The Relation between `skb->len' and `skb->data_len' and What They Represent

http://kerneltrap.org/node/16224

This writing is based on Linux kernel 2.6.21.5.

If

skb

is linear (i.e.,

skb->data_len == 0

), the length of

skb->data

is

skb->len

.

If

skb

is not linear (i.e.,

skb->data_len != 0)

, the length of

skb->data

is

(skb->len) - (skb->data_len)

for the head ONLY. The rest must see

struct skb_shared_info->frags[i].size

and

struct skb_shared_info->frag_list

, which contains a linked-list of

struct sk_buff

because, deducing from [2],

skb->data_len =
struct skb_shared_info->frags[0...struct skb_shared_info->nr_frags].size
+
size of data in struct skb_shared_info->frag_list

The rest of the data is not stored as a separate

skb

if the length of the data permits, but as an array of

struct skb_frag_struct

in

struct skb_shared_info

([4]: To allow 64K frame to be packed as single

skb

without

frag_list

).

struct skb_frag_struct

contains

struct page *

to point to the true data. If the length of the data is longer than that that can be contained in the array,

struct skb_shared_info->frag_list

will be used to contain a linked-list of

struct sk_buff

(i.e., the data undergo fragmentation because, according to [1], the

frag_list

is used to maintain a chain of SKBs organized for fragmentation purposes, it is
not used for maintaining paged data.)

As an additional information,

skb->truesize = skb->len + sizeof(struct sk_buff)

. Don't forget that

skb->len

contains the length of the total data space that the

skb

refers to taking into account

SKB_DATA_ALIGN()

and non-linear condition.

skb->len

is modified when doing

skb_pull()

,

skb_push()

or

skb_put()

.

References:

[1] DaveM's skb_data.html (http://vger.kernel.org/~davem/skb_data.html)

[2]

ip_frag_reasm()

in ip_fragment.c

[3]

__alloc_skb()

,

pskb_copy()

in skbuff.c

[4] include/linux/skbuff.h

Archive: Linux Kernel's Networking Part (skb)