linux-kernel - Re: [PATCH RFC 0/4] Add support to directly attach BPF program to ftrace

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Message-ID: <20190717012406.lugqemvubixfdd6v@ast-mbp.dhcp.thefacebook.com>
Date:   Tue, 16 Jul 2019 18:24:07 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Adrian Ratiu <adrian.ratiu@...labora.com>,
        Alexei Starovoitov <ast@...nel.org>, bpf@...r.kernel.org,
        Brendan Gregg <brendan.d.gregg@...il.com>, connoro@...gle.com,
        Daniel Borkmann <daniel@...earbox.net>,
        duyuchao <yuchao.du@...soc.com>, Ingo Molnar <mingo@...hat.com>,
        jeffv@...gle.com, Karim Yaghmour <karim.yaghmour@...rsys.com>,
        kernel-team@...roid.com, linux-kselftest@...r.kernel.org,
        Manali Shukla <manalishukla14@...il.com>,
        Manjo Raja Rao <linux@...ojrajarao.com>,
        Martin KaFai Lau <kafai@...com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Matt Mullins <mmullins@...com>,
        Michal Gregorczyk <michalgr@...com>,
        Michal Gregorczyk <michalgr@...e.com>,
        Mohammad Husain <russoue@...il.com>, namhyung@...gle.com,
        namhyung@...nel.org, netdev@...r.kernel.org,
        paul.chaignon@...il.com, primiano@...gle.com,
        Qais Yousef <qais.yousef@....com>,
        Shuah Khan <shuah@...nel.org>,
        Song Liu <songliubraving@...com>,
        Srinivas Ramana <sramana@...eaurora.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tamir Carmeli <carmeli.tamir@...il.com>,
        Yonghong Song <yhs@...com>
Subject: Re: [PATCH RFC 0/4] Add support to directly attach BPF program to
 ftrace

On Tue, Jul 16, 2019 at 07:55:00PM -0400, Joel Fernandes wrote:
> On Tue, Jul 16, 2019 at 06:41:50PM -0400, Joel Fernandes wrote:
> > On Tue, Jul 16, 2019 at 03:26:52PM -0700, Alexei Starovoitov wrote:
> > > On Tue, Jul 16, 2019 at 05:30:50PM -0400, Joel Fernandes wrote:
> > > > 
> > > > I also thought about the pinning idea before, but we also want to add support
> > > > for not just raw tracepoints, but also regular tracepoints (events if you
> > > > will). I am hesitant to add a new BPF API just for creating regular
> > > > tracepoints and then pinning those as well.
> > > 
> > > and they should be done through the pinning as well.
> > 
> > Hmm ok, I will give it some more thought.
> 
> I think I can make the new BPF API + pinning approach work, I will try to
> work on something like this and post it soon.
> 
> Also, I had a question below if you don't mind taking a look:
> 
> thanks Alexei!
> 
> > > > I don't see why a new bpf node for a trace event is a bad idea, really.
> > > 
> > > See the patches for kprobe/uprobe FD-based api and the reasons behind it.
> > > tldr: text is racy, doesn't scale, poor security, etc.
> > 
> > Is it possible to use perf without CAP_SYS_ADMIN and control security at the
> > per-event level? We are selective about who can access which event, using
> > selinux. That's how our ftrace-based tracers work. Its fine grained per-event
> > control. That's where I was going with the tracefs approach since we get that
> > granularity using the file system.

android's choice of selinux is not a factor in deciding kernel apis.
It's completely separate discusion wether disallowing particular tracepoints
for given user make sense at all.
Just because you can hack it in via selinux blocking particular
/sys/debug/tracing/ directory and convince yourself that it's somehow
makes android more secure. It doesn't mean that all new api should fit
into this model.
I think allowing one tracepoint and disallowing another is pointless
from security point of view. Tracing bpf program can do bpf_probe_read
of anything.