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

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Date:   Fri, 26 Jul 2019 14:05:24 +0100
From:   Will Deacon <will@...nel.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Anders Roxell <anders.roxell@...aro.org>,
        Kees Cook <keescook@...omium.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        catalin.marinas@....com, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/3] arm64: perf: Mark expected switch fall-through

On Fri, Jul 26, 2019 at 01:38:24PM +0100, Robin Murphy wrote:
> On 26/07/2019 13:27, Will Deacon wrote:
> > 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.
> 
> Plus, AFAICS, a 1-byte watchpoint on a 2-byte-aligned address.
> 
> Not that I know anything about this code, but it does start to look like it
> might want rewriting without the offending switch statement anyway. At a
> glance, it looks like the intended semantic might boil down to:
> 
> 	if (hw->ctrl.len > offset)
> 		return -EINVAL;

Given that it's compat code, I think it's worth staying as close to the
arch/arm/ implementation as we can. Also, beware that the
ARM_BREAKPOINT_LEN_* definitions are masks because of the BAS fields in
the debug architecture.

Will