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Re: lib/25367: arc4random state is shared across forks

The following reply was made to PR lib/25367; it has been noted by GNATS.

From: David Laight <>
Subject: Re: lib/25367: arc4random state is shared across forks
Date: Sun, 19 Aug 2012 09:01:48 +0100

 Just bumped into this one while looking at other arc4random bugs.
 There are two freebsd threads from september 2008 explaining that
 openbsd had added a getpid() call prior to every arc4random() one
 in order to detect this - butthat is slow. See:
 There is a more general problem of detecting fork() - arc4random()
 isn't the only affected code.
 Optimising getpid() (or having an optimised getpid() function) is one
 possibility. All that requires is for libc to zero the cached pid
 value in the child of fork().
 Although vfork() and others? may need special consideration.
 David Laight:

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