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    Subject[PATCH 4.9 18/31] packet: fix reserve calculation
    Date
    4.9-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Willem de Bruijn <willemb@google.com>

    [ Upstream commit 9aad13b087ab0a588cd68259de618f100053360e ]

    Commit b84bbaf7a6c8 ("packet: in packet_snd start writing at link
    layer allocation") ensures that packet_snd always starts writing
    the link layer header in reserved headroom allocated for this
    purpose.

    This is needed because packets may be shorter than hard_header_len,
    in which case the space up to hard_header_len may be zeroed. But
    that necessary padding is not accounted for in skb->len.

    The fix, however, is buggy. It calls skb_push, which grows skb->len
    when moving skb->data back. But in this case packet length should not
    change.

    Instead, call skb_reserve, which moves both skb->data and skb->tail
    back, without changing length.

    Fixes: b84bbaf7a6c8 ("packet: in packet_snd start writing at link layer allocation")
    Reported-by: Tariq Toukan <tariqt@mellanox.com>
    Signed-off-by: Willem de Bruijn <willemb@google.com>
    Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/packet/af_packet.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/net/packet/af_packet.c
    +++ b/net/packet/af_packet.c
    @@ -2918,7 +2918,7 @@ static int packet_snd(struct socket *soc
    if (unlikely(offset < 0))
    goto out_free;
    } else if (reserve) {
    - skb_push(skb, reserve);
    + skb_reserve(skb, -reserve);
    }

    /* Returns -EFAULT on error */

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