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    Subject[PATCH AUTOSEL 4.9 04/19] net: caif: remove BUG_ON(dev == NULL) in caif_xmit
    Date
    From: Du Cheng <ducheng2@gmail.com>

    [ Upstream commit 65a67792e3416f7c5d7daa47d99334cbb19a7449 ]

    The condition of dev == NULL is impossible in caif_xmit(), hence it is
    for the removal.

    Explanation:
    The static caif_xmit() is only called upon via a function pointer
    `ndo_start_xmit` defined in include/linux/netdevice.h:
    ```
    struct net_device_ops {
    ...
    netdev_tx_t (*ndo_start_xmit)(struct sk_buff *skb, struct net_device *dev);
    ...
    }
    ```

    The exhausive list of call points are:
    ```
    drivers/net/ethernet/qualcomm/rmnet/rmnet_map_command.c
    dev->netdev_ops->ndo_start_xmit(skb, dev);
    ^ ^

    drivers/infiniband/ulp/opa_vnic/opa_vnic_netdev.c
    struct opa_vnic_adapter *adapter = opa_vnic_priv(netdev);
    ^ ^
    return adapter->rn_ops->ndo_start_xmit(skb, netdev); // adapter would crash first
    ^ ^

    drivers/usb/gadget/function/f_ncm.c
    ncm->netdev->netdev_ops->ndo_start_xmit(NULL, ncm->netdev);
    ^ ^

    include/linux/netdevice.h
    static inline netdev_tx_t __netdev_start_xmit(...
    {
    return ops->ndo_start_xmit(skb, dev);
    ^
    }

    const struct net_device_ops *ops = dev->netdev_ops;
    ^
    rc = __netdev_start_xmit(ops, skb, dev, more);
    ^
    ```

    In each of the enumerated scenarios, it is impossible for the NULL-valued dev to
    reach the caif_xmit() without crashing the kernel earlier, therefore `BUG_ON(dev ==
    NULL)` is rather useless, hence the removal.

    Cc: David S. Miller <davem@davemloft.net>
    Signed-off-by: Du Cheng <ducheng2@gmail.com>
    Link: https://lore.kernel.org/r/20210503115736.2104747-20-gregkh@linuxfoundation.org
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    drivers/net/caif/caif_serial.c | 1 -
    1 file changed, 1 deletion(-)

    diff --git a/drivers/net/caif/caif_serial.c b/drivers/net/caif/caif_serial.c
    index c2dea4916e5d..32834dad0b83 100644
    --- a/drivers/net/caif/caif_serial.c
    +++ b/drivers/net/caif/caif_serial.c
    @@ -281,7 +281,6 @@ static int caif_xmit(struct sk_buff *skb, struct net_device *dev)
    {
    struct ser_device *ser;

    - BUG_ON(dev == NULL);
    ser = netdev_priv(dev);

    /* Send flow off once, on high water mark */
    --
    2.30.2
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