Messages in this thread | | | Subject | Re: issue with /dev/random? gets depleted very quick | From | Matt Mackall <> | Date | Sun, 14 Jun 2009 10:51:09 -0500 |
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[cc:ed to lkml]
On Sun, 2009-06-14 at 14:51 +0200, Folkert van Heusden wrote: > Hi, > > On an idle system (no gui, no daemons, nothing) system, /dev/random gets > empty in a matter of 20 seconds with a 2.6.26 kernel. > > My test: > > add 1000 bits to the device: > > zolder:/tmp# cat test-RNDADDENTROPY.c > #include <sys/types.h> > #include <sys/stat.h> > #include <fcntl.h> > #include <sys/ioctl.h> > #include <stdio.h> > #include <stdlib.h> > #include <asm/types.h> > #include <linux/random.h> > > int main(int argc, char *argv[]) > { > struct rand_pool_info *output; > int fd = open("/dev/random", O_WRONLY); > > output = (struct rand_pool_info *)malloc(10000); > output -> entropy_count = 1000; > output -> buf_size = 8000; > > printf("%d\n", ioctl(fd, RNDADDENTROPY, output)); > > return 0; > } > > and then check whayt is in it: > > zolder:/tmp# ./a.out ; while true ; do echo `date` `cat /proc/sys/kernel/random/entropy_avail` ; sleep 1 ; done > 0 > Sun Jun 14 14:50:44 CEST 2009 1117 > Sun Jun 14 14:50:45 CEST 2009 989 > Sun Jun 14 14:50:46 CEST 2009 925 > Sun Jun 14 14:50:47 CEST 2009 797 > Sun Jun 14 14:50:48 CEST 2009 733 > Sun Jun 14 14:50:49 CEST 2009 605 > Sun Jun 14 14:50:50 CEST 2009 541 > Sun Jun 14 14:50:51 CEST 2009 413 > Sun Jun 14 14:50:52 CEST 2009 349 > Sun Jun 14 14:50:53 CEST 2009 221 > Sun Jun 14 14:50:54 CEST 2009 157 > Sun Jun 14 14:50:55 CEST 2009 157 > > Is there something wrong with it?
Does it go below 128? If not, that's the behavior of something depleting the pool down to the anti-starvation threshold via either /dev/urandom or get_random_bytes.
On my system, I'm seeing that behavior as well. fuser reports a bunch of processes hold /dev/urandom open, but stracing them doesn't reveal a culprit. Which means there's now probably something in the kernel calling get_random_bytes continuously.
Is this a problem? It really shouldn't be. Everyone should be using /dev/urandom anyway. And the anti-starvation threshold guarantees that if there's entropy being collected, readers of /dev/random can always make forward progress.
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