int
getentropy(void *buf, size_t len)
{
if (len <= 256 && getentropy_sysctl(buf, len) == len)
return (0);
errno = EIO;
return (-1);
}
int
getentropy(void *buf, size_t len)
{
int ret = -1;
if (len > 256) {
errno = EIO;
return -1;
}
/*
* Try to get entropy with /dev/urandom
*
* This can fail if the process is inside a chroot or if file
* descriptors are exhausted.
*/
ret = getentropy_urandom(buf, len);
if (ret != -1)
return (ret);
#ifdef CTL_MAXNAME
/*
* Try to use sysctl CTL_KERN, KERN_RANDOM, RANDOM_UUID.
* sysctl is a failsafe API, so it guarantees a result. This
* should work inside a chroot, or when file descriptors are
* exhuasted.
*
* However this can fail if the Linux kernel removes support
* for sysctl. Starting in 2007, there have been efforts to
* deprecate the sysctl API/ABI, and push callers towards use
* of the chroot-unavailable fd-using /proc mechanism --
* essentially the same problems as /dev/urandom.
*
* Numerous setbacks have been encountered in their deprecation
* schedule, so as of June 2014 the kernel ABI still exists on
* most Linux architectures. The sysctl() stub in libc is missing
* on some systems. There are also reports that some kernels
* spew messages to the console.
*/
ret = getentropy_sysctl(buf, len);
if (ret != -1)
return (ret);
#endif /* CTL_MAXNAME */
/*
* Entropy collection via /dev/urandom and sysctl have failed.
*
* No other API exists for collecting entropy. See the large
* comment block above.
*
* We have very few options:
* - Even syslog_r is unsafe to call at this low level, so
* there is no way to alert the user or program.
* - Cannot call abort() because some systems have unsafe
* corefiles.
* - Could raise(SIGKILL) resulting in silent program termination.
* - Return EIO, to hint that arc4random's stir function
* should raise(SIGKILL)
* - Do the best under the circumstances....
*
* This code path exists to bring light to the issue that Linux
* does not provide a failsafe API for entropy collection.
*
* We hope this demonstrates that Linux should either retain their
* sysctl ABI, or consider providing a new failsafe API which
* works in a chroot or when file descriptors are exhausted.
*/
#undef FAIL_INSTEAD_OF_TRYING_FALLBACK
#ifdef FAIL_INSTEAD_OF_TRYING_FALLBACK
raise(SIGKILL);
#endif
ret = getentropy_fallback(buf, len);
if (ret != -1)
return (ret);
errno = EIO;
return (ret);
}
