fix(sensor): capture execve parent PID in-kernel so the fileless-systemd gate works

[maiconburn]

Problem (0.15.28 post-deploy re-audit)

fileless:systemd was still firing on Azure after 0.15.28. Root cause: the eBPF execve handler hardcoded event.ppid = 0 (sensor-ebpf/src/main.rs). Every prod execve ppid came from a userspace /proc/<pid>/status fallback, which works for long-lived processes (connect events carry ppid) but misses short-lived ones — notably systemd's sealed-executor fexecve of /proc/self/fd/N, whose /proc entry is gone before the ring reader looks. Measured in the audit: execve events were ppid=0 4995/5000. So the spec-PR1 (#1119) fileless-systemd parent-lineage gate, which needs the parent exe, was effectively inert in prod.

Fix — read the parent in-kernel

Read task_struct->real_parent->tgid at execve, mirroring the Execution Gate's BPRM_OFFSETS pattern (this crate has no vmlinux/CO-RE; it reads kernel struct fields at BTF-resolved offsets passed via a map):

• new TASK_OFFSETS map (key 0 = real_parent, key 1 = tgid), populated by the userspace loader from kernel BTF (btf_offsets::member_offset) — correct across kernels/arches.
• the execve handler reads the offsets + does two bpf_probe_read_kernel hops (task → real_parent → tgid). Offsets absent / 0 (no BTF) → returns 0 and leaves the /proc fallback in place (unchanged behaviour) — never a guessed offset.

Validated live on test001 (x86_64, kernel 6.x)

• eBPF compiles; verifier accepts the program (eBPF collector active - 27 hooks).
• TASK_OFFSETS resolves from BTF (real_parent=2504 tgid=2492).
• comm=systemd cmd=/proc/self/fd/9 now reports ppid=1 → the fileless gate resolves /proc/1/exe=/usr/lib/systemd/systemd and suppresses it (the exact target).
• Injected bash → id/true reports the shell's pid.
• Installed sensor stopped/restored cleanly during the test.

aarch64 task_struct offsets differ but are BTF-resolved identically; CI builds both arches and the verifier-load is checked on deploy (Oracle).

Test

btf_offsets::member_offset_resolves_task_struct_fields pins the exact struct + member names (a rename would silently re-inert the gate). cargo fmt + cargo clippy --workspace -- -D warnings clean; eBPF builds clean on test001. 0 em-dashes.

PR 5/5 of the Azure cloud-platform-FP epic — the eBPF data-gap that made PR1 (#1119) effective in prod. Operator chose the eBPF approach (vs a partial userspace /proc fallback) for a complete fix.

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[codecov]

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