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Date: Mon, 29 Jan 2024 13:46:07 +0100
To: Johnothan King <johnothanking AT protonmail DOT com>
Subject: Re: arc4random does not reseed after using fork()
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From: Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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Hi Johnothan,

On Jan 26 22:06, Johnothan King via Cygwin wrote:
> Hello,
> 
> While testing ksh93u+m's recently added SRANDOM variable[1], I have
> discovered a bug in Cygwin's arc4random function. After using fork(),
> arc4random does not reseed itself, which causes the results to become
> predictable[2]. Below is a test case C program exhibiting the bug:
> 
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <sys/wait.h>
> int main(void)
> {
> 	pid_t child = fork();
> 	if(child==0)
> 	{
> 		printf("%u %u %u\n", arc4random(), arc4random(), arc4random());
> 		return 0;
> 	}
> 	else if(child==-1)
> 		return 1;
> 	waitpid(child, NULL, 0);
> 	printf("%u %u %u\n", arc4random(), arc4random(), arc4random());
> 	return 0;
> }

Thanks for the testcase.  I fixed that for Cygwin 3.5.  Please
try the latest test release cygwin-3.5.0-0.617.g030a762535c1,
which should be finished building in about an hour.


Thanks,
Corinna

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