linux-kernel - Re: [PATCH v2] USB: serial: io_edgeport: mark expected switch fall-throughs

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Date:   Thu, 2 May 2019 09:28:37 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Johan Hovold <johan@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2] USB: serial: io_edgeport: mark expected switch
 fall-throughs



On 5/2/19 8:56 AM, Johan Hovold wrote:
> On Thu, May 02, 2019 at 08:22:30AM -0500, Gustavo A. R. Silva wrote:
>>
>>
>> On 5/2/19 5:26 AM, Johan Hovold wrote:
>>> On Wed, May 01, 2019 at 04:33:29PM -0500, Gustavo A. R. Silva wrote:
>>>> In preparation to enabling -Wimplicit-fallthrough, mark switch
>>>> cases where we are expecting to fall through.
>>>>
>>>> This patch fixes the following warnings:
>>>>
>>>> drivers/usb/serial/io_edgeport.c: In function ‘process_rcvd_data’:
>>>> drivers/usb/serial/io_edgeport.c:1750:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
>>>>     if (bufferLength == 0) {
>>>>        ^
>>>> drivers/usb/serial/io_edgeport.c:1755:3: note: here
>>>>    case EXPECT_HDR2:
>>>>    ^~~~
>>>> drivers/usb/serial/io_edgeport.c:1810:8: warning: this statement may fall through [-Wimplicit-fallthrough=]
>>>>      if (bufferLength == 0) {
>>>>         ^
>>>> drivers/usb/serial/io_edgeport.c:1816:3: note: here
>>>>    case EXPECT_DATA: /* Expect data */
>>>>    ^~~~
>>>>
>>>> Warning level 3 was used: -Wimplicit-fallthrough=3
>>>>
>>>> Notice that, in this particular case, the code comments are modified
>>>> in accordance with what GCC is expecting to find.
>>>>
>>>> This patch is part of the ongoing efforts to enable
>>>> -Wimplicit-fallthrough.
>>>>
>>>> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
>>>> ---
>>>> Changes in v2:
>>>>  - Warning level 3 is now used: -Wimplicit-fallthrough=3
>>>>    instead of warning level 2.
>>>>  - All warnings in the switch statement are addressed now.
>>>>
>>>> Notice that these are the last remaining fall-through warnings
>>>> in the USB subsystem. :)
>>>
>>>>  drivers/usb/serial/io_edgeport.c | 3 ++-
>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/usb/serial/io_edgeport.c b/drivers/usb/serial/io_edgeport.c
>>>> index 4ca31c0e4174..7ad10328f4e2 100644
>>>> --- a/drivers/usb/serial/io_edgeport.c
>>>> +++ b/drivers/usb/serial/io_edgeport.c
>>>> @@ -1751,7 +1751,7 @@ static void process_rcvd_data(struct edgeport_serial *edge_serial,
>>>>  				edge_serial->rxState = EXPECT_HDR2;
>>>>  				break;
>>>>  			}
>>>> -			/* otherwise, drop on through */
>>>> +			/* Fall through - otherwise, drop on through */
>>>>  		case EXPECT_HDR2:
>>>>  			edge_serial->rxHeader2 = *buffer;
>>>>  			++buffer;
>>>> @@ -1813,6 +1813,7 @@ static void process_rcvd_data(struct edgeport_serial *edge_serial,
>>>>  				}
>>>>  				/* Else, drop through */
>>>>  			}
>>>> +			/* Fall through */
>>>>  		case EXPECT_DATA: /* Expect data */
>>>
>>> Looks like you forgot to take the original review feedback you got into
>>> account:
>>>
>>> 	https://lkml.kernel.org/r/87k1zf4k24.fsf@miraculix.mork.no
>>>
>>
>> Oh, the thing is that the fall-through comments have to be placed at
>> the very bottom of the case. Also, based on that feedback, this time
>> I left the "Else, drop through" comment in place, so people can be
>> informed that such fall-through is conditional.
>>
>> What do you think about this:
>>
>> diff --git a/drivers/usb/serial/io_edgeport.c b/drivers/usb/serial/io_edgeport.c
>> index 4ca31c0e4174..52f27fc82563 100644
>> --- a/drivers/usb/serial/io_edgeport.c
>> +++ b/drivers/usb/serial/io_edgeport.c
>> @@ -1751,7 +1751,7 @@ static void process_rcvd_data(struct edgeport_serial *edge_serial,
>>                                 edge_serial->rxState = EXPECT_HDR2;
>>                                 break;
>>                         }
>> -                       /* otherwise, drop on through */
>> +                       /* Fall through - otherwise, drop on through */
>>                 case EXPECT_HDR2:
>>                         edge_serial->rxHeader2 = *buffer;
>>                         ++buffer;
>> @@ -1813,6 +1813,11 @@ static void process_rcvd_data(struct edgeport_serial *edge_serial,
>>                                 }
>>                                 /* Else, drop through */
>>                         }
>> +                       /* Beware that, currently, there are at least three
>> +                        * break statements in this case block, so the
>> +                        * fall-through marked below is NOT unconditional.
>> +                        */
>> +                       /* Fall through */
>>                 case EXPECT_DATA: /* Expect data */
>>                         if (bufferLength < edge_serial->rxBytesRemaining) {
>>                                 rxLen = bufferLength;
> 
> It's better than v2, but I thought you said you were gonna look into
> restructuring the code to maintain (or even improve) readability?
> 

At first, I thought about that, but now I don't think that's realistic.
I'd turn the if-else into a switch, and based on the history of feedback
on this patch, we will end up having the same complains about the break
statements in that new switch and the possibility of a fall-through to
case EXPECT_DATA. At the end I would still have to add a comment explaining
that the last fall-through mark in unconditional.

Thanks
--
Gustavo