XDP

eBPF Verifier Discrepancy Revealed: PREVAIL Accepts Safe Code Rejected by Linux Kernel Verifier

Researchers discovered that the same BPF object file receives conflicting verdicts from different verifiers. The program correlated_branch.c from the ebpf-samples repository passes verification under PREVAIL but is rejected by the Linux kernel verifier. The divergence occurs because the kernel verifier tracks scalar bounds separately from packet pointer offsets, while PREVAIL maintains explicit links between checked packet sizes and pointer states. The XDP function ConvergedBranch performs a bounds check via check_packet before accessing an Ethernet header, yet the kernel verifier fails to propagate the guarantee to the subsequent load instruction. Replacing the helper call with an inline comparison against data_end allows the kernel verifier to accept the program. The finding highlights that verifier rejection does not always indicate an actual safety violation in eBPF code.