On Fri, 2014-12-05 at 10:10 +0000, Jon Medhurst (Tixy) wrote: [...] > I'm worried because this whole optimised kprobes has some rather > complicated interactions, e.g. can the background thread that changes > breakpoints to jumps (or back again?) could occur at the same time > another CPU is processing a kprobe that's been hit, or is in the process > of removing a probe.
I think that is a plausible theory. We can have this situation... 1. CPU A executes a probe's 'breakpoint' instruction and the undefined instruction exception handler is triggered. 2. CPU B is executing the kprobes optimisation thread and replaces the 'breakpoint' with a branch instruction. 3. CPU A reads the invalid instruction from memory and because this is now the branch instruction it doesn't match KPROBE_ARM_BREAKPOINT_INSTRUCTION which kprobes registered to handle. This means the undefined instruction exception is treated as just that, execution of an undefined instruction. The above scenario is the exact reason why arch_disarm_kprobe is implemented to always use stop_machine to modify the code and we need to ensure the same happens with arch_optimize_kprobes. -- Tixy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/