On 6/9/23 8:52 AM, Daniel Borkmann wrote:
On 6/9/23 2:29 AM, Toke Høiland-Jørgensen wrote:
Stanislav Fomichev <sdf@xxxxxxxxxx> writes:
On 06/08, Andrii Nakryiko wrote:
On Thu, Jun 8, 2023 at 2:52 PM Stanislav Fomichev <sdf@xxxxxxxxxx> wrote:
On 06/08, Andrii Nakryiko wrote:
On Thu, Jun 8, 2023 at 10:24 AM Stanislav Fomichev <sdf@xxxxxxxxxx> wrote:
On 06/07, Daniel Borkmann wrote:
This adds a generic layer called bpf_mprog which can be reused by different
attachment layers to enable multi-program attachment and dependency resolution.
In-kernel users of the bpf_mprog don't need to care about the dependency
resolution internals, they can just consume it with few API calls.
The initial idea of having a generic API sparked out of discussion [0] from an
earlier revision of this work where tc's priority was reused and exposed via
BPF uapi as a way to coordinate dependencies among tc BPF programs, similar
as-is for classic tc BPF. The feedback was that priority provides a bad user
experience and is hard to use [1], e.g.:
I cannot help but feel that priority logic copy-paste from old tc, netfilter
and friends is done because "that's how things were done in the past". [...]
Priority gets exposed everywhere in uapi all the way to bpftool when it's
right there for users to understand. And that's the main problem with it.
The user don't want to and don't need to be aware of it, but uapi forces them
to pick the priority. [...] Your cover letter [0] example proves that in
real life different service pick the same priority. They simply don't know
any better. Priority is an unnecessary magic that apps _have_ to pick, so
they just copy-paste and everyone ends up using the same.
The course of the discussion showed more and more the need for a generic,
reusable API where the "same look and feel" can be applied for various other
program types beyond just tc BPF, for example XDP today does not have multi-
program support in kernel, but also there was interest around this API for
improving management of cgroup program types. Such common multi-program
management concept is useful for BPF management daemons or user space BPF
applications coordinating about their attachments.
Both from Cilium and Meta side [2], we've collected the following requirements
for a generic attach/detach/query API for multi-progs which has been implemented
as part of this work:
- Support prog-based attach/detach and link API
- Dependency directives (can also be combined):
- BPF_F_{BEFORE,AFTER} with relative_{fd,id} which can be {prog,link,none}
- BPF_F_ID flag as {fd,id} toggle
- BPF_F_LINK flag as {prog,link} toggle
- If relative_{fd,id} is none, then BPF_F_BEFORE will just prepend, and
BPF_F_AFTER will just append for the case of attaching
- Enforced only at attach time
- BPF_F_{FIRST,LAST}
- Enforced throughout the bpf_mprog state's lifetime
- Admin override possible (e.g. link detach, prog-based BPF_F_REPLACE)
- Internal revision counter and optionally being able to pass expected_revision
- User space daemon can query current state with revision, and pass it along
for attachment to assert current state before doing updates
- Query also gets extension for link_ids array and link_attach_flags:
- prog_ids are always filled with program IDs
- link_ids are filled with link IDs when link was used, otherwise 0
- {prog,link}_attach_flags for holding {prog,link}-specific flags
- Must be easy to integrate/reuse for in-kernel users
The uapi-side changes needed for supporting bpf_mprog are rather minimal,
consisting of the additions of the attachment flags, revision counter, and
expanding existing union with relative_{fd,id} member.
The bpf_mprog framework consists of an bpf_mprog_entry object which holds
an array of bpf_mprog_fp (fast-path structure) and bpf_mprog_cp (control-path
structure). Both have been separated, so that fast-path gets efficient packing
of bpf_prog pointers for maximum cache efficieny. Also, array has been chosen
instead of linked list or other structures to remove unnecessary indirections
for a fast point-to-entry in tc for BPF. The bpf_mprog_entry comes as a pair
via bpf_mprog_bundle so that in case of updates the peer bpf_mprog_entry
is populated and then just swapped which avoids additional allocations that
could otherwise fail, for example, in detach case. bpf_mprog_{fp,cp} arrays are
currently static, but they could be converted to dynamic allocation if necessary
at a point in future. Locking is deferred to the in-kernel user of bpf_mprog,
for example, in case of tcx which uses this API in the next patch, it piggy-
backs on rtnl. The nitty-gritty details are in the bpf_mprog_{replace,head_tail,
add,del} implementation and an extensive test suite for checking all aspects
of this API for prog-based attach/detach and link API as BPF selftests in
this series.
Kudos also to Andrii Nakryiko for API discussions wrt Meta's BPF management daemon.
[0] https://lore.kernel.org/bpf/20221004231143.19190-1-daniel@xxxxxxxxxxxxx/
[1] https://lore.kernel.org/bpf/CAADnVQ+gEY3FjCR=+DmjDR4gp5bOYZUFJQXj4agKFHT9CQPZBw@xxxxxxxxxxxxxx
[2] http://vger.kernel.org/bpfconf2023_material/tcx_meta_netdev_borkmann.pdf
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
---
MAINTAINERS | 1 +
include/linux/bpf_mprog.h | 245 +++++++++++++++++
include/uapi/linux/bpf.h | 37 ++-
kernel/bpf/Makefile | 2 +-
kernel/bpf/mprog.c | 476 +++++++++++++++++++++++++++++++++
tools/include/uapi/linux/bpf.h | 37 ++-
6 files changed, 781 insertions(+), 17 deletions(-)
create mode 100644 include/linux/bpf_mprog.h
create mode 100644 kernel/bpf/mprog.c
[...]
