• Florian Westphal's avatar
    sk_buff: add skb extension infrastructure · df5042f4
    Florian Westphal authored
    This adds an optional extension infrastructure, with ispec (xfrm) and
    bridge netfilter as first users.
    objdiff shows no changes if kernel is built without xfrm and br_netfilter
    The third (planned future) user is Multipath TCP which is still
    MPTCP needs to map logical mptcp sequence numbers to the tcp sequence
    numbers used by individual subflows.
    This DSS mapping is read/written from tcp option space on receive and
    written to tcp option space on transmitted tcp packets that are part of
    and MPTCP connection.
    Extending skb_shared_info or adding a private data field to skb fclones
    doesn't work for incoming skb, so a different DSS propagation method would
    be required for the receive side.
    mptcp has same requirements as secpath/bridge netfilter:
    1. extension memory is released when the sk_buff is free'd.
    2. data is shared after cloning an skb (clone inherits extension)
    3. adding extension to an skb will COW the extension buffer if needed.
    The "MPTCP upstreaming" effort adds SKB_EXT_MPTCP extension to store the
    mapping for tx and rx processing.
    Two new members are added to sk_buff:
    1. 'active_extensions' byte (filling a hole), telling which extensions
       are available for this skb.
       This has two purposes.
       a) avoids the need to initialize the pointer.
       b) allows to "delete" an extension by clearing its bit
       value in ->active_extensions.
       While it would be possible to store the active_extensions byte
       in the extension struct instead of sk_buff, there is one problem
       with this:
        When an extension has to be disabled, we can always clear the
        bit in skb->active_extensions.  But in case it would be stored in the
        extension buffer itself, we might have to COW it first, if
        we are dealing with a cloned skb.  On kmalloc failure we would
        be unable to turn an extension off.
    2. extension pointer, located at the end of the sk_buff.
       If the active_extensions byte is 0, the pointer is undefined,
       it is not initialized on skb allocation.
    This adds extra code to skb clone and free paths (to deal with
    refcount/free of extension area) but this replaces similar code that
    manages skb->nf_bridge and skb->sp structs in the followup patches of
    the series.
    It is possible to add support for extensions that are not preseved on
    To do this, it would be needed to define a bitmask of all extensions that
    need copy/cow semantics, and change __skb_ext_copy() to check
    ->active_extensions & SKB_EXT_PRESERVE_ON_CLONE, then just set
    ->active_extensions to 0 on the new clone.
    This isn't done here because all extensions that get added here
    need the copy/cow semantics.
    Allocate entire extension space using kmem_cache.
    Upside is that this allows better tracking of used memory,
    downside is that we will allocate more space than strictly needed in
    most cases (its unlikely that all extensions are active/needed at same
    time for same skb).
    The allocated memory (except the small extension header) is not cleared,
    so no additonal overhead aside from memory usage.
    Avoid atomic_dec_and_test operation on skb_ext_put()
    by using similar trick as kfree_skbmem() does with fclone_ref:
    If recount is 1, there is no concurrent user and we can free right away.
    Signed-off-by: default avatarFlorian Westphal <fw@strlen.de>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
ip_output.c 40.2 KB