Date: Mon, 20 Dec 2004 17:47:54 -0800 From: Brooks Davis <brooks@one-eyed-alien.net> To: "James R. Van Artsalen" <james@jrv.org> Cc: freebsd-amd64@freebsd.org Subject: Re: /usr/games/random bug? Message-ID: <20041221014754.GA13070@odin.ac.hmc.edu> In-Reply-To: <41C778CD.1080602@jrv.org>
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[-- Attachment #1 --] On Mon, Dec 20, 2004 at 07:13:49PM -0600, James R. Van Artsalen wrote: > 5-stable "/usr/games/random -e 99" seems to always return 0, which makes > it very predictable. > > 5.2.1 does this too. But i386 4.x seems to work as expected (an exit > value between 0 and 98 inclusive). > > The source code line in question is > > return (int)((denom * random()) / LONG_MAX); > > the assembler code is > > .p2align 3 > .LC17: > .long 0 > .long 1006632960 > > call random > cvtsi2sdq %rax, %xmm0 > mulsd 24(%rsp), %xmm0 > mulsd .LC17(%rip), %xmm0 > cvttsd2si %xmm0, %eax > jmp .L1 > > I'm not familiar enough with IEEE fp rules or amd64 fp coding to say why > this doesn't work, nor if this source code is even reasonable. I believe the code is bogus. The code says, compute a long (denom*random) then divide it by the largest possiable value you can store in a long. Since integer math in C truncates, the value should always be 0 unless denom*random() happens to be LONG_MAX in which case it will be 1. It's intresting that this code ever worked. A quick check indicates that this doesn not work on alpha or sparc64. Apparently no one noticed that this code has been broken for over a decade (we got it from UCB). NetBSD switched from LONG_MAX to MAXRANDOM in 2000, but apparently didn't actually read the code since it also has the bug. -- Brooks -- Any statement of the form "X is the one, true Y" is FALSE. PGP fingerprint 655D 519C 26A7 82E7 2529 9BF0 5D8E 8BE9 F238 1AD4 [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.1 (GNU/Linux) iD8DBQFBx4DJXY6L6fI4GtQRAiT5AKDH1hBsa6xruWC4p4a0I8qNq4/wtwCghcTW OV5XfgZym8U0dIVJ3B6zbLQ= =1ANS -----END PGP SIGNATURE-----home | help
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