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When the mroute socket has IP_PKTINFO > enabled, ipmr_cache_report() calls ipv4_pktinfo_prepare() on the > original skb, which reads skb->cb[] as IPCB. > > By the time the pimreg device transmits the packet, skb->cb[] is not > IPCB anymore: qdisc_pkt_len_init() overwrites IPCB(skb)->iif with > qdisc_skb_cb(skb)->pkt_len, and any qdisc installed on the device or a > tc BPF program writing skb->cb[] fills the area holding the IP options > and IPCB(skb)->flags. > > For locally generated packets routed out pimreg (they carry an rtable > dst), the mroute socket can therefore receive a bogus ipi_ifindex > (the packet length), and the l3mdev slave test in > ipv4_pktinfo_prepare() is driven by stale qdisc data. > > Clear IPCB before building the report, as done for IPv6 in > commit 238164201431 ("ip6mr: clear skb->cb[] in reg_vif_xmit()"). > The skb is freed right after, so nothing else depends on its control > block. > > Fixes: bb7403655b3c ("ipmr: support IP_PKTINFO on cache report IGMP msg") > Signed-off-by: Eric Dumazet Reviewed-by: Ido Schimmel