linux-kernel - Re: [PATCH] usbip: vhci_hcd: Mark expected switch fall-through

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Date:   Mon, 29 Apr 2019 10:05:51 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     David Laight <David.Laight@...LAB.COM>,
        Valentina Manea <valentina.manea.m@...il.com>,
        Shuah Khan <shuah@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     "linux-usb@...r.kernel.org" <linux-usb@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] usbip: vhci_hcd: Mark expected switch fall-through



On 4/29/19 9:44 AM, David Laight wrote:
> From: Gustavo A. R. Silva
>> Sent: 29 April 2019 15:40
>> In preparation to enabling -Wimplicit-fallthrough, mark switch
>> cases where we are expecting to fall through.
> ...
>> diff --git a/drivers/usb/usbip/vhci_hcd.c b/drivers/usb/usbip/vhci_hcd.c
>> index 667d9c0ec905..000ab7225717 100644
>> --- a/drivers/usb/usbip/vhci_hcd.c
>> +++ b/drivers/usb/usbip/vhci_hcd.c
>> @@ -508,6 +508,7 @@ static int vhci_hub_control(struct usb_hcd *hcd, u16 typeReq, u16 wValue,
>>  		case USB_PORT_FEAT_U1_TIMEOUT:
>>  			usbip_dbg_vhci_rh(
>>  				" SetPortFeature: USB_PORT_FEAT_U1_TIMEOUT\n");
>> +			/* Fall through */
>>  		case USB_PORT_FEAT_U2_TIMEOUT:
>>  			usbip_dbg_vhci_rh(
>>  				" SetPortFeature: USB_PORT_FEAT_U2_TIMEOUT\n");
> 
> That doesn't look right, both debug messages seem to get printed.
> 

At first sight, I thought the same way, then I took a look into
commit:

1c9de5bf428612458427943b724bea51abde520a

and noticed that the original developer properly added fall-through
comments in other places in the same switch() code, that gave me the
impression he knew what he was doing; then I noticed the following
error message in case USB_PORT_FEAT_U2_TIMEOUT:

	if (hcd->speed != HCD_USB3) {
		pr_err("USB_PORT_FEAT_U1/2_TIMEOUT req not "
		       "supported for USB 2.0 roothub\n");
		goto error;
	}

this error message is what makes me think the fall-through is
intentional; otherwise I think it would look like this instead:

	if (hcd->speed != HCD_USB3) {
		pr_err("USB_PORT_FEAT_U2_TIMEOUT req not "
		       "supported for USB 2.0 roothub\n");
		goto error;
	}

Thanks
--
Gustavo