RE: [PATCH v2] libbpf: Cleanup the kprobe_event on failed add_kprobe_event_legacy()

[John Fastabend]

Chuang W wrote:
> Before the 0bc11ed5ab60 commit ("kprobes: Allow kprobes coexist with
> livepatch"), in a scenario where livepatch and kprobe coexist on the
> same function entry, the creation of kprobe_event using
> add_kprobe_event_legacy() will be successful, at the same time as a
> trace event (e.g. /debugfs/tracing/events/kprobe/XX) will exist, but
> perf_event_open() will return an error because both livepatch and kprobe
> use FTRACE_OPS_FL_IPMODIFY.
> 
> With this patch, whenever an error is returned after
> add_kprobe_event_legacy(), this ensures that the created kprobe_event is
> cleaned.
> 
> Signed-off-by: Chuang W <nashuiliang@xxxxxxxxx>
> Signed-off-by: Jingren Zhou <zhoujingren@xxxxxxxxxxxxxx>
> ---
>  tools/lib/bpf/libbpf.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)

I think we want to improve the commit message otherwise I'm sure we will
stumble on this in the future and from just above its tricky to follow.
I would suggest almost verbatim the description you gave in reply to
my question. Just cut'n'pasting your text together with minor edit
glue,

"
 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! The new kprobe API (i.e. perf kprobe API)
 aggregates register_kprobe and enable_kprobe, internally fixes the
 issue on failed enable_kprobe.

 To fix: before the 0bc11ed5ab60 commit ("kprobes: Allow kprobes coexist with
 livepatch"), in a scenario where livepatch and kprobe coexist on the
 same function entry, the creation of kprobe_event using
 add_kprobe_event_legacy() will be successful, at the same time as a
 trace event (e.g. /debugfs/tracing/events/kprobe/XX) will exist, but
 perf_event_open() will return an error because both livepatch and kprobe
 use FTRACE_OPS_FL_IPMODIFY.
 
 With this patch, whenever an error is returned after
 add_kprobe_event_legacy(), this ensures that the created kprobe_event is
 cleaned.
"

Thanks,
John