Re: [PATCH] libbpf: Remove kprobe_event on failed kprobe_open_legacy

[Jiri Olsa]

On Sat, Jun 18, 2022 at 01:31:01PM +0800, chuang wrote:
> Hi John,
> 
> On Sat, Jun 18, 2022 at 12:13 PM John Fastabend
> <john.fastabend@xxxxxxxxx> wrote:
> >
> > Chuang W wrote:
> > > In a scenario where livepatch and aggrprobe coexist, the creating
> > > kprobe_event using tracefs API will succeed, a trace event (e.g.
> > > /debugfs/tracing/events/kprobe/XX) will exist, but perf_event_open()
> > > will return an error.
> >
> > This seems a bit strange from API side. I'm not really familiar with
> > livepatch, but I guess this is UAPI now so fixing add_kprobe_event_legacy
> > to fail is not an option?
> >
> 
> The legacy kprobe API (i.e. tracefs API) has two steps:
> 
> 1) register_kprobe
> $ echo 'p:mykprobe XXX' > /sys/kernel/debug/tracing/kprobe_events
> This will create a trace event of mykprobe and register a disable
> kprobe that waits to be activated.
> 
> 2) enable_kprobe
> 2.1) using syscall perf_event_open
> as the following code, perf_event_kprobe_open_legacy (file:
> tools/lib/bpf/libbpf.c):
> ---
> attr.type = PERF_TYPE_TRACEPOINT;
> pfd = syscall(__NR_perf_event_open, &attr,
>               pid < 0 ? -1 : pid, /* pid */
>               pid == -1 ? 0 : -1, /* cpu */
>               -1 /* group_fd */,  PERF_FLAG_FD_CLOEXEC);
> ---
> In the implementation code of perf_event_open, enable_kprobe() will be executed.
> 2.2) using shell
> $ echo 1 > /sys/kernel/debug/tracing/events/kprobes/mykprobe/enable
> As with perf_event_open, enable_kprobe() will also be executed.
> 
> When using the same function XXX, kprobe and livepatch cannot coexist,
> that is, step 2) will return an error (ref: arm_kprobe_ftrace()),

just curious.. is that because of ipmodify flag on ftrace_ops?
AFAICS that be a poblem just for kretprobes, cc-ing Masami

thanks,
jirka


> however, step 1) is ok!
> However, the new kprobe API (i.e. perf kprobe API) aggregates
> register_kprobe and enable_kprobe, internally fixes the issue on
> failed enable_kprobe.
> But above all, for the legacy kprobe API, I think it should remove
> kprobe_event on failed add_kprobe_event_legacy() in
> perf_event_kprobe_open_legacy (file: tools/lib/bpf/libbpf.c).