Re: [RFC PATCH bpf-next 8/8] tools/bpftool: show PIDs with FDs open against BPF map/prog/link/btf

From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Andrii Nakryiko <andriin@fb.com>, bpf <bpf@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>,
	Alexei Starovoitov <ast@fb.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <kernel-team@fb.com>, Hao Luo <haoluo@google.com>,
	Song Liu <songliubraving@fb.com>,
	Quentin Monnet <quentin@isovalent.com>
Subject: Re: [RFC PATCH bpf-next 8/8] tools/bpftool: show PIDs with FDs open against BPF map/prog/link/btf
Date: Sat, 13 Jun 2020 19:14:19 -0300	[thread overview]
Message-ID: <20200613221419.GB7488@kernel.org> (raw)
In-Reply-To: <CAEf4BzaHVRxkiDbTGashiuakXFBRYvDsQmJ0O08xFijKXiAwSg@mail.gmail.com>

Em Fri, Jun 12, 2020 at 10:57:59PM -0700, Andrii Nakryiko escreveu:
> On Fri, Jun 12, 2020 at 8:45 PM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Fri, Jun 12, 2020 at 03:31:50PM -0700, Andrii Nakryiko wrote:
> > > Add bpf_iter-based way to find all the processes that hold open FDs against
> > > BPF object (map, prog, link, btf). Add new flag (-o, for "ownership", given
> > > -p is already taken) to trigger collection and output of these PIDs.
> > >
> > > Sample output for each of 4 BPF objects:
> > >
> > > $ sudo ./bpftool -o prog show
> > > 1992: cgroup_skb  name egress_alt  tag 9ad187367cf2b9e8  gpl
> > >         loaded_at 2020-06-12T14:18:10-0700  uid 0
> > >         xlated 48B  jited 59B  memlock 4096B  map_ids 2074
> > >         btf_id 460
> > >         pids: 913709,913732,913733,913734
> > > 2062: cgroup_device  tag 8c42dee26e8cd4c2  gpl
> > >         loaded_at 2020-06-12T14:37:52-0700  uid 0
> > >         xlated 648B  jited 409B  memlock 4096B
> > >         pids: 1
> > >
> > > $ sudo ./bpftool -o map show
> > > 2074: array  name test_cgr.bss  flags 0x400
> > >         key 4B  value 8B  max_entries 1  memlock 8192B
> > >         btf_id 460
> > >         pids: 913709,913732,913733,913734
> > >
> > > $ sudo ./bpftool -o link show
> > > 82: cgroup  prog 1992
> > >         cgroup_id 0  attach_type egress
> > >         pids: 913709,913732,913733,913734
> > > 86: cgroup  prog 1992
> > >         cgroup_id 0  attach_type egress
> > >         pids: 913709,913732,913733,913734
> >
> > This is awesome.

Indeed.

> Thanks.
> 
> >
> > Why extra flag though? I think it's so useful that everyone would want to see

Agreed.

> No good reason apart from "being safe by default". If turned on by
> default, bpftool would need to probe for bpf_iter support first. I can
> add probing and do this by default.

I think this is the way to go.

> > this by default. Also the word 'pid' has kernel meaning or user space meaning?
> > Looks like kernel then bpftool should say 'tid'.
> 
> No, its process ID in user-space sense. See task->tgid in
> pid_iter.bpf.c. I figured thread ID isn't all that useful.
> 
> > Could you capture comm as well and sort it by comm, like:
> >
> > $ sudo ./bpftool link show
> > 82: cgroup  prog 1992
> >         cgroup_id 0  attach_type egress
> >         systemd(1), firewall(913709 913732), logger(913733 913734)
> 
> Yep, comm is useful, I'll add that. Grouping by comm is kind of a
> pain, though, plus usually there will be one process only. So let me
> start with doing comm (pid) for each PID independently. I think that
> will be as good in practice.

-- 

- Arnaldo