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Grepping the function name shows call sites with NULL getting passed as skb. You may see that we do check skb != NULL before we timestamp it. But later in the process we depend skb == NULL implying that tx_fixup != NULL That is the combination that must not happen. If it happens, though simply bailing out seems to the wrong answer. > Yes And I just learned that during the cdc_ncm_driver.probe, it is possible to pass a NULL SKB to usbnet_start_xmit(). How can that happen? And if it happens is tx_fixup set? Regards Oliver