Re: [PATCH net-next 3/4] batman-adv: keep skb crc32 helper local in BLA

[Sven Eckelmann <sven@narfation.org>]

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On Saturday, 27 September 2025 22:55:52 CEST Eric Biggers wrote:
> Hi,
> 
> On Tue, Sep 16, 2025 at 02:24:40PM +0200, Simon Wunderlich wrote:
> > +static __be32 batadv_skb_crc32(struct sk_buff *skb, u8 *payload_ptr)
> > +{
> > +	unsigned int to = skb->len;
> > +	unsigned int consumed = 0;
> > +	struct skb_seq_state st;
> > +	unsigned int from;
> > +	unsigned int len;
> > +	const u8 *data;
> > +	u32 crc = 0;
> > +
> > +	from = (unsigned int)(payload_ptr - skb->data);
> > +
> > +	skb_prepare_seq_read(skb, from, to, &st);
> > +	while ((len = skb_seq_read(consumed, &data, &st)) != 0) {
> > +		crc = crc32c(crc, data, len);
> > +		consumed += len;
> > +	}
> > +
> > +	return htonl(crc);
> > +}
> 
> Has using skb_crc32c() been considered here?

No. At the time this was written (v3.8), skb_crc32c (v6.16) didnt exist. Also 
its predecessor __skb_checksum only started its existence in v3.13. And no one 
noticed it as candidate to replace batadv_skb_crc32 with

And this patch here was just moving the function between two places - so not 
introducing new code.

Do you want to submit a patch to integrate your new function in batman-adv? I 
only performed a quick-and-dirty test to see if it returns the same results 
and it seemed to do its job fine.

diff --git c/net/batman-adv/Kconfig w/net/batman-adv/Kconfig
index c299e2bc..58c408b7 100644
--- c/net/batman-adv/Kconfig
+++ w/net/batman-adv/Kconfig
@@ -35,6 +35,7 @@ config BATMAN_ADV_BLA
 	bool "Bridge Loop Avoidance"
 	depends on BATMAN_ADV && INET
 	select CRC16
+	select NET_CRC32C
 	default y
 	help
 	  This option enables BLA (Bridge Loop Avoidance), a mechanism
diff --git c/net/batman-adv/bridge_loop_avoidance.c w/net/batman-adv/bridge_loop_avoidance.c
index b992ba12..eef40b6f 100644
--- c/net/batman-adv/bridge_loop_avoidance.c
+++ w/net/batman-adv/bridge_loop_avoidance.c
@@ -12,7 +12,6 @@
 #include <linux/compiler.h>
 #include <linux/container_of.h>
 #include <linux/crc16.h>
-#include <linux/crc32.h>
 #include <linux/err.h>
 #include <linux/errno.h>
 #include <linux/etherdevice.h>
@@ -1585,45 +1584,11 @@ int batadv_bla_init(struct batadv_priv *bat_priv)
 	return 0;
 }
 
-/**
- * batadv_skb_crc32() - calculate CRC32 of the whole packet and skip bytes in
- *  the header
- * @skb: skb pointing to fragmented socket buffers
- * @payload_ptr: Pointer to position inside the head buffer of the skb
- *  marking the start of the data to be CRC'ed
- *
- * payload_ptr must always point to an address in the skb head buffer and not to
- * a fragment.
- *
- * Return: big endian crc32c of the checksummed data
- */
-static __be32 batadv_skb_crc32(struct sk_buff *skb, u8 *payload_ptr)
-{
-	unsigned int to = skb->len;
-	unsigned int consumed = 0;
-	struct skb_seq_state st;
-	unsigned int from;
-	unsigned int len;
-	const u8 *data;
-	u32 crc = 0;
-
-	from = (unsigned int)(payload_ptr - skb->data);
-
-	skb_prepare_seq_read(skb, from, to, &st);
-	while ((len = skb_seq_read(consumed, &data, &st)) != 0) {
-		crc = crc32c(crc, data, len);
-		consumed += len;
-	}
-
-	return htonl(crc);
-}
-
 /**
  * batadv_bla_check_duplist() - Check if a frame is in the broadcast dup.
  * @bat_priv: the bat priv with all the mesh interface information
  * @skb: contains the multicast packet to be checked
- * @payload_ptr: pointer to position inside the head buffer of the skb
- *  marking the start of the data to be CRC'ed
+ * @payload_offset: offset in the skb, marking the start of the data to be CRC'ed
  * @orig: originator mac address, NULL if unknown
  *
  * Check if it is on our broadcast list. Another gateway might have sent the
@@ -1638,16 +1603,18 @@ static __be32 batadv_skb_crc32(struct sk_buff *skb, u8 *payload_ptr)
  * Return: true if a packet is in the duplicate list, false otherwise.
  */
 static bool batadv_bla_check_duplist(struct batadv_priv *bat_priv,
-				     struct sk_buff *skb, u8 *payload_ptr,
+				     struct sk_buff *skb, int payload_offset,
 				     const u8 *orig)
 {
 	struct batadv_bcast_duplist_entry *entry;
 	bool ret = false;
+	int payload_len;
 	int i, curr;
 	__be32 crc;
 
 	/* calculate the crc ... */
-	crc = batadv_skb_crc32(skb, payload_ptr);
+	payload_len = skb->len - payload_offset;
+	crc = htonl(skb_crc32c(skb, payload_offset, payload_len, 0));
 
 	spin_lock_bh(&bat_priv->bla.bcast_duplist_lock);
 
@@ -1727,7 +1694,7 @@ static bool batadv_bla_check_duplist(struct batadv_priv *bat_priv,
 static bool batadv_bla_check_ucast_duplist(struct batadv_priv *bat_priv,
 					   struct sk_buff *skb)
 {
-	return batadv_bla_check_duplist(bat_priv, skb, (u8 *)skb->data, NULL);
+	return batadv_bla_check_duplist(bat_priv, skb, 0, NULL);
 }
 
 /**
@@ -1745,12 +1712,10 @@ bool batadv_bla_check_bcast_duplist(struct batadv_priv *bat_priv,
 				    struct sk_buff *skb)
 {
 	struct batadv_bcast_packet *bcast_packet;
-	u8 *payload_ptr;
 
 	bcast_packet = (struct batadv_bcast_packet *)skb->data;
-	payload_ptr = (u8 *)(bcast_packet + 1);
 
-	return batadv_bla_check_duplist(bat_priv, skb, payload_ptr,
+	return batadv_bla_check_duplist(bat_priv, skb, sizeof(*bcast_packet),
 					bcast_packet->orig);
 }
 

Regards,
	Sven

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