linux-kernel - Re: [PATCHv3 1/5] efi/runtime-wrappers: detect FW irq flag corruption

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Message-ID: <20160425142735.GA14974@leverpostej>
Date:	Mon, 25 Apr 2016 15:27:35 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Matt Fleming <matt@...eblueprint.co.uk>
Cc:	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	Robin Murphy <robin.murphy@....com>,
	"linux-efi@...r.kernel.org" <linux-efi@...r.kernel.org>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCHv3 1/5] efi/runtime-wrappers: detect FW irq flag corruption

On Mon, Apr 25, 2016 at 03:24:35PM +0100, Matt Fleming wrote:
> On Mon, 25 Apr, at 04:18:41PM, Ard Biesheuvel wrote:
> > On 25 April 2016 at 16:15, Matt Fleming <matt@...eblueprint.co.uk> wrote:
> > > On Mon, 25 Apr, at 03:12:01PM, Robin Murphy wrote:
> > >> >+static void efi_call_virt_check_flags(unsigned long flags, const char *call)
> > >> >+{
> > >> >+    unsigned long cur_flags;
> > >> >+    bool mismatch;
> > >> >+
> > >> >+    local_save_flags(cur_flags);
> > >> >+
> > >> >+    mismatch = !!((cur_flags ^ flags) & ARCH_EFI_IRQ_FLAGS_MASK);
> > >>
> > >> nit: the assignment itself is already a conversion to bool, so the
> > >> excitement is redundant here.
> > >
> > > This was intentional. I asked Mark to make this change so that it's
> > > explicit for the developer that we're performing the type conversion.
> > 
> > But replacing an implicit boolean cast with an explicit one makes
> > little sense, no? Don't we simply want '!= 0' here if you need a
> > boolean expression?
> 
> Aha but '!!' is fewer characters to type!!
> 
> I'm not that bothered as long as we don't stuff an int into a bool
> without giving the programmer some idea we're doing that. It's not
> about the compiler getting it wrong, more about a developer
> introducing a bug when they change the code in the future. 
> 
> Unless anyone objects, I'll fix this up to use '!= 0' when I apply it.

I have no strong preference so long as the code is correct.

Another option is to get rid of the bool entirely:

	flags ^= cur_flags;
	if (!WARN_ON(flags & ARCH_EFI_IRQ_FLAGS_MASK))
		return;

Mark.