Re: N_HDLC line discipline: Race condition

[Dianne Skoll <dianne@skoll.ca>]

On Tue, 21 May 2024 12:47:00 +0200
Jiri Slaby <jirislaby@kernel.org> wrote:

> I believe it is a correct behavior after all. As you use pty for 
> testing, the "framing" is lost during the pty-to-pty pass on the
> flush to ldisc path (receive_buf()).

That might be what's happening, but I don't think it matches the documentation
in n_hdlc.c.  If you read the comment block near the top of the file,
it says this:

 * All HDLC data is frame oriented which means:
 *
 * 1. tty write calls represent one complete transmit frame of data
 *    The device driver should accept the complete frame or none of
 *    the frame (busy) in the write method. Each write call should have
 *    a byte count in the range of 2-65535 bytes (2 is min HDLC frame
 *    with 1 addr byte and 1 ctrl byte). The max byte count of 65535
 *    should include any crc bytes required. For example, when using
 *    CCITT CRC32, 4 crc bytes are required, so the maximum size frame
 *    the application may transmit is limited to 65531 bytes. For CCITT
 *    CRC16, the maximum application frame size would be 65533.
 *
 *
 * 2. receive callbacks from the device driver represents
 *    one received frame. The device driver should bypass
 *    the tty flip buffer and call the line discipline receive
 *    callback directly to avoid fragmenting or concatenating
 *    multiple frames into a single receive callback.
 *
 *    The HDLC line discipline queues the receive frames in separate
 *    buffers so complete receive frames can be returned by the
 *    tty read calls.
 *
 * 3. tty read calls returns an entire frame of data or nothing.
[...] */

Point 2 says that the driver should avoid fragmenting frames, or concatenating
frames into a single receive callback.  Doesn't this imply that frame
boundaries should be preserved when you read() data, which happens reliably
when you add a small delay between write()'s?

Regards,

Dianne.