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

[chuang]

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()),
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).