Re: [PATCH net v9 05/14] rxrpc: Fix sendmsg length

[netdev-bot+sashiko@kernel.org]

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- [Low] "rxrpc: Fix sendmsg length" carries a `Fixes:` tag and `cc:
  stable@vger.kernel.org` while its own commit message states…

--

rxrpc: Fix sendmsg length

This makes rxrpc_send_data() consistently use its len argument rather than
msg->msg_iter.count, decrementing len as data is copied, and limits each
copy to the smaller of len and msg_data_left(msg).

> diff --git a/net/rxrpc/sendmsg.c b/net/rxrpc/sendmsg.c
> index 1d66e98081624..565799548102c 100644
> --- a/net/rxrpc/sendmsg.c
> +++ b/net/rxrpc/sendmsg.c
> @@ -379,9 +379,9 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
>  
>  	ret = -EMSGSIZE;
>  	if (call->tx_total_len != -1) {
> -		if (len - copied > call->tx_total_len)
> +		if (len > call->tx_total_len)
>  			goto maybe_error;
> -		if (!more && len - copied != call->tx_total_len)
> +		if (!more && len != call->tx_total_len)
>  			goto maybe_error;
>  	}

[Severity: Low]
This isn't a bug in the code, but a question about the tagging.  The
commit carries:

    Fixes: 382d7974de31 ("RxRPC: Use iov_iter_count() in rxrpc_send_data() instead of the len argument")
    cc: stable@vger.kernel.org

while the message itself says:

    Note that, currently, all the callers have len and msg->msg_iter.count the
    same and so the problem won't occur.

Walking the callers agrees with that statement.  rxrpc_do_sendmsg()
forwards the sendmsg() len that the syscall also used to build msg_iter.
In fs/afs/rxrpc.c, afs_make_call() and afs_send_simple_reply() pass
msg_data_left(&msg):

	ret = rxrpc_kernel_send_data(call->net->socket, rxcall, &msg,
				     msg_data_left(&msg),
				     afs_notify_end_request_tx);

and afs_send_empty_reply() passes 0 for a zero-length iterator.  In
net/rxrpc/rxperf.c, rxperf_process_call() passes the same local len used
to build the iterator:

		iov_iter_bvec(&msg.msg_iter, WRITE, &bv, 1, len);
		msg.msg_flags = MSG_MORE;
		n = rxrpc_kernel_send_data(rxperf_socket, call->rxcall, &msg,
					   len, rxperf_notify_end_reply_tx);

So no in-tree caller can diverge.  Given there is no symptom, reproducer
or affected caller named, are the Fixes: and cc: stable tags intended
here?

What stable trees would pick up instead is a rewrite of the sendmsg data
loop: len becomes mutable and is decremented in the loop, the copy bound
becomes min3(), the loop termination test gains a second clause, and the
-EMSGSIZE re-check that is re-executed via the reload label now tests the
decremented len instead of len - copied.

>  
> @@ -405,7 +405,7 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
>  			 * the security header is going to be in the padded
>  			 * region (enc blocksize), but the trailer is not.
>  			 */
> -			remain = more ? INT_MAX : msg_data_left(msg);
> +			remain = more ? INT_MAX : len;
>  			txb = call->conn->security->alloc_txbuf(call, remain, sk->sk_allocation);
>  			if (!txb) {
>  				ret = -ENOMEM;
> @@ -416,8 +416,8 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
>  		_debug("append");
>  
>  		/* append next segment of data to the current buffer */
> -		if (msg_data_left(msg) > 0) {
> -			size_t copy = umin(txb->space, msg_data_left(msg));
> +		if (len > 0) {
> +			size_t copy = min3(txb->space, len, msg_data_left(msg));
>  
>  			_debug("add %zu", copy);
>  			if (!copy_from_iter_full(txb->data + txb->offset,
> @@ -428,6 +428,7 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
>  			txb->len += copy;
>  			txb->offset += copy;
>  			copied += copy;
> +			len -= copy;
>  			if (call->tx_total_len != -1)
>  				call->tx_total_len -= copy;
>  		}
> @@ -439,8 +440,8 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
>  
>  		/* add the packet to the send queue if it's now full */
>  		if (!txb->space ||
> -		    (msg_data_left(msg) == 0 && !more)) {
> -			if (msg_data_left(msg) == 0 && !more)
> +		    (len == 0 && !more)) {
> +			if (len == 0 && !more)
>  				txb->flags |= RXRPC_LAST_PACKET;
>  
>  			ret = call->security->secure_packet(call, txb);
> @@ -449,7 +450,7 @@ static int rxrpc_send_data(struct rxrpc_sock *rx,
>  			rxrpc_queue_packet(rx, call, txb, notify_end_tx);
>  			txb = NULL;
>  		}
> -	} while (msg_data_left(msg) > 0);
> +	} while (len > 0 && msg_data_left(msg) > 0);

Making len mutable also appears to be a prerequisite for the later patch
in the series, "rxrpc: Fix packet encryption error handling", whose rewind
path tests:

	if (copied && !more && !len) {

which only behaves as intended once len is decremented here.  Could that
dependency be mentioned in the commit message?

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