Re: [PATCH net v3 1/1] ipmr: only copy pktinfo to cache reports

[Ido Schimmel <idosch@nvidia.com>]

On Tue, Aug 04, 2026 at 06:31:26PM -0700, Jakub Kicinski wrote:
> This is an AI-generated review of your patch. The human sending this
> email says "Looks legit, let's fix both in one patch?"

I suggested [1], but the bug in the IGMPMSG_WHOLEPKT /
IGMPMSG_WRVIFWHOLE branch needs a different Fixes tag (it precedes
bb7403655b3c) and Sashiko [2][3] found another bug in this code:

ipv4_pktinfo_prepare() modifies the control block of 'pkt', which can
result in this packet being processed incorrectly after
ipmr_cache_report() returns. I *think* that [4] takes care of it, but
all of this needs to be explained in the commit message which should
carry the appropriate Fixes tags (bb7403655b3c + 1da177e4c3f4).

Note that this upcall is not fast path, so the extra work shouldn't be a
problem.

Is this what you prefer?

[1] https://lore.kernel.org/all/20260729091227.GA1258429@shredder/
[2] https://sashiko.dev/#/patchset/1b809975e5bd9c0a1dd6fdd1db534e701fe5a4b6.1785379072.git.zhilinz%40nebusec.ai
[3] https://netdev-ai.bots.linux.dev/sashiko/#/patchset/1b809975e5bd9c0a1dd6fdd1db534e701fe5a4b6.1785379072.git.zhilinz%40nebusec.ai
[4]
diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index 1d9a4ac14fce..cdc84eb05c79 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -1057,6 +1057,7 @@ static void ipmr_cache_resolve(struct net *net, struct mr_table *mrt,
 static int ipmr_cache_report(const struct mr_table *mrt,
 			     struct sk_buff *pkt, vifi_t vifi, int assert)
 {
+	unsigned char pkt_cb[sizeof(pkt->cb)];
 	const int ihl = ip_hdrlen(pkt);
 	struct sock *mroute_sk;
 	struct igmphdr *igmp;
@@ -1112,8 +1113,6 @@ static int ipmr_cache_report(const struct mr_table *mrt,
 		msg = (struct igmpmsg *)skb_network_header(skb);
 		msg->im_vif = vifi;
 		msg->im_vif_hi = vifi >> 8;
-		ipv4_pktinfo_prepare(mroute_sk, pkt, false);
-		memcpy(skb->cb, pkt->cb, sizeof(skb->cb));
 		/* Add our header.
 		 * Note that code, csum and group fields are cleared.
 		 */
@@ -1124,6 +1123,12 @@ static int ipmr_cache_report(const struct mr_table *mrt,
 		skb->transport_header = skb->network_header;
 	}
 
+	memcpy(pkt_cb, pkt->cb, sizeof(pkt_cb));
+	ipv4_pktinfo_prepare(mroute_sk, pkt, false);
+	memset(skb->cb, 0, sizeof(skb->cb));
+	*PKTINFO_SKB_CB(skb) = *PKTINFO_SKB_CB(pkt);
+	memcpy(pkt->cb, pkt_cb, sizeof(pkt_cb));
+
 	igmpmsg_netlink_event(mrt, skb);
 
 	/* Deliver to mrouted */