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index a7b5e91dd768..207f8a37b327 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -1102,7 +1102,14 @@ enum bpf_link_type {
*/
#define BPF_F_ALLOW_OVERRIDE (1U << 0)
#define BPF_F_ALLOW_MULTI (1U << 1)
+/* Generic attachment flags. */
#define BPF_F_REPLACE (1U << 2)
+#define BPF_F_BEFORE (1U << 3)
+#define BPF_F_AFTER (1U << 4)
[..]
+#define BPF_F_FIRST (1U << 5)
+#define BPF_F_LAST (1U << 6)
I'm still not sure whether the hard semantics of first/last is really
useful. My worry is that some prog will just use BPF_F_FIRST which
would prevent the rest of the users.. (starting with only
F_BEFORE/F_AFTER feels 'safer'; we can iterate later on if we really
need first/laste).
Without FIRST/LAST some scenarios cannot be guaranteed to be safely
implemented. E.g., if I have some hard audit requirements and I need
to guarantee that my program runs first and observes each event, I'll
enforce BPF_F_FIRST when attaching it. And if that attachment fails,
then server setup is broken and my application cannot function.
In a setup where we expect multiple applications to co-exist, it
should be a rule that no one is using FIRST/LAST (unless it's
absolutely required). And if someone doesn't comply, then that's a bug
and has to be reported to application owners.
But it's not up to the kernel to enforce this cooperation by
disallowing FIRST/LAST semantics, because that semantics is critical
for some applications, IMO.
Maybe that's something that should be done by some other mechanism?
(and as a follow up, if needed) Something akin to what Toke
mentioned with another program doing sorting or similar.
The goal of this API is to avoid needing some extra special program to
do this sorting
Otherwise, those first/last are just plain simple old priority bands;
only we have two now, not u16.
I think it's different. FIRST/LAST has to be used judiciously, of
course, but when they are needed, they will have no alternative.
Also, specifying FIRST + LAST is the way to say "I want my program to
be the only one attached". Should we encourage such use cases? No, of
course. But I think it's fair for users to be able to express this.
I'm mostly coming from the observability point: imagine I have my fancy
tc_ingress_tcpdump program that I want to attach as a first program to debug
some issue, but it won't work because there is already a 'first' program
installed.. Or the assumption that I'd do F_REPLACE | F_FIRST ?
If your production setup requires that some important program has to
be FIRST, then yeah, your "let me debug something" program shouldn't
interfere with it (assuming that FIRST requirement is a real
requirement and not someone just thinking they need to be first; but
that's up to user space to decide). Maybe the solution for you in that
case would be freplace program installed on top of that stubborn FIRST
program? And if we are talking about local debugging and development,
then you are a sysadmin and you should be able to force-detach that
program that is getting in the way.
I'm not really concerned about our production environment. It's pretty
controlled and restricted and I'm pretty certain we can avoid doing
something stupid. Probably the same for your env.
I'm mostly fantasizing about upstream world where different users don't
know about each other and start doing stupid things like F_FIRST where
they don't really have to be first. It's that "used judiciously" part
that I'm a bit skeptical about :-D
But in the end how is that different from just attaching themselves blindly
into the first position (e.g. with before and relative_fd as 0 or the fd/id
of the current first program) - same, they don't really have to be first.
How would that not result in doing something stupid? ;) To add to Andrii's
earlier DDoS mitigation example ... think of K8s environment: one project
is implementing DDoS mitigation with BPF, another one wants to monitor/
sample traffic to user space with BPF. Both install as first position by
default (before + 0). In K8s, there is no built-in Pod dependency management
so you cannot guarantee whether Pod A comes up before Pod B. So you'll end
up in a situation where sometimes the monitor runs before the DDoS mitigation
and on some other nodes it's vice versa. The other case where this gets
broken (assuming a node where we get first the DDoS mitigation, then the
monitoring) is when you need to upgrade one of the Pods: monitoring Pod
gets a new stable update and is being re-rolled out, then it inserts
itself before the DDoS mitigation mechanism, potentially causing outage.
With the first/last mechanism these two situations cannot happen. The DDoS
mitigation software uses first and the monitoring uses before + 0, then no
matter the re-rollouts or the ordering in which Pods come up, it's always
at the expected/correct location.
Because even with this new ordering scheme, there still should be
some entity to do relative ordering (systemd-style, maybe CNI?).
Just to add, in K8s there can be multiple CNIs chained together, and there
is also no common management daemon as you have in G or Meta. So yes, K8s is
special snowflake, but everyone outside of the big hyperscalers are relying
on it as a platform, so we do need to have a solution for the trivial, above-
mentioned scenario if we drop the first/last.
And if it does the ordering, I don't really see why we need
F_FIRST/F_LAST.
I can see I'm a bit late to the party, but FWIW I agree with this:
FIRST/LAST will definitely be abused if we add it. It also seems to me
See above on the issues w/o the first/last. How would you work around them
in practice so they cannot happen?
to be policy in the kernel, which would be much better handled in
userspace like we do for so many other things. So we should rather
expose a hook to allow userspace to set the policy, as we've discussed
before; I definitely think we should add that at some point! Although
obviously it doesn't have to be part of this series...
Imo, it would be better if we could avoid that.. it feels like we're
trying to shoot sparrows with cannon, e.g. when this API gets reused
for other attach hooks, then for each of them you need yet another
policy program. I don't think that's a good user experience, and I
presume this is then single-user program, thus you'll run into the same
race in the end - whichever management daemon or application gets to
install this policy program first wins. This is potentially just
shifting the same issue one level higher, imo.
Thanks,
Daniel
