linux-kernel - Re: [PATCH] ARC: Fix _

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: <C2D7FE5348E1B147BCA15975FBA23075140F26@IN01WEMBXA.internal.synopsys.com>
Date:	Thu, 29 Aug 2013 05:55:11 +0000
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Mischa Jonker <Mischa.Jonker@...opsys.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Joern Rennecke" <joern.rennecke@...ecosm.com>,
	joe perches <joe@...ches.com>
Subject: Re: [PATCH] ARC: Fix __udelay parentheses

On 08/29/2013 12:00 AM, Mischa Jonker wrote:
> Make sure that usecs is casted to long long, to ensure that the
> (usecs * 4295 * HZ) multiplication is 64 bit.
>
> Initially, the (usecs * 4295 * HZ) part was done as a 32 bit
> multiplication, with the result casted to 64 bit. This led to some bits
> falling off.
>
> Signed-off-by: Mischa Jonker <mjonker@...opsys.com>
> ---
>  arch/arc/include/asm/delay.h |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arc/include/asm/delay.h b/arch/arc/include/asm/delay.h
> index 442ce5d..8d35fe1 100644
> --- a/arch/arc/include/asm/delay.h
> +++ b/arch/arc/include/asm/delay.h
> @@ -56,8 +56,8 @@ static inline void __udelay(unsigned long usecs)
>  	/* (long long) cast ensures 64 bit MPY - real or emulated
>  	 * HZ * 4295 is pre-evaluated by gcc - hence only 2 mpy ops
>  	 */
> -	loops = ((long long)(usecs * 4295 * HZ) *
> -		 (long long)(loops_per_jiffy)) >> 32;
> +	loops = (((long long) usecs) * 4295 * HZ *
> +		  (long long) loops_per_jiffy) >> 32;
>  
>  	__delay(loops);
>  }

The intent of writing orig code was to generate only 1 MPYHU insn (32*32 =
high-part-64) for the whole math, at any optimization level whatsoever. If the
first MPY is overflowing, u r likely spinning for > 10,000 usec (10ms) which is 1
scheduling tick on ARC - not good - presumably for hardware debug. It would be
better to use a tight loop there and throw it out later.

The API abuse would only be caught for const @usecs case. Maybe we need to add a
WARN_ON() there.

OTOH, if we really want to fix this, it would be cleaner to rewrite this as
    loops = ((u64)usecs * 4295 * HZ * loops_per_jiffy) >> 32;

Since one factor is upcasted, all are promoted to 64 bit. And we leave the
optimizations to whims of gcc.

@Joern, I would assume that long long vs u64 (or unsigned long long) doesn't
matter in this particular case.

-Vineet
--
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/