On Wed, 24 Jul 2019 11:19:42 -0400 Joe Lawrence <joe.lawre...@redhat.com> wrote:
> 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. OK, now I think we can pass a flag to kprobe_register() to modify regs->ip or not. Then we can introduce 2 different ftrace_ops for IPMODIFY or just requires REGS. > (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? NO, that flag has been shared among all ftrace-based kprobes, and checked when registering. So what we need is to introduce a new kprobe flag which states that this kprobe doesn't modify regs->ip. And kprobe prepare 2 ftrace_ops 1 is for IPMODIFY and 1 is for !IPMODIFY. > > 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). It is not supported yet. But I can make it. wait a bit :) Thank you, > > Regards, > > -- Joe -- Masami Hiramatsu <mhira...@kernel.org>
