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

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Date:   Thu, 2 May 2019 08:22:30 -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 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;



Thanks
--
Gustavo