netdev - Re: [PATCH v2 4/7] skbuff: introduce skb_gso_validate

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Date:	Tue, 31 May 2016 12:07:54 -0700
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:	Netdev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org,
	Xin Long <lucien.xin@...il.com>,
	Neil Horman <nhorman@...driver.com>,
	David Laight <David.Laight@...lab.com>,
	Vlad Yasevich <vyasevich@...il.com>,
	Daniel Borkmann <daniel@...earbox.net>,
	Florian Westphal <fw@...len.de>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v2 4/7] skbuff: introduce skb_gso_validate_mtu

On Tue, May 31, 2016 at 11:55 AM, Marcelo Ricardo Leitner
<marcelo.leitner@...il.com> wrote:
> skb_gso_network_seglen is not enough for checking fragment sizes if
> skb is using GSO_BY_FRAGS as we have to check frag per frag.
>
> This patch introduces skb_gso_validate_mtu, based on the former, which
> will wrap the use case inside it as all calls to skb_gso_network_seglen
> were to validate if it fits on a given TMU, and improve the check.
>
> Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> Tested-by: Xin Long <lucien.xin@...il.com>
> ---
>  include/linux/skbuff.h |  1 +
>  net/core/skbuff.c      | 31 +++++++++++++++++++++++++++++++
>  net/ipv4/ip_forward.c  |  2 +-
>  net/ipv4/ip_output.c   |  2 +-
>  net/ipv6/ip6_output.c  |  2 +-
>  net/mpls/af_mpls.c     |  2 +-
>  6 files changed, 36 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 1f713541cb2fc232cb0e8417232cb9942409c9fc..2109c2dc9767d454b2cd08696af039b6bcd1ace7 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -2992,6 +2992,7 @@ void skb_split(struct sk_buff *skb, struct sk_buff *skb1, const u32 len);
>  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);
>  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);
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 97c32c75e704af1f31b064e8f1e0475ff1505d67..5ca562b56ec39d39e1225d96547e242732518ffe 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -4392,6 +4392,37 @@ 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 skb
> + *
> + * 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)
> +{
> +       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;
> +
> +       /* Undo this so we can re-use header sizes */
> +       hlen -= GSO_BY_FRAGS;

Isn't this just "hlen = 0"?  If so you could probably just remove this
line and the references to hlen below and instead just loop through
verifying skb_headlen() instead of adding a value that should be 0.

> +       skb_walk_frags(skb, iter) {
> +               if (hlen + skb_headlen(iter) > mtu)
> +                       return false;
> +       }
> +
> +       return true;
> +}
> +EXPORT_SYMBOL_GPL(skb_gso_validate_mtu);
> +
>  static struct sk_buff *skb_reorder_vlan_header(struct sk_buff *skb)
>  {
>         if (skb_cow(skb, skb_headroom(skb)) < 0) {