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

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Date:   Fri, 26 Jul 2019 13:27:29 +0100
From:   Will Deacon <will@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Anders Roxell <anders.roxell@...aro.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 01:13:54PM +0100, Mark Rutland wrote:
> 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.

I still don't think it's quite right though, since it forbids a 2-byte
watchpoint on a byte-aligned address.

I think the arm64 code matches what we had on 32-bit prior to
d968d2b801d8 ("ARM: 7497/1: hw_breakpoint: allow single-byte watchpoints
on all addresses"), so we should have one patch bringing us up to speed
with that change, and then another annotating the fallthroughs.

Will