Re: [PATCH] tty: n_hdlc: fix duplicated prefix on partial writes

[sashiko-bot@kernel.org]

> n_hdlc_send_frames() passes the whole queued frame to tty->ops->write().
> When the driver accepts only a prefix and returns a positive short count,
> the current code puts the whole buffer back at the head of tx_buf_list
> without adjusting its length.  The wakeup callback later sends the frame
> again from byte 0, so the prefix already committed to the peer's flip
> buffer is transmitted twice:
> 
>   DUPLICATED PREFIX: magic occurrence 5 at offset 19968, frame index 4
>   drained 36864 bytes, target 28672
> 
> n_hdlc_tty_write() still returns the full count because the ldisc owns
> the queued frame; the short write is an internal transport event.  Drop
> the written prefix from the buffer and requeue only the remaining bytes,
> so the next wakeup continues where the driver stopped.  Full writes, the
> -ERESTARTSYS rollback, other errors, and a zero-byte write keep their
> existing behavior.  Clamp a driver report larger than the frame to the
> frame length.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM
> Signed-off-by: Cheng Yongkang <teel4res@gmail.com>

Sashiko has reviewed this patch and found no issues.

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