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From: Steven Rostedt <rostedt@goodmis.org>
To: Alexei Starovoitov <ast@fb.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.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: Re: [PATCH bpf-next] bpf, capabilities: introduce CAP_BPF
Date: Wed, 2 Oct 2019 19:00:27 -0400
Message-ID: <20191002190027.4e204ea8@gandalf.local.home> (raw)
In-Reply-To: <a98725c6-a7db-1d9f-7033-5ecd96438c8d@fb.com>

On Wed, 2 Oct 2019 17:18:21 +0000
Alexei Starovoitov <ast@fb.com> wrote:

> >> It's an interesting idea, but I don't think it can work.
> >> Please see bpf_trace_printk implementation in kernel/trace/bpf_trace.c
> >> It's a lot more than string printing.  
> > 
> > Well, trace_printk() is just string printing. I was thinking that the
> > bpf_trace_printk() could just use a vsnprintf() into a temporary buffer
> > (like trace_printk() does), and then call the trace event to write it
> > out.  
> 
> are you proposing to replicate get_trace_buf() functionality
> into bpf_trace_printk?

No, do you need bpf_trace_printk() to run in all contexts?
trace_printk() does the get_trace_buf() dance so that it can be called
without locks and from any context including NMIs.

> So print into temp string buffer is done twice?
> I'm not excited about such hack.
> And what's the goal? so that trace_bpf_print(string_msg);
> can go through _run-time_ check whether that particular trace event
> was allowed in tracefs ?

No, just to use a standard event instead of hacking into
trace_printk().

> That's not how file system acls are typically designed.
> The permission check is at open(). Not at write().
> If I understood you correctly you're proposing to check permissions
> at bpf program run-time which is no good.
> 
> bpf_trace_printk() already has one small buffer for
> probe_kernel_read-ing an unknown string to pass into %s.
> That's not ftrace. That's core tracing. That aspect is covered by 
> CAP_TRACING as well.

Then use that buffer.

> 
> 
> >>  
> >>> The user could then just enable the trace event from the file system. I
> >>> could also work on making instances work like /tmp does (with the
> >>> sticky bit) in creation. That way people with write access to the
> >>> instances directory, can make their own buffers that they can use (and
> >>> others can't access).  
> >>
> >> We tried instances in bcc in the past and eventually removed all the
> >> support. The overhead of instances is too high to be usable.  
> > 
> > What overhead? An ftrace instance should not have any more overhead than
> > the root one does (it's the same code). Or are you talking about memory
> > overhead?  
> 
> Yes. Memory overhead. Human users doing cat/echo into tracefs won't be
> creating many instances, so that's the only practical usage of them.

If it's a real event, it can go into any of the ftrace buffers (top
level or instance), but it gives you the choice.

> 
> >   
> >>  
> >>>
> >>>      
> >>>>
> >>>> 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).

-- Steve

  reply index

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20190827205213.456318-1-ast@kernel.org>
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 [this message]
2019-10-03 16:18                                 ` trace_printk issue. Was: " Alexei Starovoitov
2019-10-03 16:41                                   ` 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

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