[PATCH net] drivers/net/wan: lapb: Replace the skb->len checks with pskb_may_pull

[PATCH net] drivers/net/wan: lapb: Replace the skb->len checks with pskb_may_pull

[Xie He]

The purpose of these skb->len checks in these drivers is to ensure that
there is a header in the skb available to be read and pulled.

However, we already have the pskb_may_pull function for this purpose.

The pskb_may_pull function also correctly handles non-linear skbs.

(Also delete the word "check" in the comments because pskb_may_pull may
do things other than simple checks in the case of non-linear skbs.)

Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Martin Schiller <ms@dev.tdt.de>
Signed-off-by: Xie He <xie.he.0141@gmail.com>
---
 drivers/net/wan/hdlc_x25.c  | 4 ++--
 drivers/net/wan/lapbether.c | 4 ++--
 drivers/net/wan/x25_asy.c   | 4 ++--
 3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/net/wan/hdlc_x25.c b/drivers/net/wan/hdlc_x25.c
index f52b9fed0593..891a7a918b29 100644
--- a/drivers/net/wan/hdlc_x25.c
+++ b/drivers/net/wan/hdlc_x25.c
@@ -108,9 +108,9 @@ static netdev_tx_t x25_xmit(struct sk_buff *skb, struct net_device *dev)
 	int result;
 
 	/* There should be a pseudo header of 1 byte added by upper layers.
-	 * Check to make sure it is there before reading it.
+	 * Make sure it is there before reading it.
 	 */
-	if (skb->len < 1) {
+	if (!pskb_may_pull(skb, 1)) {
 		kfree_skb(skb);
 		return NETDEV_TX_OK;
 	}
diff --git a/drivers/net/wan/lapbether.c b/drivers/net/wan/lapbether.c
index b6be2454b8bd..e0cf692357d6 100644
--- a/drivers/net/wan/lapbether.c
+++ b/drivers/net/wan/lapbether.c
@@ -158,9 +158,9 @@ static netdev_tx_t lapbeth_xmit(struct sk_buff *skb,
 		goto drop;
 
 	/* There should be a pseudo header of 1 byte added by upper layers.
-	 * Check to make sure it is there before reading it.
+	 * Make sure it is there before reading it.
 	 */
-	if (skb->len < 1)
+	if (!pskb_may_pull(skb, 1))
 		goto drop;
 
 	switch (skb->data[0]) {
diff --git a/drivers/net/wan/x25_asy.c b/drivers/net/wan/x25_asy.c
index c418767a890a..b935a3e1d5f6 100644
--- a/drivers/net/wan/x25_asy.c
+++ b/drivers/net/wan/x25_asy.c
@@ -308,9 +308,9 @@ static netdev_tx_t x25_asy_xmit(struct sk_buff *skb,
 	}
 
 	/* There should be a pseudo header of 1 byte added by upper layers.
-	 * Check to make sure it is there before reading it.
+	 * Make sure it is there before reading it.
 	 */
-	if (skb->len < 1) {
+	if (!pskb_may_pull(skb, 1)) {
 		kfree_skb(skb);
 		return NETDEV_TX_OK;
 	}
-- 
2.25.1

Re: [PATCH net] drivers/net/wan: lapb: Replace the skb->len checks with pskb_may_pull

[David Miller]

From: Xie He <xie.he.0141@gmail.com>
Date: Sat,  3 Oct 2020 17:46:19 -0700

> The purpose of these skb->len checks in these drivers is to ensure that
> there is a header in the skb available to be read and pulled.
> 
> However, we already have the pskb_may_pull function for this purpose.
> 
> The pskb_may_pull function also correctly handles non-linear skbs.
> 
> (Also delete the word "check" in the comments because pskb_may_pull may
> do things other than simple checks in the case of non-linear skbs.)
> 
> Cc: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
> Cc: Martin Schiller <ms@dev.tdt.de>
> Signed-off-by: Xie He <xie.he.0141@gmail.com>

This is excessive.

On transmit the header will be in the linear area, especially
if it is only one byte in size.

You're adding a lot of extra checks, function calls, etc. which are
frankly unnecessary.

I'm not applying this unless you can prove that a non-linear single
header byte is possible in these code paths.  And you'll need to add
such proof to your commit message.

Thank you.