Re: [PATCH v1] can: j1939: j1939_sk_send_loop_abort(): improved error queue handling in J1939 Socket

[Marc Kleine-Budde]

On 26.05.2023 10:19:46, Oleksij Rempel wrote:
> This patch addresses an issue within the j1939_sk_send_loop_abort()
> function in the j1939/socket.c file, specifically in the context of
> Transport Protocol (TP) sessions.
> 
> Without this patch, when a TP session is initiated and a Clear To Send
> (CTS) frame is received from the remote side requesting one data packet,
> the kernel dispatches the first Data Transport (DT) frame and then waits
> for the next CTS. If the remote side doesn't respond with another CTS,
> the kernel aborts due to a timeout. This leads to the user-space
> receiving an EPOLLERR on the socket, and the socket becomes active.
> 
> However, when trying to read the error queue from the socket with
> sock.recvmsg(, , socket.MSG_ERRQUEUE), it returns -EAGAIN,
> given that the socket is non-blocking. This situation results in an
> infinite loop: the user-space repeatedly calls epoll(), epoll() returns
> the socket file descriptor with EPOLLERR, but the socket then blocks on
> the recv() of ERRQUEUE.
> 
> This patch introduces an additional check for the J1939_SOCK_ERRQUEUE
> flag within the j1939_sk_send_loop_abort() function. If the flag is set,
> it indicates that the application has subscribed to receive error queue
> messages. In such cases, the kernel can communicate the current transfer
> state via the error queue. This allows for the function to return early,
> preventing the unnecessary setting of the socket into an error state,
> and breaking the infinite loop. It is crucial to note that a socket
> error is only needed if the application isn't using the error queue, as,
> without it, the application wouldn't be aware of transfer issues.
> 
> Fixes: 9d71dd0c7009 ("can: add support of SAE J1939 protocol")
> Reported-by: David Jander <david@xxxxxxxxxxx>
> Tested-by: David Jander <david@xxxxxxxxxxx>
> Signed-off-by: Oleksij Rempel <o.rempel@xxxxxxxxxxxxxx>

Applied to linux-can, added stable on Cc.

Thanks,
Marc

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