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 [thread overview]
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
next prev parent reply other threads:[~2019-10-02 23:00 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 [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
2019-08-28 10:38 ` kbuild test robot
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20191002190027.4e204ea8@gandalf.local.home \
--to=rostedt@goodmis.org \
--cc=Kernel-team@fb.com \
--cc=alexei.starovoitov@gmail.com \
--cc=ast@fb.com \
--cc=ast@kernel.org \
--cc=bpf@vger.kernel.org \
--cc=daniel@iogearbox.net \
--cc=davem@davemloft.net \
--cc=jannh@google.com \
--cc=jmorris@namei.org \
--cc=keescook@chromium.org \
--cc=linux-api@vger.kernel.org \
--cc=linux-security-module@vger.kernel.org \
--cc=luto@amacapital.net \
--cc=luto@kernel.org \
--cc=mhiramat@kernel.org \
--cc=netdev@vger.kernel.org \
--cc=peterz@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).