eBPF: Preventing Garbage HTTP Payloads When Reading Kernel Scatter-Gather Buffers

An intermittent HTTP payload corruption issue in the nettap eBPF capture agent was traced to incomplete handling of the kernel’s scatter-gather `iov_iter` buffers in `tcp_recvmsg`, rather than to the monitored applications or protocol dissector. The original fexit-based implementation could read an iterator after the kernel had advanced it, causing multi-segment responses to be read from only the final segment and resulting in buffer over-reads and garbage trailing data. Kernel-version differences further complicated the issue because `iter_type` enum values, `iov`/`__iov` field names, and `tcp_recvmsg` function signatures vary across releases, often failing silently because eBPF user-memory reads tolerate invalid pointers. The revised approach uses CO-RE facilities to resolve version-specific enum values and fields, selects compatible probe signatures through BTF metadata, records each segment’s original base pointer and capacity at fentry in task-local BPF storage, and retrieves them at fexit after the return value is known. It then reads across saved segments while limiting each read to the lesser of the remaining returned byte count and the segment capacity, avoiding stale-buffer data, although unsupported iterator types and reads beyond a 32-segment limit remain known limitations.

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