[PATCH net-next 04/11] gre: make gre_parse_header() report a drop reason
From: Anton Danilov
Date: Mon Aug 31 2026 - 17:54:24 EST
gre_parse_header() returns -EINVAL for six different reasons and its
callers turn that into a plain kfree_skb(). The only detail they could
get so far was the csum_err flag, which none of them actually reads:
both ip_gre and ip6_gre declare it, pass it in and then ignore it.
Replace that dead output parameter with an enum skb_drop_reason one and
let the two receive paths report what happened. Two reasons are added:
- SKB_DROP_REASON_GRE_INVALID_HDR, for a header carrying an unsupported
version or the routing bit,
- SKB_DROP_REASON_GRE_CSUM, for a checksum error, next to the existing
TCP_CSUM, UDP_CSUM, ICMP_CSUM and IP_CSUM.
The header pull failures reuse SKB_DROP_REASON_HDR_TRUNC, which
documents exactly this case, and gre_rcv() in the demux reuses
pskb_may_pull_reason() and SKB_DROP_REASON_UNHANDLED_PROTO.
A NULL reason keeps the meaning a NULL csum_err had: the caller is not
interested in it and the checksum must not be verified. This matters
for the ICMP error handlers, which only get a part of the original
packet and must not drop it when the checksum does not validate.
Tunnel lookup failures still report SKB_DROP_REASON_NOT_SPECIFIED here;
they are addressed in the following patches.
Assisted-by: Claude-Code:claude-opus-5
Signed-off-by: Anton Danilov <littlesmilingcloud@xxxxxxxxx>
---
include/net/dropreason-core.h | 9 +++++++
include/net/gre.h | 2 +-
net/ipv4/gre_demux.c | 51 ++++++++++++++++++++++++++---------
net/ipv4/ip_gre.c | 6 ++---
net/ipv6/ip6_gre.c | 6 ++---
5 files changed, 55 insertions(+), 19 deletions(-)
diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h
index 40f94ecb912a..8efa682601f0 100644
--- a/include/net/dropreason-core.h
+++ b/include/net/dropreason-core.h
@@ -130,6 +130,8 @@
FN(RECURSION_LIMIT) \
FN(IP_TUNNEL_CFG_OPTS_MISMATCH) \
FN(IP_TUNNEL_OLD_SEQ) \
+ FN(GRE_INVALID_HDR) \
+ FN(GRE_CSUM) \
FNe(MAX)
/**
@@ -622,6 +624,13 @@ enum skb_drop_reason {
* numbering.
*/
SKB_DROP_REASON_IP_TUNNEL_OLD_SEQ,
+ /**
+ * @SKB_DROP_REASON_GRE_INVALID_HDR: the GRE header is invalid, e.g.
+ * an unsupported version or the routing bit is set.
+ */
+ SKB_DROP_REASON_GRE_INVALID_HDR,
+ /** @SKB_DROP_REASON_GRE_CSUM: GRE checksum error */
+ SKB_DROP_REASON_GRE_CSUM,
/**
* @SKB_DROP_REASON_MAX: the maximum of core drop reasons, which
* shouldn't be used as a real 'reason' - only for tracing code gen
diff --git a/include/net/gre.h b/include/net/gre.h
index b55f67ecd2fc..a63f26c3f78e 100644
--- a/include/net/gre.h
+++ b/include/net/gre.h
@@ -33,7 +33,7 @@ int gre_add_protocol(const struct gre_protocol *proto, u8 version);
int gre_del_protocol(const struct gre_protocol *proto, u8 version);
int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
- bool *csum_err, __be16 proto, int nhs);
+ enum skb_drop_reason *reason, __be16 proto, int nhs);
static inline bool netif_is_gretap(const struct net_device *dev)
{
diff --git a/net/ipv4/gre_demux.c b/net/ipv4/gre_demux.c
index 96fd7dc6d82d..598678f42ef5 100644
--- a/net/ipv4/gre_demux.c
+++ b/net/ipv4/gre_demux.c
@@ -58,26 +58,43 @@ EXPORT_SYMBOL_GPL(gre_del_protocol);
/* Fills in tpi and returns header length to be pulled.
* Note that caller must use pskb_may_pull() before pulling GRE header.
+ *
+ * @reason is only written when the header is rejected, so the caller has
+ * to initialise it before the call.
+ *
+ * A NULL @reason means that the caller is not interested in the drop
+ * reason, and also that the checksum must not be verified. This is what
+ * the ICMP error handlers need, as they only get a part of the original
+ * packet.
