On Mon, Nov 11, 2013 at 10:51:52AM +0000, Masami Hiramatsu wrote: > (2013/11/11 16:54), Masami Hiramatsu wrote: > >>>> In fact, how do you avoid a race with hardware breakpoints? E.g., > >>>> somebody > >>>> places a hardware breakpoint on an instruction in the kernel for which > >>>> kprobes has patched in a brk. We take the hardware breakpoint, disable > >>>> the > >>>> breakpoint and set up a single step before returning to the brk. The brk > >>>> then traps, but we must take care not to disable single-step and/or > >>>> unmask > >>>> debug exceptions, because that will cause the hardware breakpoint code to > >>>> re-arm its breakpoint before we've stepped off the brk instruction. > >>> > >>> Hmm, frankly to say, this kind of race issue is not seriously discussed > >>> on x86 too, since kgdb is still a special tool (not used on the production > >>> system). > >>> I think under such situation kgdb operator must have full control of the > >>> system, and he can (and has to) avoid such kind of race. > >> Masami, > >> > >> Hmm I think in same lines, but not sure if we expect kprobes to be > >> able to work fool-proof along with kgdb or hw breakpoints ? > > > > For hw breakpoint, yes, we finally get check each other to safely > > use it even if one rejects the other one at some points(address). > > Since the hw breakpoint is already open for normal user via perf, > > we should do it. But the policy still needs to be discussed. > > OK, I've ensured that the hw_breakpoint (from perf) can work > with kprobes (from ftrace) at the same address on x86. > So if arm64 already support hw_breakpoint on perf, kprobes should > work with it.
Single-stepping on x86 is different to the step behaviour on arm64 afaik. On ARM, we have to manually remove the breakpoint, perform a single-step, then add the breakpoint again. If we re-enable debug exceptions in the kprobe handler, the step will complete early and we'll never step off the breakpoint. Sandeepa: I think you need to retry Masami's test on the arm64 model, since I'm fairly sure it won't work as expected without some additional code. Will -- 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/
