Hi Masami, I wanted to revisit FTRACE_OPS_FL_IPMODIFY blocking of kprobes and livepatch, at least in cases where kprobe pre_handlers don't modify regs->ip. (We've discussed this previously at part of a kpatch github issue #47: https://github.com/dynup/kpatch/issues/47) The particular use case I was wondering about was perf probing a particular function, then attempting to livepatch that same function: % uname -r 5.3.0-rc1+ % dmesg -C % perf probe --add cmdline_proc_show Added new event: probe:cmdline_proc_show (on cmdline_proc_show) You can now use it in all perf tools, such as: perf record -e probe:cmdline_proc_show -aR sleep 1 % perf record -e probe:cmdline_proc_show -aR sleep 30 & [1] 1007 % insmod samples/livepatch/livepatch-sample.ko insmod: ERROR: could not insert module samples/livepatch/livepatch-sample.ko: Device or resource busy % dmesg [ 440.913962] livepatch_sample: tainting kernel with TAINT_LIVEPATCH [ 440.917123] livepatch_sample: module verification failed: signature and/or required key missing - tainting kernel [ 440.942493] livepatch: enabling patch 'livepatch_sample' [ 440.943445] livepatch: failed to register ftrace handler for function 'cmdline_proc_show' (-16) [ 440.944576] livepatch: failed to patch object 'vmlinux' [ 440.945270] livepatch: failed to enable patch 'livepatch_sample' [ 440.946085] livepatch: 'livepatch_sample': unpatching complete This same behavior holds in reverse, if we want to probe a livepatched function: % insmod samples/livepatch/livepatch-sample.ko % perf probe --add cmdline_proc_show Added new event: probe:cmdline_proc_show (on cmdline_proc_show) You can now use it in all perf tools, such as: perf record -e probe:cmdline_proc_show -aR sleep 1 % perf record -e probe:cmdline_proc_show -aR sleep 30 Error: The sys_perf_event_open() syscall returned with 16 (Device or resource busy) for event (probe:cmdline_proc_show). /bin/dmesg | grep -i perf may provide additional information. Now, if I read kernel/trace/trace_kprobe.c :: kprobe_dispatcher() correctly, it's only going to return !0 (indicating a modified regs->ip) when kprobe_perf_func() returns !0, i.e. regs->ip changes over a call to trace_call_bpf(). Aside: should kprobe_ftrace_handler() check that FTRACE_OPS_FL_IPMODIFY is set when a pre_handler returns !0? In kpatch #47, Josh suggested: - If a kprobe handler needs to modify IP, user sets KPROBE_FLAG_IPMODIFY flag to register_kprobe, and then kprobes sets FTRACE_OPS_FL_IPMODIFY when registering with ftrace for that probe. - If KPROBE_FLAG_IPMODIFY is not used, kprobe_ftrace_handler() can detect when a kprobe handler changes regs->ip and restore it to its original value (regs->ip = ip). Is this something that could still be supported? In cases like perf probe, could we get away with not setting FTRACE_OPS_FL_IPMODIFY? The current way that we're applying that flag, kprobes and livepatch are mutually exclusive (for the same function). Regards, -- Joe
