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Subject: Re: svn commit: r297633 - in head:
 sys/cddl/contrib/opensolaris/uts/common/fs/zfs sys/fs/ext2fs sys/kern sys/sys
 sys/ufs/ffs sys/ufs/ufs sys/vm usr.bin/rctl
To: Edward Tomasz Napierala <trasz@FreeBSD.org>, src-committers@freebsd.org,
 svn-src-all@freebsd.org, svn-src-head@freebsd.org
References: <201604070423.u374NP0Z021115@repo.freebsd.org>
From: Pieter de Goeje <pieter@degoeje.nl>
Message-ID: <570762E4.6080706@degoeje.nl>
Date: Fri, 8 Apr 2016 09:51:00 +0200
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Op 2016-04-07 om 06:23 schreef Edward Tomasz Napierala:
> +static uint64_t
> +xmul(uint64_t a, uint64_t b)
> +{
> +	uint64_t c;
> +
> +	if (a == 0 || b == 0)
> +		return (0);
> +
> +	c = a * b;
> +
> +	if (c < a || c < b)
> +		return (UINT64_MAX);

If the intent is to check for overflow, then this check is insufficient. 
It fails for example if a = 2^32+1 and b = 2^32.

This works for all cases, assuming a != 0:

if(UINT64_MAX / a > b)
	return (UINT64_MAX);

If the extra division is too expensive, GCC and clang provide 
__builtin_mul_overflow().

--
Pieter de Goeje