Re: [PATCH net-next 3/4] bpf: add support for persistent maps/progs

[Hannes Frederic Sowa]

Hello Alexei,

On Mon, Oct 19, 2015, at 22:48, Alexei Starovoitov wrote:
> On 10/19/15 1:03 PM, Hannes Frederic Sowa wrote:
> >
> > I doubt it will stay a lightweight feature as it should not be in the
> > responsibility of user space to provide those debug facilities.
> 
> It feels we're talking past each other.
> I want to solve 'persistent map' problem.
> debugging of maps/progs, hierarchy, etc are all nice to have,
> but different issues.

I understand that. Main problem is to persist fds with attached maps,
sure.

I am not a big fan of the kernel referencing or persisting resources on
behalf of its own. IMHO they should be attached to user space programs.
So I am still in favor for user space, but I see that most people are
not comfortable with that. So a way to make this kind of persistence in
the kernel as introspectable as possible is the main goal of the idea we
are sharing here.

I bet commercial software will make use of this ebpf framework, too. And
the kernel always helped me and gave me a way to see what is going on,
debug which part of my operating system universe interacts with which
other part. Merely dropping file descriptors with data attached to them
in an filesystem seems not to fulfill my need at all. I would love to
see where resources are referenced and why, like I am nowadays.

Btw.: has anybody had a look at kdbus if it allows user space to much
more easily handle those file descriptors (as an alternative to
af_unix). I haven't, yet.

> In case of persistent maps I imagine unprivileged process would want
> to use it eventually as well, so this requirement already kills cdev
> approach for me, since I don't think we ever let unprivileged apps
> create cdev with syscall.

TTY code is creating nodes on behalf of users, I check if this could
work for bpf cdevs as well.

> > The bpf syscall is still used to create the pseudo nodes. If they should
> > be persistent they just get registered in the sysfs class hierarchy.
> 
> nope. they should not. sysfs is debugging/tunning facility.
> There is absolutely no need for bpf to plug into sysfs.
> 
> >> Doing 'resource stats' via sysfs requires bpf to add to sysfs, which
> >> is not this cdev approach.
> >
> > This is not yet part of the patch, but I think this would be added.
> > Daniel?
> 
> please don't. I'm strongly against adding unnecessary bloat.
> 
> > I don't think there are broad differences. But in case a namespaces uses
> > huge number of maps with tons of data, the admin in the initial
> > namespace might want to debug that without searching all mountpoints and
> > find dependencies between processes etc. IMHO sysfs approach can be
> > better extended here.
> 
> sure, then we can force all bpffs to have the same hierarchy and mounted
> in /sys/kernel/bpf location. That would be the same.
> 
> It feels you're pushing for cdev only because of that potential
> debugging need. Did you actually face that need? I didn't and
> don't like to add 'nice to have' feature until real need comes.

Given that we want to monitor the load of a hashmap for graphing
purposes. Or liberate some hashmaps from its restriction on number of
keys and make upper bounds configurable by admins who know the
dimensions of their systems and not some software deep down buried in
the bpf syscall where I might not have access to source code. In tc
force e.g. hashmaps to do garbage collection because we cannot be sure
that under DoS attacks user space clean up gets scheduled early enough
if ebpf adds flows to hashtables. I do see need to expand and implement
some kind of policy in the future.

> >> Also I don't buy the point of reinventing sysfs. bpffs is not doing
> >> sysfs. I don't want to see _every_ bpf object in sysfs. It's way too
> >> much overhead. Classic doesn't have sysfs and everyone have been
> >> using it just fine.
> >
> > But classic bpf does not have persistence for maps and data. ;) There is
> > a 1:1 relationship between socket and bpf_prog for example.
> 
> single task in seccomp can have a chain of bpf progs, so hierarchy
> is already there.

And it would be great to inspect them.

> > But how can the filesystem be extended in terms of tunables and
> > information? File attributes? Wouldn't it need the same infrastructure
> > otherwise as sysfs? Some third-party lookup filesystem or ioctl? This
> > char dev approach also pins maps and progs while giving more policy in
> > hand of central user space programs we are currently using (udev,
> > systemd, whatever, etc.).
> 
> tunables for bpf maps? There are no such things today.
> I think you're implying that we can add rhashtable type of map, so
> admin can tune thresholds ? Ouch. I think if we add it, its parameters
> will be specified by the user that is creating the map only. There will
> be no tunables exposed to sysfs and there should be no way of creating
> maps via sysfs.

I am fine with creating maps only by bpf syscall. But to hide
configuration details or at least not be really able to query them
easily seems odd to me. If we go with the ebpffs how could those
attributes be added?

Thanks,
Hannes
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