short read from /dev/urandom

[Ulrich Drepper]

The /dev/urandom device is advertised as always returning the requested 
number of bytes.  Yet, it fails to do this under some situations. 
Compile this

int main (void)
{
  alarm (100);
  char buf[32];
  int fd = open ("/dev/urandom", 0);
  while (1)
    if (read (fd, buf, sizeof buf) != sizeof (buf))
      abort ();
}

with

  gcc -o r r.c -g -O2 -pg


Note the -pg at the end to enable profiling.  Running this code fails 
for me after less than a second.

The relevant code in the kernel is this 
(drivers/char/random.c:extract_entropy)

        while (nbytes) {
                /*
                 * Check if we need to break out or reschedule....
                 */
                if ((flags & EXTRACT_ENTROPY_USER) && need_resched()) {
                        if (signal_pending(current)) {
                                if (ret == 0)
                                        ret = -ERESTARTSYS;
                                break;
                        }


Here the loop is left prematurely if a signal is pending.

One solution is to redefine the /dev/urandom interface.  The problem is 
that this will cause program to fail.  I know since I found this while 
debugging such a program.

--
â Ulrich Drepper â Red Hat, Inc. â 444 Castro St â Mountain View, CA â

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