netdev - Re: [PATCH v2] libbpf: Cleanup the kprobe_event on failed add_kprobe_event

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Date:   Thu, 23 Jun 2022 14:09:41 +0800
From:   Chuang W <nashuiliang@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jingren Zhou <zhoujingren@...iglobal.com>
Subject: Re: [PATCH v2] libbpf: Cleanup the kprobe_event on failed add_kprobe_event_legacy()

Hi, John. Thanks, I will resubmit V3 soon, and supplement the commit
information.

On Thu, Jun 23, 2022 at 11:37 AM John Fastabend
<john.fastabend@...il.com> wrote:
>
> 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@...il.com>
> > Signed-off-by: Jingren Zhou <zhoujingren@...iglobal.com>
> > ---
> >  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