linux-kernel - Re: [PATCH v2 14/22] arm64: Cleanup HWCAP handling

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20151008130026.GI17192@e104818-lin.cambridge.arm.com>
Date:	Thu, 8 Oct 2015 14:00:26 +0100
From:	Catalin Marinas <catalin.marinas@....com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	mark.rutland@....com, Vladimir.Murzin@....com,
	steve.capper@...aro.org, ard.biesheuvel@...aro.org,
	marc.zyngier@....com, andre.przywara@....com,
	"Suzuki K. Poulose" <suzuki.poulose@....com>, will.deacon@....com,
	linux-kernel@...r.kernel.org, edward.nevill@...aro.org,
	aph@...hat.com, james.morse@....com, dave.martin@....com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 14/22] arm64: Cleanup HWCAP handling

On Thu, Oct 08, 2015 at 12:17:09PM +0100, Russell King - ARM Linux wrote:
> On Thu, Oct 08, 2015 at 12:10:00PM +0100, Catalin Marinas wrote:
> > On Mon, Oct 05, 2015 at 06:02:03PM +0100, Suzuki K. Poulose wrote:
> > > +static bool cpus_have_hwcap(const struct arm64_cpu_capabilities *cap)
> > > +{
> > > +	switch(cap->hwcap_type) {
> > > +	case CAP_HWCAP:
> > > +		return !!(elf_hwcap & cap->hwcap);
> > > +#ifdef CONFIG_COMPAT
> > > +	case CAP_COMPAT_HWCAP:
> > > +		return !!(compat_elf_hwcap & (u32)cap->hwcap);
> > > +	case CAP_COMPAT_HWCAP2:
> > > +		return !!(compat_elf_hwcap2 & (u32)cap->hwcap);
> > > +#endif
> > > +	default:
> > > +		BUG();
> > > +		return false;
> > > +	}
> > > +}
> > 
> > Apart from the multiple returns, you don't really need !! since the
> > return type is bool already.
> 
> That's wrong.  a & b doesn't return 0 or 1, but the bitwise-and result.

a & b is indeed a bitwise operation and, in this particular case, its
type is an unsigned long. However, because the return type of the
function is a bool, the result of the bitwise operation (unsigned long)
is converted to a bool.

The above may be true only for gcc, I haven't checked other compilers,
nor the standard (AFAIK, it appeared in C99).

On AArch64, the compiler generates something like:

	tst	x0, x1
	cset	w0, ne
	ret

On AArch32, Thumb-2, I get:

	tst	r2, r3
	ite	ne
	movne	r0, #1
	moveq	r0, #0
	bx	lr

So a bool type function always returns 0 or 1 and does the appropriate
conversion.

> http://yarchive.net/comp/linux/bool.html
> 
> especially hpa's response.

This seems to be more about a union of int and bool rather than
automatic type conversion. But I can see in the simple test that Linus
did towards the end of the thread that x86 does something similar with
converting a char to a bool:

	testb	%al, %al
	setne	%al
	ret

I stand by my original comment.

-- 
Catalin
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/