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

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Date:   Fri, 26 Jul 2019 13:13:54 +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 01:10:57PM +0100, Mark Rutland wrote:
> 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.

Hmm... I now see I spoke too soon, and this is making the 1-byte
breakpoint work at a 3-byte offset.

Given that:

Acked-by: Mark Rutland <mark.rutland@....com>

... and the fixes and stable tags are appropriate for that portion of
the patch.

Sorry for the noise.

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
> >