[PATCH net-next v3] tun: use pskb_may_pull_reason() and remove SKB_DROP_REASON_HDR_TRUNC

[Anton Danilov <littlesmilingcloud@gmail.com>]

tun_get_user() is the only user of SKB_DROP_REASON_HDR_TRUNC. It sets
it when pskb_may_pull() fails to pull the Ethernet header of a frame
written to a TAP device.

pskb_may_pull_reason() reports why the pull failed: PKT_TOO_SMALL if
the packet is shorter than requested, NOMEM if pulling the data into
the linear area fails. Use it for both header pulls in tun_get_user(),
on TAP and on TUN devices, and remove the now unused
SKB_DROP_REASON_HDR_TRUNC. The pull of the IP version byte on a TUN
device with IFF_NO_PI was reported as NOT_SPECIFIED and now reports
PKT_TOO_SMALL, for example when only the virtio-net header is written.

After a successful pull drop_reason is SKB_NOT_DROPPED_YET. The later
error paths without a reason of their own, the unknown IP version on
TUN, the virtio-net header conversion and the path that finds the
queue detached, relied on the SKB_DROP_REASON_NOT_SPECIFIED
initializer. Fall back to SKB_DROP_REASON_NOT_SPECIFIED at the free_skb
label, as vxlan_rcv() does, so that these paths, and any later one
without a reason of its own, keep reporting it.

The errors returned to user space do not change.

Suggested-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/netdev/20260923145931.GA2859441@shredder/
Assisted-by: LLM
Signed-off-by: Anton Danilov <littlesmilingcloud@gmail.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
---
v3:
- Rebased on net-next. Since commit 44378d02c5aa ("net: always dissect
  GSO packets in __virtio_net_hdr_to_skb()"), the virtio-net header
  conversion runs after the header pulls, so the commit message lists
  it among the paths covered by the fallback. No code changes.
- Collected tags.

v2: https://lore.kernel.org/netdev/20261003132340.4175269-1-littlesmilingcloud@gmail.com/
- Also convert the pull of the IP version byte on TUN devices (Willem).
- Dropped the Reviewed-by tags, as the IFF_TUN conversion is new.

v1: https://lore.kernel.org/netdev/20261001093925.3529772-1-littlesmilingcloud@gmail.com/

 drivers/net/tun.c             | 8 +++++---
 include/net/dropreason-core.h | 7 -------
 2 files changed, 5 insertions(+), 10 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 8b82a7fe6ec7..d4152c9ba8df 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1903,7 +1903,8 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 		if (tun->flags & IFF_NO_PI) {
 			u8 ip_version;
 
-			if (!pskb_may_pull(skb, 1)) {
+			drop_reason = pskb_may_pull_reason(skb, 1);
+			if (drop_reason) {
 				err = -EINVAL;
 				goto drop;
 			}
@@ -1926,9 +1927,9 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 		skb->protocol = pi.proto;
 		break;
 	case IFF_TAP:
-		if (!pskb_may_pull(skb, ETH_HLEN)) {
+		drop_reason = pskb_may_pull_reason(skb, ETH_HLEN);
+		if (drop_reason) {
 			err = -ENOMEM;
-			drop_reason = SKB_DROP_REASON_HDR_TRUNC;
 			goto drop;
 		}
 		skb_set_network_header(skb, ETH_HLEN);
@@ -2065,6 +2066,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 		dev_core_stats_rx_dropped_inc(tun->dev);
 
 free_skb:
+	drop_reason = drop_reason ?: SKB_DROP_REASON_NOT_SPECIFIED;
 	if (!IS_ERR_OR_NULL(skb))
 		kfree_skb_reason(skb, drop_reason);
 
diff --git a/include/net/dropreason-core.h b/include/net/dropreason-core.h
index 664249924219..fa2fe8990557 100644
--- a/include/net/dropreason-core.h
+++ b/include/net/dropreason-core.h
@@ -82,7 +82,6 @@
 	FN(DEV_READY)			\
 	FN(FULL_RING)			\
 	FN(NOMEM)			\
-	FN(HDR_TRUNC)			\
 	FN(TAP_FILTER)			\
 	FN(TAP_TXFILTER)		\
 	FN(ICMP_CSUM)			\
@@ -427,12 +426,6 @@ enum skb_drop_reason {
 	SKB_DROP_REASON_FULL_RING,
 	/** @SKB_DROP_REASON_NOMEM: error due to OOM */
 	SKB_DROP_REASON_NOMEM,
-	/**
-	 * @SKB_DROP_REASON_HDR_TRUNC: failed to trunc/extract the header from
-	 * networking data, e.g., failed to pull the protocol header from
-	 * frags via pskb_may_pull()
-	 */
-	SKB_DROP_REASON_HDR_TRUNC,
 	/**
 	 * @SKB_DROP_REASON_TAP_FILTER: dropped by (ebpf) filter directly attached
 	 * to tun/tap, e.g., via TUNSETFILTEREBPF
-- 
2.47.3