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

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Date:   Thu, 2 May 2019 15:56:36 +0200
From:   Johan Hovold <johan@...nel.org>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Johan Hovold <johan@...nel.org>,
        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 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?

Johan