netdev - Re: [PATCH v2 linux-trace 1/8] tracing: attach eBPF programs to tracepoints and syscalls

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Date:	Wed, 28 Jan 2015 23:04:52 -0800
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 linux-trace 1/8] tracing: attach eBPF programs to
 tracepoints and syscalls

On Wed, Jan 28, 2015 at 10:41 PM, Namhyung Kim <namhyung@...nel.org> wrote:
>
> I think it's not a problem of bpf.  An user process can be killed
> anytime while it enabed events without bpf.  The only thing it should
> care is the auto-unload IMHO.

ok. I think it does indeed make sense to decouple the logic.
We can add 'auto_enable' file to achieve desired Ctrl-C behavior.
While the 'auto_enable' file is open the event will be enabled
and writes to 'enable' file will be ignored.
As soon as file closes, the event is auto-disabled.
Then user space will use 'bpf' file to attach/auto-unload
and 'auto_enable' file together.
Seem there would be a use for such 'auto_enable'
without bpf as well.

> I'm okay for not calling bpf program in NMI but not for disabling events.
>
> Suppose an user was collecting an event (including in NMI) and then
> [s]he also wanted to run a bpf program.  So [s]he wrote a program
> always return 1.  But after attaching the program, it didn't record
> the event in NMI..  Isn't that a problem?

ok, I think 'if (in_nmi()) return 1;' will work then, right?
Or you're thinking something else ?

> Right.  I think bpf programs belong to a user process but events are
> global resource.  Maybe you also need to consider attaching bpf
> program via perf (ioctl?) interface..

yes. I did. Please see my reply to Masami.
ioctl only works for tracepoints.
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