[PATCH 5.10.y 1/2] skbuff: introduce skb_pull_data

[Sasha Levin <sashal@kernel.org>]

From: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

[ Upstream commit 13244cccc2b61ec715f0ac583d3037497004d4a5 ]

Like skb_pull but returns the original data pointer before pulling the
data after performing a check against sbk->len.

This allows to change code that does "struct foo *p = (void *)skb->data;"
which is hard to audit and error prone, to:

        p = skb_pull_data(skb, sizeof(*p));
        if (!p)
                return;

Which is both safer and cleaner.

Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Stable-dep-of: 47778d2c2087 ("Bluetooth: HIDP: reject frames without a transaction header")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/linux/skbuff.h |  2 ++
 net/core/skbuff.c      | 24 ++++++++++++++++++++++++
 2 files changed, 26 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 667de403b2ebb..01eb7bc4e84b0 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2317,6 +2317,8 @@ static inline void *skb_pull_inline(struct sk_buff *skb, unsigned int len)
 	return unlikely(len > skb->len) ? NULL : __skb_pull(skb, len);
 }
 
+void *skb_pull_data(struct sk_buff *skb, size_t len);
+
 void *__pskb_pull_tail(struct sk_buff *skb, int delta);
 
 static inline void *__pskb_pull(struct sk_buff *skb, unsigned int len)
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 90a63c356affb..83bf2e5bed2b2 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1976,6 +1976,30 @@ void *skb_pull(struct sk_buff *skb, unsigned int len)
 }
 EXPORT_SYMBOL(skb_pull);
 
+/**
+ *	skb_pull_data - remove data from the start of a buffer returning its
+ *	original position.
+ *	@skb: buffer to use
+ *	@len: amount of data to remove
+ *
+ *	This function removes data from the start of a buffer, returning
+ *	the memory to the headroom. A pointer to the original data in the buffer
+ *	is returned after checking if there is enough data to pull. Once the
+ *	data has been pulled future pushes will overwrite the old data.
+ */
+void *skb_pull_data(struct sk_buff *skb, size_t len)
+{
+	void *data = skb->data;
+
+	if (skb->len < len)
+		return NULL;
+
+	skb_pull(skb, len);
+
+	return data;
+}
+EXPORT_SYMBOL(skb_pull_data);
+
 /**
  *	skb_trim - remove end from a buffer
  *	@skb: buffer to alter
-- 
2.53.0