*/
int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
- bool *csum_err, __be16 proto, int nhs)
+ enum skb_drop_reason *reason, __be16 proto, int nhs)
{
const struct gre_base_hdr *greh;
__be32 *options;
int hdr_len;
- if (unlikely(!pskb_may_pull(skb, nhs + sizeof(struct gre_base_hdr))))
+ if (unlikely(!pskb_may_pull(skb, nhs + sizeof(struct gre_base_hdr)))) {
+ if (reason)
+ *reason = SKB_DROP_REASON_HDR_TRUNC;
return -EINVAL;
+ }
greh = (struct gre_base_hdr *)(skb->data + nhs);
- if (unlikely(greh->flags & (GRE_VERSION | GRE_ROUTING)))
+ if (unlikely(greh->flags & (GRE_VERSION | GRE_ROUTING))) {
+ if (reason)
+ *reason = SKB_DROP_REASON_GRE_INVALID_HDR;
return -EINVAL;
+ }
gre_flags_to_tnl_flags(tpi->flags, greh->flags);
hdr_len = gre_calc_hlen(tpi->flags);
- if (!pskb_may_pull(skb, nhs + hdr_len))
+ if (!pskb_may_pull(skb, nhs + hdr_len)) {
+ if (reason)
+ *reason = SKB_DROP_REASON_HDR_TRUNC;
return -EINVAL;
+ }
greh = (struct gre_base_hdr *)(skb->data + nhs);
tpi->proto = greh->protocol;
@@ -87,8 +104,8 @@ int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
if (!skb_checksum_simple_validate(skb)) {
skb_checksum_try_convert(skb, IPPROTO_GRE,
null_compute_pseudo);
- } else if (csum_err) {
- *csum_err = true;
+ } else if (reason) {
+ *reason = SKB_DROP_REASON_GRE_CSUM;
return -EINVAL;
}
@@ -116,8 +133,11 @@ int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
val = skb_header_pointer(skb, nhs + hdr_len,
sizeof(_val), &_val);
- if (!val)
+ if (!val) {
+ if (reason)
+ *reason = SKB_DROP_REASON_HDR_TRUNC;
return -EINVAL;
+ }
tpi->proto = proto;
if ((*val & 0xF0) != 0x40)
hdr_len += 4;
@@ -132,8 +152,11 @@ int gre_parse_header(struct sk_buff *skb, struct tnl_ptk_info *tpi,
greh->protocol == htons(ETH_P_ERSPAN2)) {
struct erspan_base_hdr *ershdr;
- if (!pskb_may_pull(skb, nhs + hdr_len + sizeof(*ershdr)))
+ if (!pskb_may_pull(skb, nhs + hdr_len + sizeof(*ershdr))) {
+ if (reason)
+ *reason = SKB_DROP_REASON_HDR_TRUNC;
return -EINVAL;
+ }
ershdr = (struct erspan_base_hdr *)(skb->data + nhs + hdr_len);
tpi->key = cpu_to_be32(get_session_id(ershdr));
@@ -146,15 +169,19 @@ EXPORT_SYMBOL(gre_parse_header);
static int gre_rcv(struct sk_buff *skb)
{
const struct gre_protocol *proto;
+ enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED;
u8 ver;
int ret;
- if (!pskb_may_pull(skb, 12))
+ reason = pskb_may_pull_reason(skb, 12);
+ if (reason)
goto drop;
ver = skb->data[1]&0x7f;
- if (ver >= GREPROTO_MAX)
+ if (ver >= GREPROTO_MAX) {
+ reason = SKB_DROP_REASON_UNHANDLED_PROTO;
goto drop;
+ }
rcu_read_lock();
proto = rcu_dereference(gre_proto[ver]);
@@ -167,11 +194,11 @@ static int gre_rcv(struct sk_buff *skb)
drop_nohandler:
rcu_read_unlock();
dev_core_stats_rx_nohandler_inc(skb->dev);
- kfree_skb(skb);
+ kfree_skb_reason(skb, SKB_DROP_REASON_UNHANDLED_PROTO);
return NET_RX_DROP;
drop:
dev_core_stats_rx_dropped_inc(skb->dev);
- kfree_skb(skb);
+ kfree_skb_reason(skb, reason);
return NET_RX_DROP;
}
diff --git a/net/ipv4/ip_gre.c b/net/ipv4/ip_gre.c
index 82309efd417e..1894c5746a73 100644
--- a/net/ipv4/ip_gre.c
+++ b/net/ipv4/ip_gre.c
@@ -439,8 +439,8 @@ static int ipgre_rcv(struct sk_buff *skb, const struct tnl_ptk_info *tpi,
static int gre_rcv(struct sk_buff *skb)
{
+ enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED;
struct tnl_ptk_info tpi;
- bool csum_err = false;
int hdr_len;
#ifdef CONFIG_NET_IPGRE_BROADCAST
@@ -451,7 +451,7 @@ static int gre_rcv(struct sk_buff *skb)
}
#endif
- hdr_len = gre_parse_header(skb, &tpi, &csum_err, htons(ETH_P_IP), 0);
+ hdr_len = gre_parse_header(skb, &tpi, &reason, htons(ETH_P_IP), 0);
if (hdr_len < 0)
goto drop;
@@ -469,7 +469,7 @@ static int gre_rcv(struct sk_buff *skb)
icmp_send(skb, ICMP_DEST_UNREACH, ICMP_PORT_UNREACH, 0);
drop:
dev_core_stats_rx_dropped_inc(skb->dev);
- kfree_skb(skb);
+ kfree_skb_reason(skb, reason);
return 0;
}
diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index 69c51f1a5bf0..736eefdb528f 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -569,11 +569,11 @@ static int ip6erspan_rcv(struct sk_buff *skb,
static int gre_rcv(struct sk_buff *skb)
{
+ enum skb_drop_reason reason = SKB_DROP_REASON_NOT_SPECIFIED;
struct tnl_ptk_info tpi;
- bool csum_err = false;
int hdr_len;
- hdr_len = gre_parse_header(skb, &tpi, &csum_err, htons(ETH_P_IPV6), 0);
+ hdr_len = gre_parse_header(skb, &tpi, &reason, htons(ETH_P_IPV6), 0);
if (hdr_len < 0)
goto drop;
@@ -594,7 +594,7 @@ static int gre_rcv(struct sk_buff *skb)
icmpv6_send(skb, ICMPV6_DEST_UNREACH, ICMPV6_PORT_UNREACH, 0);
drop:
dev_core_stats_rx_dropped_inc(skb->dev);
- kfree_skb(skb);
+ kfree_skb_reason(skb, reason);
return 0;
}
--
2.47.3