From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Alexei Starovoitov <ast@fb.com>,
Kees Cook <keescook@chromium.org>,
Andy Lutomirski <luto@amacapital.net>,
Andy Lutomirski <luto@kernel.org>,
Alexei Starovoitov <ast@kernel.org>,
LSM List <linux-security-module@vger.kernel.org>,
James Morris <jmorris@namei.org>, Jann Horn <jannh@google.com>,
Peter Zijlstra <peterz@infradead.org>,
Masami Hiramatsu <mhiramat@kernel.org>,
"David S. Miller" <davem@davemloft.net>,
Daniel Borkmann <daniel@iogearbox.net>,
Network Development <netdev@vger.kernel.org>,
bpf <bpf@vger.kernel.org>, Kernel Team <Kernel-team@fb.com>,
Linux API <linux-api@vger.kernel.org>
Subject: trace_printk issue. Was: [PATCH bpf-next] bpf, capabilities: introduce CAP_BPF
Date: Thu, 3 Oct 2019 09:18:40 -0700 [thread overview]
Message-ID: <20191003161838.7lz746aa2lzl7qi4@ast-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <20191002190027.4e204ea8@gandalf.local.home>
On Wed, Oct 02, 2019 at 07:00:27PM -0400, Steven Rostedt wrote:
> > >>>>
> > >>>> Both 'trace' and 'trace_pipe' have quirky side effects.
> > >>>> Like opening 'trace' file will make all parallel trace_printk() to be ignored.
> > >>>> While reading 'trace_pipe' file will clear it.
> > >>>> The point that traditional 'read' and 'write' ACLs don't map as-is
> > >>>> to tracefs, so I would be careful categorizing things into
> > >>>> confidentiality vs integrity only based on access type.
> > >>>
> > >>> What exactly is the bpf_trace_printk() used for? I may have other ideas
> > >>> that can help.
> > >>
> > >> It's debugging of bpf programs. Same is what printk() is used for
> > >> by kernel developers.
> > >>
> > >
> > > How is it extracted? Just read from the trace or trace_pipe file?
> >
> > yep. Just like kernel devs look at dmesg when they sprinkle printk.
> > btw, if you can fix 'trace' file issue that stops all trace_printk
> > while 'trace' file is open that would be great.
> > Some users have been bitten by this behavior. We even documented it.
>
> The behavior is documented as well in the ftrace documentation. That's
> why we suggest the trace_pipe redirected into a file so that you don't
> lose data (unless the writer goes too fast). If you prefer a producer
> consumer where you lose newer events (like perf does), you can turn off
> overwrite mode, and it will drop events when the buffer is full (see
> options/overwrite).
I think dropping last events is just as bad. Is there a mode to overwrite old
and keep the last N (like perf does) ?
That aside having 'trace' file open should NOT drop trace_printks.
My point that bpf_trace_printk is just as important to bpf developers as
printk to kernel developers.
Imagine kernel developer losing their printk-s only because they typed
"dmesg" in another terminal?
It's completely unexpected and breaks developer trust in debugging mechanism.
Peter Wu brought this issue to my attention in
commit 55c33dfbeb83 ("bpf: clarify when bpf_trace_printk discards lines").
And later sent similar doc fix to ftrace.rst.
To be honest if I knew of this trace_printk quirk I would not have picked it
as a debugging mechanism for bpf.
I urge you to fix it.
next prev parent reply other threads:[~2019-10-03 17:20 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-27 20:52 [PATCH bpf-next] bpf, capabilities: introduce CAP_BPF Alexei Starovoitov
2019-08-27 23:01 ` Andy Lutomirski
2019-08-27 23:21 ` Steven Rostedt
2019-08-27 23:34 ` Andy Lutomirski
2019-08-28 0:44 ` Steven Rostedt
2019-08-28 1:12 ` Andy Lutomirski
2019-08-28 2:22 ` Steven Rostedt
2019-08-28 0:38 ` Alexei Starovoitov
2019-08-28 3:30 ` Masami Hiramatsu
2019-08-28 4:47 ` Alexei Starovoitov
2019-08-28 0:34 ` Alexei Starovoitov
2019-08-28 0:55 ` Andy Lutomirski
2019-08-28 2:00 ` Andy Lutomirski
2019-08-28 4:49 ` Alexei Starovoitov
2019-08-28 6:20 ` Andy Lutomirski
2019-08-28 23:38 ` Alexei Starovoitov
2019-08-29 0:58 ` Andy Lutomirski
2019-08-28 4:43 ` Alexei Starovoitov
2019-08-28 6:12 ` Andy Lutomirski
2019-08-28 22:55 ` Alexei Starovoitov
2019-08-29 0:45 ` Andy Lutomirski
2019-08-29 0:53 ` Andy Lutomirski
2019-08-29 4:07 ` Alexei Starovoitov
2019-09-28 23:37 ` Steven Rostedt
2019-09-30 18:31 ` Kees Cook
2019-10-01 1:22 ` Alexei Starovoitov
2019-10-01 22:10 ` Steven Rostedt
2019-10-01 22:18 ` Alexei Starovoitov
2019-10-01 22:47 ` Steven Rostedt
2019-10-02 17:18 ` Alexei Starovoitov
2019-10-02 23:00 ` Steven Rostedt
2019-10-03 16:18 ` Alexei Starovoitov [this message]
2019-10-03 16:41 ` trace_printk issue. Was: " Steven Rostedt
2019-10-04 19:56 ` Alexei Starovoitov
2019-10-03 6:12 ` Masami Hiramatsu
2019-10-03 16:20 ` Alexei Starovoitov
2019-08-28 7:14 ` Peter Zijlstra
2019-08-28 22:08 ` Alexei Starovoitov
2019-08-29 13:34 ` Steven Rostedt
2019-08-29 15:43 ` Andy Lutomirski
2019-08-29 17:23 ` Alexei Starovoitov
2019-08-29 17:36 ` Andy Lutomirski
2019-08-29 17:49 ` Steven Rostedt
2019-08-29 17:19 ` Alexei Starovoitov
2019-08-29 17:47 ` Steven Rostedt
2019-08-28 10:38 ` kbuild test robot
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