This series continues the "Signed BPF programs" work and adds the missing pieces needed for an LSM to do policy enforcement and addresses the concerns raised by the developers of Hornet.

One signing scheme, please.

BPF does not need a second signing scheme. It needs a policy framework that consumes the verdict the existing signing pipeline produces. Two parallel signing stacks is harmful UX for Cilium, bpftrace, systemd, distros, and everyone shipping signed lskels. Hornet has been NACK'd repeatedly by the BPF maintainers [1][2] on layering and TOCTOU grounds.

What this series adds

• prog->aux->sig (verdict + keyring) and prog->aux->is_kernel, populated by the syscall path before security_bpf_prog_load fires.
• bpf_loader_verify_metadata kfunc -- the metadata check is now kernel C code, not BPF bytecode. The verifier injects the calling prog->aux as an implicit argument via KF_IMPLICIT_ARGS.
• Loader-side prog BTF with BPF_PSEUDO_KFUNC_CALL_PROG_BTF so the kfunc CALL is reproducible across build hosts and resolved at load time.
• security_bpf_prog_load_post_integrity LSM hook, fired by the kfunc on a successful metadata check.
• IPE properties (bpf_signature, bpf_keyring, bpf_kernel) and two ops (BPF_PROG_LOAD, BPF_PROG_LOAD_POST_INTEGRITY).

This series address concerns raised by the Hornet developers:

• The metadata hash check should be in kernel C, not BPF bytecode -- Blaise Boscaccy [3]:

  The bpf_loader_verify_metadata kfunc moves the hash check from inline BPF instructions into kernel C code.

• LSMs cannot observe the verification result at hook time -- Paul Moore [4]:

  prog->aux->sig.verdict and sig.keyring are populated before any LSM hook runs. Furthermore, security_bpf_prog_load_post_integrity hook fires after the in-kernel hash check for consumers that want to observe or gate the post-integrity transition.

[1] Alexei Starovoitov, NACK on Hornet (TOCTOU + layering), https://lore.kernel.org/all/CAADnVQJ1CRvTXBU771KaYzrx-vRaWF+k164DcFOqOsCxmuL+ig@mail.gmail.com/ [2] Daniel Borkmann, NACK on Hornet v3, https://lore.kernel.org/all/[email protected]/ [3] Blaise Boscaccy, Hornet v6 (C-side hash verification rationale), https://lore.kernel.org/all/[email protected]/ [4] Paul Moore, push for post-verifier observability, https://lore.kernel.org/all/CACYkzJ4+=3owK+ELD9Nw7Rrm-UajxXEw8kVtOTJJ+SNAXpsOpw@mail.gmail.com/