Re: [RFC PATCH bpf-next 05/16] bpf: create file or anonymous dumpers

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Wed, Apr 15, 2020 at 06:48:13PM -0700, Andrii Nakryiko wrote:
> On Wed, Apr 15, 2020 at 9:46 AM Alexei Starovoitov
> <alexei.starovoitov@gmail.com> wrote:
> >
> > On Tue, Apr 14, 2020 at 09:45:08PM -0700, Andrii Nakryiko wrote:
> > > >
> > > > > FD is closed, dumper program is detached and dumper is destroyed
> > > > > (unless pinned in bpffs, just like with any other bpf_link.
> > > > > 3. At this point bpf_dumper_link can be treated like a factory of
> > > > > seq_files. We can add a new BPF_DUMPER_OPEN_FILE (all names are for
> > > > > illustration purposes) command, that accepts dumper link FD and
> > > > > returns a new seq_file FD, which can be read() normally (or, e.g.,
> > > > > cat'ed from shell).
> > > >
> > > > In this case, link_query may not be accurate if a bpf_dumper_link
> > > > is created but no corresponding bpf_dumper_open_file. What we really
> > > > need to iterate through all dumper seq_file FDs.
> > >
> > > If the goal is to iterate all the open seq_files (i.e., bpfdump active
> > > sessions), then bpf_link is clearly not the right approach. But I
> > > thought we are talking about iterating all the bpfdump programs
> > > attachments, not **sessions**, in which case bpf_link is exactly the
> > > right approach.
> >
> > That's an important point. What is the pinned /sys/kernel/bpfdump/tasks/foo ?
> 
> Assuming it's not a rhetorical question, foo is a pinned bpf_dumper
> link (in my interpretation of all this).

It wasn't rhetorical question and your answer is differrent from mine :)
It's not a link. It's a template of seq_file. It's the same as
$ stat /proc/net/ipv6_route
  File: ‘/proc/net/ipv6_route’
  Size: 0         	Blocks: 0          IO Block: 1024   regular empty file

> > Every time 'cat' opens it a new seq_file is created with new FD, right ?
> 
> yes
> 
> > Reading of that file can take infinite amount of time, since 'cat' can be
> > paused in the middle.
> 
> yep, correct (though most use case probably going to be very short-lived)
> 
> > I think we're dealing with several different kinds of objects here.
> > 1. "template" of seq_file that is seen with 'ls' in /sys/kernel/bpfdump/
> 
> Let's clarify here again, because this can be interpreted differently.
> 
> Are you talking about, e.g., /sys/fs/bpfdump/task directory that
> defines what class of items should be iterated? Or you are talking
> about named dumper: /sys/fs/bpfdump/task/my_dumper?

the latter.

> 
> If the former, I agree that it's not a link. If the latter, then
> that's what we've been so far calling "a named bpfdumper". Which is
> what I argue is a link, pinned in bpfdumpfs (*not bpffs*).

It cannot be a link, since link is only a connection between
kernel object and bpf prog.
Whereas seq_file is such kernel object.

> 
> For named dumper:
> 1. load bpfdump prog
> 2. attach prog to bpfdump "provider" (/sys/fs/bpfdump/task), get
> bpf_link anon FD back
> 3. pin link in bpfdumpfs (e.g., /sys/fs/bpfdump/task/my_dumper)
> 4. each open() of /sys/fs/bpfdump/task/my_dumper produces new
> bpfdumper session/seq_file
> 
> For anon dumper:
> 1. load bpfdump prog
> 2. attach prog to bpfdump "provider" (/sys/fs/bpfdump/task), get
> bpf_link anon FD back
> 3. give bpf_link FD to some new API (say, BPF_DUMP_NEW_SESSION or
> whatever name) to create seq_file/bpfdumper session, which will create
> FD that can be read(). One can do that many times, each time getting
> its own bpfdumper session.

I slept on it and still fundamentally disagree that seq_file + bpf_prog
is a derivative of link. Or in OoO terms it's not a child class of bpf_link.
seq_file is its own class that should contain bpf_link as one of its
members, but it shouldn't be derived from 'class bpf_link'.

In that sense Yonghong proposed api (raw_tp_open to create anon seq_file+prog
and obj_pin to create a template of named seq_file+prog) are the best fit.
Implementation wise his 'struct extra_priv_data' needs to include
'struct bpf_link' instead of 'struct bpf_prog *prog;' directly.

So evertime 'cat' opens named seq_file there is bpf_link registered in IDR.
Anon seq_file should have another bpf_link as well.

My earlier suggestion to disallow get_fd_from_id for such links is wrong.
It's fine to get an FD to such link, but it shouldn't prevent destruction
of seq_file. 'cat' will close named seq_file and 'struct extra_priv_data' class
should do link_put. If some other process did get_fd_from_id then such link will
become dangling. Just like removal of netdev will make dangling xdp links.