Re: [PATCH v3 1/2] net: create skb_gso_validate_mac_len()

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

Hi,

On Tue, Jan 30, 2018 at 12:14:46PM +1100, Daniel Axtens wrote:
> If you take a GSO skb, and split it into packets, will the MAC
> length (L2 + L3 + L4 headers + payload) of those packets be small
> enough to fit within a given length?
> 
> Move skb_gso_mac_seglen() to skbuff.h with other related functions
> like skb_gso_network_seglen() so we can use it, and then create
> skb_gso_validate_mac_len to do the full calculation.
> 
> Signed-off-by: Daniel Axtens <dja@axtens.net>
> ---
>  include/linux/skbuff.h | 16 +++++++++++++
>  net/core/skbuff.c      | 65 +++++++++++++++++++++++++++++++++++++++-----------
>  net/sched/sch_tbf.c    | 10 --------
>  3 files changed, 67 insertions(+), 24 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index b8e0da6c27d6..242d6773c7c2 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -3287,6 +3287,7 @@ int skb_shift(struct sk_buff *tgt, struct sk_buff *skb, int shiftlen);
>  void skb_scrub_packet(struct sk_buff *skb, bool xnet);
>  unsigned int skb_gso_transport_seglen(const struct sk_buff *skb);
>  bool skb_gso_validate_mtu(const struct sk_buff *skb, unsigned int mtu);
> +bool skb_gso_validate_mac_len(const struct sk_buff *skb, unsigned int len);
>  struct sk_buff *skb_segment(struct sk_buff *skb, netdev_features_t features);
>  struct sk_buff *skb_vlan_untag(struct sk_buff *skb);
>  int skb_ensure_writable(struct sk_buff *skb, int write_len);
> @@ -4120,6 +4121,21 @@ static inline unsigned int skb_gso_network_seglen(const struct sk_buff *skb)
>  	return hdr_len + skb_gso_transport_seglen(skb);
>  }
>  
> +/**
> + * skb_gso_mac_seglen - Return length of individual segments of a gso packet
> + *
> + * @skb: GSO skb
> + *
> + * skb_gso_mac_seglen is used to determine the real size of the
> + * individual segments, including MAC/L2, Layer3 (IP, IPv6) and L4
> + * headers (TCP/UDP).
> + */
> +static inline unsigned int skb_gso_mac_seglen(const struct sk_buff *skb)
> +{
> +	unsigned int hdr_len = skb_transport_header(skb) - skb_mac_header(skb);
> +	return hdr_len + skb_gso_transport_seglen(skb);
> +}
> +
>  /* Local Checksum Offload.
>   * Compute outer checksum based on the assumption that the
>   * inner checksum will be offloaded later.
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 01e8285aea73..55d84ab7d093 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -4914,36 +4914,73 @@ unsigned int skb_gso_transport_seglen(const struct sk_buff *skb)
>  EXPORT_SYMBOL_GPL(skb_gso_transport_seglen);
>  
>  /**
> - * skb_gso_validate_mtu - Return in case such skb fits a given MTU
> + * skb_gso_size_check - check the skb size, considering GSO_BY_FRAGS
>   *
> - * @skb: GSO skb
> - * @mtu: MTU to validate against
> + * There are a couple of instances where we have a GSO skb, and we
> + * want to determine what size it would be after it is segmented.
>   *
> - * skb_gso_validate_mtu validates if a given skb will fit a wanted MTU
> - * once split.
> + * We might want to check:
> + * -    L3+L4+payload size (e.g. IP forwarding)
> + * - L2+L3+L4+payload size (e.g. sanity check before passing to driver)
> + *
> + * This is a helper to do that correctly considering GSO_BY_FRAGS.
> + *
> + * @seg_len: The segmented length (from skb_gso_*_seglen). In the
> + *           GSO_BY_FRAGS case this will be [header sizes + GSO_BY_FRAGS].
> + *
> + * @max_len: The maximum permissible length.
> + *
> + * Returns true if the segmented length <= max length.
>   */
> -bool skb_gso_validate_mtu(const struct sk_buff *skb, unsigned int mtu)
> -{
> +static inline bool skb_gso_size_check(const struct sk_buff *skb,
> +				      unsigned int seg_len,
> +				      unsigned int max_len) {
>  	const struct skb_shared_info *shinfo = skb_shinfo(skb);
>  	const struct sk_buff *iter;
> -	unsigned int hlen;
> -
> -	hlen = skb_gso_network_seglen(skb);
>  
>  	if (shinfo->gso_size != GSO_BY_FRAGS)
> -		return hlen <= mtu;
> +		return seg_len <= max_len;
>  
>  	/* Undo this so we can re-use header sizes */
> -	hlen -= GSO_BY_FRAGS;
> +	seg_len -= GSO_BY_FRAGS;
>  
>  	skb_walk_frags(skb, iter) {
> -		if (hlen + skb_headlen(iter) > mtu)
> +		if (seg_len + skb_headlen(iter) > max_len)
>  			return false;
>  	}
>  
>  	return true;
>  }
> -EXPORT_SYMBOL_GPL(skb_gso_validate_mtu);
> +
> +/**
> + * skb_gso_validate_mtu - Return in case such skb fits a given MTU
> + *
> + * @skb: GSO skb
> + * @mtu: MTU to validate against
> + *
> + * skb_gso_validate_mtu validates if a given skb will fit a wanted MTU
> + * once split.
> + */
> +bool skb_gso_validate_mtu(const struct sk_buff *skb, unsigned int mtu)
> +{
> +	return skb_gso_size_check(skb, skb_gso_network_seglen(skb), mtu);
> +}
> +EXPORT_SYMBOL_GPL(skb_gso_validate_network_len);

This export is not matching the function name.