052a5418e8
Not all mbufs passed up from device drivers are M_WRITABLE(). In particular, the Chelsio T4/T5 driver uses a feature called "buffer packing" to receive multiple frames in a single receive buffer. The mbufs for these frames all share the same external storage so are treated as read-only by the rest of the stack when multiple frames are in flight. Previously tcp_respond() would blindly overwrite read-only mbufs when INVARIANTS was disabled or panic with an assertion failure if INVARIANTS was enabled. Note that the new case is a bit of a mix of the two other cases in tcp_respond(). The TCP and IP headers must be copied explicitly into the new mbuf instead of being inherited (similar to the m == NULL case), but the addresses and ports must be swapped in the reply (similar to the m != NULL case). Reviewed by: glebius
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