[PATCH 5.15.y 1/2] net: add pskb_may_pull_reason() helper

[Sherry Yang <sherry.yang@oracle.com>]

From: Eric Dumazet <edumazet@google.com>

[ Upstream commit 1fb2d41501f38192d8a19da585cd441cf8845697 ]

pskb_may_pull() can fail for two different reasons.

Provide pskb_may_pull_reason() helper to distinguish
between these reasons.

It returns:

SKB_NOT_DROPPED_YET           : Success
SKB_DROP_REASON_PKT_TOO_SMALL : packet too small
SKB_DROP_REASON_NOMEM         : skb->head could not be resized

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: 8bd67ebb50c0 ("net: bridge: xmit: make sure we have at least eth header len bytes")
Signed-off-by: Sasha Levin <sashal@kernel.org>
[Sherry: bp to 5.15.y. Minor conflicts due to missing commit
d427c8999b07 ("net-next: skbuff: refactor pskb_pull") which is not
necessary in 5.15.y. Ignore context change.
Signed-off-by: Sherry Yang <sherry.yang@oracle.com>
---
 include/linux/skbuff.h | 19 +++++++++++++++----
 1 file changed, 15 insertions(+), 4 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index b230c422dc3b..f92e8fe4f5eb 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2465,13 +2465,24 @@ static inline void *pskb_pull(struct sk_buff *skb, unsigned int len)
 	return unlikely(len > skb->len) ? NULL : __pskb_pull(skb, len);
 }
 
-static inline bool pskb_may_pull(struct sk_buff *skb, unsigned int len)
+static inline enum skb_drop_reason
+pskb_may_pull_reason(struct sk_buff *skb, unsigned int len)
 {
 	if (likely(len <= skb_headlen(skb)))
-		return true;
+		return SKB_NOT_DROPPED_YET;
+
 	if (unlikely(len > skb->len))
-		return false;
-	return __pskb_pull_tail(skb, len - skb_headlen(skb)) != NULL;
+		return SKB_DROP_REASON_PKT_TOO_SMALL;
+
+	if (unlikely(!__pskb_pull_tail(skb, len - skb_headlen(skb))))
+		return SKB_DROP_REASON_NOMEM;
+
+	return SKB_NOT_DROPPED_YET;
+}
+
+static inline bool pskb_may_pull(struct sk_buff *skb, unsigned int len)
+{
+	return pskb_may_pull_reason(skb, len) == SKB_NOT_DROPPED_YET;
 }
 
 void skb_condense(struct sk_buff *skb);
-- 
2.46.0