Subject: Re: [PATCH RFC net-next v4 2/7] net: introduce skb_crc32c_csum_help

From: Marcelo Ricardo Leitner <[email-masked]>
Date: Thu, 27 Apr 2017 09:29:14 -0300

On Thu, Apr 20, 2017 at 03:38:08PM +0200, Davide Caratti wrote:
> skb_crc32c_csum_help is like skb_checksum_help, but it is designed for
> checksumming SCTP packets using crc32c (see RFC3309), provided that
> libcrc32c.ko has been loaded before. In case libcrc32c is not loaded,
> invoking skb_crc32c_csum_help on a skb results in one the following
> printouts:
> 
> warn_crc32c_csum_update: attempt to compute crc32c without libcrc32c.ko
> warn_crc32c_csum_combine: attempt to compute crc32c without libcrc32c.ko
> 
> Signed-off-by: Davide Caratti <[email-masked]>
> ---
>  include/linux/netdevice.h |  1 +
>  include/linux/skbuff.h    |  3 ++-
>  net/core/dev.c            | 40 ++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 43 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index b0aa089..bf84a67 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -3898,6 +3898,7 @@ void netdev_rss_key_fill(void *buffer, size_t len);
>  
>  int dev_get_nest_level(struct net_device *dev);
>  int skb_checksum_help(struct sk_buff *skb);
> +int skb_crc32c_csum_help(struct sk_buff *skb);
>  struct sk_buff *__skb_gso_segment(struct sk_buff *skb,
>  				  netdev_features_t features, bool tx_path);
>  struct sk_buff *skb_mac_gso_segment(struct sk_buff *skb,
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index ba3ae21..ec4551b 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -193,7 +193,8 @@
>   *     accordingly. Note the there is no indication in the skbuff that the
>   *     CHECKSUM_PARTIAL refers to an SCTP checksum, a driver that supports
>   *     both IP checksum offload and SCTP CRC offload must verify which offload
> - *     is configured for a packet presumably by inspecting packet headers.
> + *     is configured for a packet presumably by inspecting packet headers; in
> + *     case, skb_crc32c_csum_help is provided to compute CRC on SCTP packets.
>   *
>   *   NETIF_F_FCOE_CRC - This feature indicates that a device is capable of
>   *     offloading the FCOE CRC in a packet. To perform this offload the stack
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 5d33e2b..c7aec95 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -140,6 +140,7 @@
>  #include <linux/hrtimer.h>
>  #include <linux/netfilter_ingress.h>
>  #include <linux/crash_dump.h>
> +#include <linux/sctp.h>
>  
>  #include "net-sysfs.h"
>  
> @@ -2606,6 +2607,45 @@ int skb_checksum_help(struct sk_buff *skb)
>  }
>  EXPORT_SYMBOL(skb_checksum_help);
>  
> +int skb_crc32c_csum_help(struct sk_buff *skb)
> +{
> +	__le32 crc32c_csum;
> +	int ret = 0, offset;
> +
> +	if (skb->ip_summed != CHECKSUM_PARTIAL)
> +		goto out;
> +
> +	if (unlikely(skb_is_gso(skb)))
> +		goto out;
> +
> +	/* Before computing a checksum, we should make sure no frag could
> +	 * be modified by an external entity : checksum could be wrong.
> +	 */
> +	if (unlikely(skb_has_shared_frag(skb))) {
> +		ret = __skb_linearize(skb);
> +		if (ret)
> +			goto out;
> +	}
> +
> +	offset = skb_checksum_start_offset(skb);
> +	crc32c_csum = cpu_to_le32(~__skb_checksum(skb, offset,
> +						  skb->len - offset, ~(__u32)0,
> +						  crc32c_csum_stub));
> +	offset += offsetof(struct sctphdr, checksum);
> +	BUG_ON(offset >= skb_headlen(skb));

I suggest using WARN_ON_ONCE() here and returning an error instead. Will
still allow debugging and won't disrupt the system.

> +
> +	if (skb_cloned(skb) &&
> +	    !skb_clone_writable(skb, offset + sizeof(__le32))) {
> +		ret = pskb_expand_head(skb, 0, 0, GFP_ATOMIC);
> +		if (ret)
> +			goto out;
> +	}

We could do this check (including the BUG_ON/WARN check above) before
the actual crc32 calc. This can fail, and if it does, we will have
calculated it in vain. Note how offset doesn't really depend on the
checksum result.

I know skb_checksum_help also does it this way, maybe it was because of
some cache optimization on the offset += checksum offset  operation?

> +	*(__le32 *)(skb->data + offset) = crc32c_csum;
> +	skb->ip_summed = CHECKSUM_NONE;
> +out:
> +	return ret;
> +}
> +
>  __be16 skb_network_protocol(struct sk_buff *skb, int *depth)
>  {
>  	__be16 type = skb->protocol;
> -- 
> 2.7.4
> 
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