement irq counter(trace_hardirqs_on()) should be longjmp_break_handler(), which recovers flags register with other registers from kcb->jprobe_saved_regs, and IRET recovers IF. (maybe it doesn't count inc/dec correctly, isn't it?)
> > I'm considering to remove disabling-irq itself from jprobe. > > (Frankly to say, I would like to remove jprobe itself...) > > That would obviously also solve all problems :-)
Yeah, for long term it needs to be removed :)
Thank you,
-- Masami Hiramatsu <mhiramat@kernel.org>
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