Issue #22527 - bpf_lxc.c: do drop notifications at drop source

[bleggett]

Fixes: #22527

#22527 points out that there may be some corner cases where packet drops are not properly logged.

This is largely possible because a macro-defined func is reused in several places, and send_drop_notify is assumed to be deferred up the chain - but not all parents would actually invoke it, and by definition it seems clearer/less error prone to always explicitly issue drop notification at the source, where the drop is decided.

This is a smidge more verbose, but it avoids the problem of bad assumptions or hard-to-catch mistakes causing missing drop notifications.

Additionally - I might be missing something - but I couldn't see that the macro expansions of tail_ipv4_ct_ingress/tail_ipv6_ct_ingress actually ever get invoked? Am I missing something there? was missing something

(I couldn't find any formal C style guidelines declared for this repo, so if I'm violating some understood assumption or doing something very dumb, please comment)

Fix missing drop notifications on conntrack lookup failures when IPv4 and IPv6 are both enabled or socket-level load balancing is disabled.

[pchaigno]

Thanks for tackling this! 🚀

I've left a few inline comments below. I also have a few global comments:

• The context that you gave in the pull request description should be in the commit message.
• For the commit title, we usually follow the format of bpf: [Description]. I think a good title would be bpf: Fix missing drop notifications on ct lookup failures.
• I usually try to avoid referencing GitHub issues in git commits because it creates a lot of noise on the target issue. Not everyone does the same though, so I'm fine either way 🙂
• I've added a release note given this is a bug and will appear as such in the release notes.

[pchaigno]

Thx - I resynced with main just now as well.

Thanks, but I'd actually avoid doing that unless there are conflicts or it fixes some of the CI flakes. Every time we rebase the PR, we'll have to rerun tests from scratch and one of them takes up to 3h 😬 😅

[pchaigno]

/test

[pchaigno]

Thanks a lot for the contribution @bleggett!