On Mon, Sep 27, 2021 at 11:20 AM John Fastabend
[off-list ref] wrote:

Dave Marchevsky wrote:

quoted

On 9/23/21 10:02 PM, Andrii Nakryiko wrote:

quoted

On Thu, Sep 23, 2021 at 6:27 PM Dave Marchevsky [off-list ref] wrote:

quoted

On 9/23/21 4:51 PM, Alexei Starovoitov wrote:

quoted

On Mon, Sep 20, 2021 at 08:11:10AM -0700, Dave Marchevsky wrote:

quoted

The verifier currently logs some useful statistics in
print_verification_stats. Although the text log is an effective feedback
tool for an engineer iterating on a single application, it would also be
useful to enable tracking these stats in a more structured form for
fleetwide or historical analysis, which this patchset attempts to do.

A concrete motivating usecase which came up in recent weeks:

A team owns a complex BPF program, with various folks extending its
functionality over the years. An engineer tries to make a relatively
simple addition but encounters "BPF program is too large. Processed
1000001 insn".

Their changes bumped the processed insns from 700k to over the limit and
there's no obvious way to simplify. They must now consider a large
refactor in order to incorporate the new feature. What if there was some
previous change which bumped processed insns from 200k->700k which
_could_ be modified to stress verifier less? Tracking historical
verifier stats for each version of the program over the years would
reduce manual work necessary to find such a change.


Although parsing the text log could work for this scenario, a solution
that's resilient to log format and other verifier changes would be
preferable.

This patchset adds a bpf_prog_verif_stats struct - containing the same
data logged by print_verification_stats - which can be retrieved as part
of bpf_prog_info. Looking for general feedback on approach and a few
specific areas before fleshing it out further:

* None of my usecases require storing verif_stats for the lifetime of a
  loaded prog, but adding to bpf_prog_aux felt more correct than trying
  to pass verif_stats back as part of BPF_PROG_LOAD
* The verif_stats are probably not generally useful enough to warrant
  inclusion in fdinfo, but hoping to get confirmation before removing
  that change in patch 1
* processed_insn, verification_time, and total_states are immediately
  useful for me, rest were added for parity with
     print_verification_stats. Can remove.
* Perhaps a version field would be useful in verif_stats in case future
  verifier changes make some current stats meaningless
* Note: stack_depth stat was intentionally skipped to keep patch 1
  simple. Will add if approach looks good.

Sorry for the delay. LPC consumes a lot of mental energy :)

I see the value of exposing some of the verification stats as prog_info.
Let's look at the list:
struct bpf_prog_verif_stats {
       __u64 verification_time;
       __u32 insn_processed;
       __u32 max_states_per_insn;
       __u32 total_states;
       __u32 peak_states;
       __u32 longest_mark_read_walk;
};
verification_time is non deterministic. It varies with frequency
and run-to-run. I don't see how alerting tools can use it.

Makes sense to me, will get rid of it.

quoted

insn_processed is indeed the main verification metric.
By now it's well known and understood.

max_states_per_insn, total_states, etc were the metrics I've studied
carefully with pruning, back tracking and pretty much every significant
change I did or reiviewed in the verifier. They're useful to humans
and developers, but I don't see how alerting tools will use them.

So it feels to me that insn_processed alone will be enough to address the
monitoring goal.

For the concrete usecase in my original message insn_processed would be
enough. For the others - I thought there might be value in gathering
those "fleetwide" to inform verifier development, e.g.:

"Hmm, this team's libbpf program has been regressing total_states over
past few {kernel, llvm} rollouts, but they haven't been modifying it.
Let's try to get a minimal repro, send to bpf@vger, and contribute to
selftests if it is indeed hitting a weird verifier edge case"

So for those I'm not expecting them to be useful to alert on or be a
number that the average BPF program writer needs to care about.

Of course this is hypothetical as I haven't tried to gather such data
and look for interesting patterns. But these metrics being useful to
you when looking at significant verifier changes is a good sign.

One reason to not add all those fields is to not end up with
meaningless stats (in the future) in UAPI. One way to work around that
is to make it "unstable" by providing it through raw_tracepoint as
internal kernel struct.

Basically, the proposal would be: add new tracepoint for when BPF
program is verified, either successfully or not. As one of the
parameters provide stats struct which is internal to BPF verifier and
is not exposed through UAPI.

Such tracepoint actually would be useful more generally as well, e.g.,
to monitor which programs are verified in the fleet, what's the rate
of success/failure (to detect verifier regression), what are the stats
(verification time actually would be good to have there, again for
stats and detecting regression), etc, etc.

WDYT?

Seems reasonable to me - and attaching a BPF program to the tracepoint to
grab data is delightfully meta :)

I'll do a pass on alternate implementation with _just_ tracepoint, no
prog_info or fdinfo, can add minimal or full stats to those later if
necessary.

We can also use a hook point here to enforce policy on allowing the
BPF program to load or not using the stats here. For now basic
insn is a good start to allow larger/smaller programs to be loaded,
but we might add other info like call bitmask, features, types, etc.
If one of the arguments is the bpf_attr struct we can just read
lots of useful program info out directly.

We would need something different from a tracepoint though to let
it return a reject|accept code. How about a new hook type that
has something similar to sockops that lets us just return an
accept or reject code?

By doing this we can check loader signatures here to be sure the
loader is signed or otherwise has correct permissions to be loading
whatever type of bpf program is here.

For signing and generally preventing some BPF programs from loading
(e.g., if there is some malicious BPF program that takes tons of
memory to be validated), wouldn't you want to check that before BPF
verifier spent all those resources on verification? So maybe there
will be another hook before BPF prog is validated for that? Basically,
if you don't trust any BPF program unless it is signed, I'd expect you
check signature before BPF verifier does its heavy job.

Thanks,
John