net/tcp: Fix clear condition in ofoseg input

[wengzhe]

Summary

Fix clear condition in TCP ofoseg input (when receiving an out-of-ordered ACK)

We have a case that an http server gives out-of-ordered ACKs, and NuttX client makes ofosegs with length 0, trying to rebuild / put them into ofosegs array, which is not intended (no available data and should be skipped). This breaks later logic and finally crashed in tcp_ofoseg_bufsize (ofosegs[i].data is NULL, which should never happen in normal logic).

Note:

• iob_trimhead won't return NULL when it's applying on normal IOB.
  • Keep dev->d_iob == NULL to avoid iob_trimhead changed.
• iob_free_chain will do nothing when applied to NULL.

Patch also included:

• mm/iob: Don't return NULL in iob_pack
  • The direct reason which causes ofosegs[i].data become NULL in tcp_ofoseg_bufsize when there are several ofosegs with length 0.

Impact

TCP Out-of-order segment process, especially when receiving an out-of-ordered ACK.

Testing

Manually.

[alancassis]

@wengzhe please explain the issue you were facing (like you started to do in the Testing comment: "We have a case that an http server gives an out-of-ordered ACK, and triggers problem on NuttX client."). Also please include it in the log message

[wengzhe]

@wengzhe please explain the issue you were facing (like you started to do in the Testing comment: "We have a case that an http server gives an out-of-ordered ACK, and triggers problem on NuttX client."). Also please include it in the log message

Done.

[xiaoxiang781216]

@wengzhe please rebase your change.