Re: [PATCH net-next 03/12] net: mctp: usblib: Move RX transfer processing to a new mctp-usblib

[Paolo Abeni <pabeni@redhat.com>]

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net: mctp: usblib: Move RX transfer processing to a new mctp-usblib

This commit extracts common USB receive transfer processing for MCTP into a
new library file (mctp-usblib.c). This is intended to support both the
current host-side driver and a future gadget driver.

> diff --git a/drivers/net/mctp/mctp-usblib.c b/drivers/net/mctp/mctp-usblib.c
> new file mode 100644
> index 0000000000000..9b86eb4310ce3
> --- /dev/null
> +++ b/drivers/net/mctp/mctp-usblib.c
[ ... ]
> +/*
> + * Prepare a transfer buffer for future completion; *bufp and *lenp will
> + * be populated on success.
> + */
> +int mctp_usblib_rx_prepare(struct net_device *netdev,
> +			   struct mctp_usblib_rx *rx,
> +			   void **bufp, size_t *lenp, gfp_t gfp)
> +{
> +	const unsigned int len = MCTP_USB_1_0_XFER_SIZE;
> +	struct sk_buff *skb;
> +
> +	skb = __netdev_alloc_skb(netdev, len, gfp);
> +	if (!skb)
> +		return -ENOMEM;
> +
> +	rx->skb = skb;
> +
> +	*bufp = skb_tail_pointer(skb);
> +	*lenp = skb_tailroom(skb);

[Severity: High]
Does assigning the full skb_tailroom() here create a risk of a USB host
controller hang?

Because of slab alignment and padding, skb_tailroom() often exceeds the
exact requested allocation length (e.g., returning 640 bytes instead of
512). If the MCTP device sends exactly a full USB High-Speed packet
(512 bytes) and does not append a Zero Length Packet (ZLP), the USB
host controller might not terminate the transfer since 512 is a multiple
of wMaxPacketSize and the requested 640 bytes haven't been met.

Could this instead use the exact protocol max transfer size (e.g., passing
the len variable instead of skb_tailroom(skb)) to ensure the controller
completes the transfer correctly?

> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(mctp_usblib_rx_prepare);
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