linux-kernel - Re: [PATCH 1/3] arm64: perf: Mark expected switch fall-through

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Message-ID: <20190726121056.GA26088@lakrids.cambridge.arm.com>
Date:   Fri, 26 Jul 2019 13:10:57 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Anders Roxell <anders.roxell@...aro.org>
Cc:     will@...nel.org, catalin.marinas@....com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 1/3] arm64: perf: Mark expected switch fall-through

On Fri, Jul 26, 2019 at 01:27:16PM +0200, Anders Roxell wrote:
> When fall-through warnings was enabled by default, commit d93512ef0f0e
> ("Makefile: Globally enable fall-through warning"), the following
> warnings was starting to show up:
> 
> ../arch/arm64/kernel/hw_breakpoint.c: In function ‘hw_breakpoint_arch_parse’:
> ../arch/arm64/kernel/hw_breakpoint.c:540:7: warning: this statement may fall
>  through [-Wimplicit-fallthrough=]
>     if (hw->ctrl.len == ARM_BREAKPOINT_LEN_1)
>        ^
> ../arch/arm64/kernel/hw_breakpoint.c:542:3: note: here
>    case 2:
>    ^~~~
> ../arch/arm64/kernel/hw_breakpoint.c:544:7: warning: this statement may fall
>  through [-Wimplicit-fallthrough=]
>     if (hw->ctrl.len == ARM_BREAKPOINT_LEN_2)
>        ^
> ../arch/arm64/kernel/hw_breakpoint.c:546:3: note: here
>    default:
>    ^~~~~~~
> 
> Rework so that the compiler doesn't warn about fall-through. Rework so
> the code looks like the arm code. Since the comment in the function
> indicates taht this is supposed to behave the same way as arm32 because

Typo: s/taht/that/

> it handles 32-bit tasks also.
> 
> Cc: stable@...r.kernel.org # v3.16+
> Fixes: 6ee33c2712fc ("ARM: hw_breakpoint: correct and simplify alignment fixup code")
> Signed-off-by: Anders Roxell <anders.roxell@...aro.org>

The patch itself looks fine, but I don't think this needs a CC to
stable, nor does it require that fixes tag, as there's no functional
problem.

If anything, it fixes:

  d93512ef0f0e (" Makefile: Globally enable fall-through warning")

... given the commit message for that patch states:

  Now that all the fall-through warnings have been addressed in the
  kernel, enable the fall-through warning globally.

... and the existence of this patch implies otherwise.

IIUC that patch isn't even in mainline yet, but given this is simple I
imagine that Will and Catalin might be happy to pick this up for the
next rc.

Thanks,
Mark.

> ---
>  arch/arm64/kernel/hw_breakpoint.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
> index dceb84520948..ea616adf1cf1 100644
> --- a/arch/arm64/kernel/hw_breakpoint.c
> +++ b/arch/arm64/kernel/hw_breakpoint.c
> @@ -535,14 +535,17 @@ int hw_breakpoint_arch_parse(struct perf_event *bp,
>  		case 0:
>  			/* Aligned */
>  			break;
> -		case 1:
> -			/* Allow single byte watchpoint. */
> -			if (hw->ctrl.len == ARM_BREAKPOINT_LEN_1)
> -				break;
>  		case 2:
>  			/* Allow halfword watchpoints and breakpoints. */
>  			if (hw->ctrl.len == ARM_BREAKPOINT_LEN_2)
>  				break;
> +			/* Fall through */
> +		case 1:
> +		case 3:
> +			/* Allow single byte watchpoint. */
> +			if (hw->ctrl.len == ARM_BREAKPOINT_LEN_1)
> +				break;
> +			/* Fall through */
>  		default:
>  			return -EINVAL;
>  		}
> -- 
> 2.20.1
>