ip_tunnel: make __iptunnel_pull_header() return a drop reason

__iptunnel_pull_header() returns -ENOMEM whenever it fails. It can
fail in two pskb_may_pull() calls, one for the tunnel header and one
for the inner Ethernet header of ETH_P_TEB, and in the skb_unclone()
done for GSO packets. pskb_may_pull() fails when the packet is shorter
than the requested length as well as when pulling from the frags cannot
allocate, so a truncated packet and an allocation failure look the same
to the callers. The ones that report a drop reason can only pick
SKB_DROP_REASON_NOMEM, as vxlan_rcv() does, and so would the GRE
receive paths once they report drop reasons. In ip6_gre, gre_rcv()
pulls the header before the tunnel lookup, so a packet from any sender
whose ETH_P_TEB inner Ethernet header or WCCPv2 extra word is cut short
would be reported as an out of memory condition.

Make __iptunnel_pull_header() and iptunnel_pull_header() return the
reason pskb_may_pull_reason() already computes, SKB_DROP_REASON_NOMEM
when skb_unclone() fails, and SKB_NOT_DROPPED_YET on success. A failure
is still non-zero, so the callers that only test the result keep
working. Three callers, in ip_gre and ip6_gre, test it with "< 0"
instead; the enum has no negative values, so that test would always be
false, and the compiler does not warn about it. Make them test for a
non-zero value. vxlan_rcv() keeps reporting NOMEM for any failure until
the next patch has it report the returned reason.

Suggested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Anton Danilov <littlesmilingcloud@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20261005201417.906344-3-littlesmilingcloud@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 6 insertions, 4 deletions

diff --git a/include/net/ip_tunnels.h b/include/net/ip_tunnels.h
index 7102aa11fae2..5cccf4c0e691 100644
--- a/include/net/ip_tunnels.h
+++ b/include/net/ip_tunnels.h
@@ -614,11 +614,13 @@ static inline u8 ip_tunnel_ecn_encap(u8 tos, const struct iphdr *iph,
 	return INET_ECN_encapsulate(tos, inner);
 }
 
-int __iptunnel_pull_header(struct sk_buff *skb, int hdr_len,
-			   __be16 inner_proto, bool raw_proto, bool xnet);
+enum skb_drop_reason
+__iptunnel_pull_header(struct sk_buff *skb, int hdr_len,
+		       __be16 inner_proto, bool raw_proto, bool xnet);
 
-static inline int iptunnel_pull_header(struct sk_buff *skb, int hdr_len,
-				       __be16 inner_proto, bool xnet)
+static inline enum skb_drop_reason
+iptunnel_pull_header(struct sk_buff *skb, int hdr_len,
+		     __be16 inner_proto, bool xnet)
 {
 	return __iptunnel_pull_header(skb, hdr_len, inner_proto, false, xnet);
 }