Re: [PATCH bpf-next v2 2/7] bpf: Add fd-based tcx multi-prog infra with link support

From: Jamal Hadi Salim <jhs@mojatatu.com>
To: Daniel Borkmann <daniel@iogearbox.net>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>,
	ast@kernel.org, andrii@kernel.org,  martin.lau@linux.dev,
	razor@blackwall.org, sdf@google.com,  john.fastabend@gmail.com,
	kuba@kernel.org, dxu@dxuuu.xyz, joe@cilium.io,  toke@kernel.org,
	davem@davemloft.net, bpf@vger.kernel.org,
	 netdev@vger.kernel.org
Subject: Re: [PATCH bpf-next v2 2/7] bpf: Add fd-based tcx multi-prog infra with link support
Date: Tue, 4 Jul 2023 18:38:21 -0400	[thread overview]
Message-ID: <CAM0EoMndiP6c20Q9g+dSFMh+XPJCdCAUzjHPXm6-4mmNJtAH3A@mail.gmail.com> (raw)
In-Reply-To: <15ab0ba7-abf7-b9c3-eb5e-7a6b9fd79977@iogearbox.net>

On Tue, Jul 4, 2023 at 6:01 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> On 7/4/23 11:36 PM, Jamal Hadi Salim wrote:
> > On Thu, Jun 8, 2023 at 5:25 PM Andrii Nakryiko
> > <andrii.nakryiko@gmail.com> wrote:
> >> On Thu, Jun 8, 2023 at 12:46 PM Jamal Hadi Salim <jhs@mojatatu.com> wrote:
> >>> On Thu, Jun 8, 2023 at 6:12 AM Daniel Borkmann <daniel@iogearbox.net> wrote:
> >>>> On 6/8/23 3:25 AM, Jamal Hadi Salim wrote:
> [...]
> >>>> BPF links are supported for XDP today, just tc BPF is one of the few
> >>>> remainders where it is not the case, hence the work of this series. What
> >>>> XDP lacks today however is multi-prog support. With the bpf_mprog concept
> >>>> that could be addressed with that common/uniform api (and Andrii expressed
> >>>> interest in integrating this also for cgroup progs), so yes, various hook
> >>>> points/program types could benefit from it.
> >>>
> >>> Is there some sample XDP related i could look at?  Let me describe our
> >>> use case: lets say we load an ebpf program foo attached to XDP of a
> >>> netdev  and then something further upstream in the stack is consuming
> >>> the results of that ebpf XDP program. For some reason someone, at some
> >>> point, decides to replace the XDP prog with a different one - and the
> >>> new prog does a very different thing. Could we stop the replacement
> >>> with the link mechanism you describe? i.e the program is still loaded
> >>> but is no longer attached to the netdev.
> >>
> >> If you initially attached an XDP program using BPF link api
> >> (LINK_CREATE command in bpf() syscall), then subsequent attachment to
> >> the same interface (of a new link or program with BPF_PROG_ATTACH)
> >> will fail until the current BPF link is detached through closing its
> >> last fd.
> >
> > So this works as advertised. The problem is however not totally solved
> > because it seems we need a process that's alive to hold the ownership.
> > If we had a daemon then that would solve it i think (we dont).
> > Alternatively,  you pin the link. The pinning part can be
> > circumvented, unless i misunderstood i,e anybody with the right
> > permissions can remove it.
> >
> > Am I missing something?
>
> It would be either of those depending on the use case, and for pinning
> removal, it would require right permissions/acls. Keep in mind that for
> your application you can also use your own bpffs mount, so you don't
> need to use the default /sys/fs/bpf one in hostns.

This helps for sure - doesnt 100% solve it. It would really be nice if
we could tie in a kerberos-like ticketing system for ownership of the
mount or something even more fine grained like a link. Doesnt have to
be kerberos but anything that would allow a digest of some verifiable
credentials/token to be handed to the kernel for authorization...

cheers,
jamal

> Thanks,
> Daniel