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

From: Daniel Axtens <[email-masked]>
Date: Wed, 31 Jan 2018 00:08:59 +1100

Marcelo Ricardo Leitner <[email-masked]> writes:

> 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 <[email-masked]>
>> ---
>>  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.
Oh darn it! I fixed this locally and then forgot to run
format-patch. Will fix it in the next spin.

Thanks!

Regards,
Daniel