Date: Sat, 21 May 2016 21:54:22 -0500 From: Pedro Giffuni <pfg@freebsd.org> To: Bruce Evans <brde@optusnet.com.au> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r300384 - head/sys/compat/ndis Message-ID: <214223B1-89C7-403B-84F1-8F9B539D29C5@freebsd.org> In-Reply-To: <20160522112323.N1388@besplex.bde.org> References: <201605220029.u4M0TPI6050123@repo.freebsd.org> <20160522112323.N1388@besplex.bde.org>
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> Il giorno 21/mag/2016, alle ore 21:15, Bruce Evans = <brde@optusnet.com.au> ha scritto: >=20 > On Sun, 22 May 2016, Pedro F. Giffuni wrote: >=20 >> Log: >> ndis(4): adjustments for our random() specific implementation. >>=20 >> - Revert r300377: The implementation claims to return a value >> within the range. [1] >> - Adjust the value for the case of a zero seed, whihc according >> to standards should be equivalent to a seed of value 1. >=20 > This was already correct. Standards don't say that a seed of 0 for > srand() means the same as a seed of 1 passed to srand(). They say > that if srand() is not called, then the sequence shall be the same > as if it had been called with a seed of 1. This is already > implemented by using an internal initial value of the seed that is > the result of srand(1). >=20 OK. I misunderstood the standard. I will revert the if seed thing. (Tomorrow). > The commit gives the following bugs: > - srand() is less random. srand(0) now gives the same sequence as > srand(0) > - srand(0) is not binary compatible. >=20 > My previous mail pointed out the version in libc is the kernel version > with some bugs fixed. It also has some regressions. In the libc > version, srand(seed) sets the internal seed almost directly (it just > adds 1 for technical reasons). The kernel version advances the seed > through 50 iterations of random(). This doesn't really increase > randomness but it makes the linearity relation in the LCG less = obvious. >=20 Merging both is out of scope from what I expected to be a much simpler cleanup for ndis(4). I would have liked to replace completely random() with something like Mersenne Twister + unpredictable seed but that is basically a waste of time: developers seem sufficiently happy with random() and when not arc4random() can be used. :-/ Thanks for all this feedback. Pedro. >=20
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