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

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Date:   Sat, 28 Oct 2017 18:47:39 -0500
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Bjørn Mork <bjorn@...k.no>
Cc:     Johan Hovold <johan@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] USB: serial: io_edgeport: mark expected switch
 fall-throughs


Quoting Bjørn Mork <bjorn@...k.no>:

> "Gustavo A. R. Silva" <garsilva@...eddedor.com> writes:
>
>> In preparation to enabling -Wimplicit-fallthrough, mark switch cases
>> where we are expecting to fall through.
>>
>> Notice that in this particular case I replaced "...drop on through"
>> comments with a proper "fall through" comment on its own line, which
>> is what GCC is expecting to find.
>
> Sounds to me like GCC is the wrong tool for this.  But I would of course
> not mind if it was *just* the text.  However, as your patch cleary
> shows, the simplified logic leads to real problems:
>
>> @@ -1819,8 +1819,8 @@ static void process_rcvd_data(struct  
>> edgeport_serial *edge_serial,
>>  					edge_serial->rxState = EXPECT_DATA;
>>  					break;
>>  				}
>> -				/* Else, drop through */
>>  			}
>> +			/* fall through */
>>  		case EXPECT_DATA: /* Expect data */
>>  			if (bufferLength < edge_serial->rxBytesRemaining) {
>>  				rxLen = bufferLength;
>
>
> The original comment clearly marked a *conditional* fall through at the
> correct place.  Your patch makes it appear as if there is an
> unconditional fall through here.  There is not.  The fallthrough only
> applies to one of a number of nested if blocks. There are no less than
> 3 break statements in the same case block.
>

I see.
You are right.

> Not a big deal maybe, just as the lack of any "fall through" comment
> isn't a big deal in the first place.  But this change is clearly making
> this code harder to read, and the change is therefore harmful IMHO.
>
> If you can't make -Wimplicit-fallthrough work without removing such
> precise fallthrough markings, then my proposal is to drop it and use
> some other tool.
>

I will talk with the hardening guys to see what we can do about this.

I appreciate for your comments.
Thanks
--
Gustavo A. R. Silva