Work in progress on how to deal with the inherit unreliability of

/dev/urandom. Does well in the fallback case. Get it in tree so
it can be worked on.
ok otto@ deraadt@

src/lib/libcrypto/arc4random/getentropy_linux.c

@@ -0,0 +1,439 @@
+/*	$OpenBSD: getentropy_linux.c,v 1.1 2014/06/20 19:08:11 beck Exp $	*/
+
+/*
+ * Copyright (c) 2014 Theo de Raadt <deraadt@openbsd.org>
+ * Copyright (c) 2014 Bob Beck <beck@obtuse.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#define	_POSIX_C_SOURCE 199309L
+#define	_GNU_SOURCE     1
+#include <sys/types.h>
+#include <sys/param.h>
+#include <sys/ioctl.h>
+#include <sys/resource.h>
+#include <sys/syscall.h>
+#include <sys/sysctl.h>
+#include <sys/statvfs.h>
+#include <sys/socket.h>
+#include <sys/mount.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/time.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <termios.h>
+#include <fcntl.h>
+#include <signal.h>
+#include <string.h>
+#include <errno.h>
+#include <unistd.h>
+#include <time.h>
+#include <openssl/sha.h>
+
+#include <linux/random.h>
+#include <sys/vfs.h>
+
+#define REPEAT 5
+#define min(a, b) (((a) < (b)) ? (a) : (b))
+
+#define HASHX(a, b) \
+	do { \
+		if ((a)) \
+			HASHD(errno); \
+		else \
+			HASHD(b); \
+	} while (0)
+
+#define HASHD(xxx)	(SHA512_Update(&ctx, (char *)&(xxx), sizeof (xxx)))
+
+int	getentropy(void *buf, size_t len);
+
+extern int main(int, char *argv[]);
+static int gotdata(char *buf, size_t len);
+
+/*
+ * XXX Should be replaced with a proper entropy measure.
+ */
+static int
+gotdata(char *buf, size_t len)
+{
+	char	any_set = 0;
+	size_t	i;
+
+	for (i = 0; i < len; ++i)
+		any_set |= buf[i];
+	if (any_set == 0)
+		return -1;
+	return 0;
+}
+
+static int
+getentropy_urandom(void *buf, size_t len)
+{
+	struct stat st;
+	size_t i;
+	int fd, cnt;
+	int save_errno = errno;
+
+start:
+#ifdef O_CLOEXEC
+	fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC, 0);
+	if (fd == -1) {
+		if (errno == EINTR)
+			goto start;
+		goto nodevrandom;
+	}
+#else
+	fd = open("/dev/urandom", O_RDONLY, 0);
+	if (fd == -1) {
+		if (errno == EINTR)
+			goto start;
+			goto nodevrandom;
+	}
+	fcntl(fd, F_SETFD, fcntl(fd, F_GETFD) | FD_CLOEXEC);
+#endif
+
+	/* Lightly verify that the device node looks sane */
+	if (fstat(fd, &st) == -1 || !S_ISCHR(st.st_mode)) {
+		close(fd);
+		goto nodevrandom;
+	}
+	if (ioctl(fd, RNDGETENTCNT, &cnt) == -1) {
+		close(fd);
+		goto nodevrandom;
+	}
+	for (i = 0; i < len; ) {
+		size_t wanted = len - i;
+		ssize_t ret = read(fd, buf + i, wanted);
+
+		if (ret == -1) {
+			if (errno == EAGAIN || errno == EINTR)
+				continue;
+			close(fd);
+			goto nodevrandom;
+		}
+		i += ret;
+	}
+	close(fd);
+	if (gotdata(buf, len) == 0) {
+		errno = save_errno;
+		return 0;		/* satisfied */
+	}
+nodevrandom:
+	errno = EIO;
+	return -1;
+}
+
+#ifdef RANDOM_UUID
+static int
+getentropy_sysctl(void *buf, size_t len)
+{
+	static const int mib[] = { CTL_KERN, KERN_RANDOM, RANDOM_UUID };
+	size_t i, chunk;
+	int save_errno = errno;
+
+	for (i = 0; i < len; ) {
+		chunk = min(len - i, 16);
+
+		/* SYS__sysctl because some systems already removed sysctl() */
+		struct __sysctl_args args = {
+			.name = mib,
+			.nlen = 3,
+			.oldval = &buf[i],
+			.oldlenp = &chunk,
+		};
+		if (syscall(SYS__sysctl, &args) != 0)
+			goto sysctlfailed;
+		i += chunk;
+	}
+	if (gotdata(buf, len) == 0) {
+		errno = save_errno;
+		return (0);			/* satisfied */
+	}
+sysctlfailed:
+	errno = EIO;
+	return -1;
+}
+#endif /* RANDOM_UUID */
+
+static int cl[] = {
+	CLOCK_REALTIME,
+#ifdef CLOCK_MONOTONIC
+	CLOCK_MONOTONIC,
+#endif
+#ifdef CLOCK_MONOTONIC_RAW
+	CLOCK_MONOTONIC_RAW,
+#endif
+#ifdef CLOCK_TAI
+	CLOCK_TAI,
+#endif
+#ifdef CLOCK_VIRTUAL
+	CLOCK_VIRTUAL,
+#endif
+#ifdef CLOCK_UPTIME
+	CLOCK_UPTIME,
+#endif
+#ifdef CLOCK_PROCESS_CPUTIME_ID
+	CLOCK_PROCESS_CPUTIME_ID,
+#endif
+#ifdef CLOCK_THREAD_CPUTIME_ID
+	CLOCK_THREAD_CPUTIME_ID,
+#endif
+};
+
+static int
+getentropy_fallback(void *buf, size_t len)
+{
+	uint8_t results[SHA512_DIGEST_LENGTH];
+	int save_errno = errno, e, m, pgsiz = getpagesize(), repeat;
+	static int counter;
+	struct timespec ts;
+	struct timeval tv;
+	struct rusage ru;
+	sigset_t sigset;
+	struct stat st;
+	SHA512_CTX ctx;
+	pid_t pid;
+	size_t i, ii;
+	void *p;
+
+	for (i = 0; i < len; ) {
+		SHA512_Init(&ctx);
+		for (repeat = 0; repeat < REPEAT; repeat++) {
+
+		HASHX((e = gettimeofday(&tv, NULL)) == -1, tv);
+		if (e != -1) {
+			counter += (int)tv.tv_sec;
+			counter += (int)tv.tv_usec;
+		}
+
+		for (ii = 0; ii < sizeof(cl)/sizeof(cl[0]); ii++)
+			HASHX(clock_gettime(cl[ii], &ts) == -1, ts);
+
+		HASHX((pid = getpid()) == -1, pid);
+		HASHX((pid = getsid(pid)) == -1, pid);
+		HASHX((pid = getppid()) == -1, pid);
+		HASHX((pid = getpgid(0)) == -1, pid);
+		HASHX((m = getpriority(0, 0)) == -1, m);
+
+		ts.tv_sec = 0;
+		ts.tv_nsec = 1;
+		(void) nanosleep(&ts, NULL);
+
+		HASHX(sigpending(&sigset) == -1, sigset);
+		HASHX(sigprocmask(SIG_BLOCK, NULL, &sigset) == -1, sigset);
+
+		HASHD(main);		/* an address in the main program */
+		HASHD(getentropy);		/* an address in this library */
+		HASHD(printf);		/* an address in libc */
+		p = (void *)&p;
+		HASHD(p);			/* an address on stack */
+		p = (void *)&errno;
+		HASHD(p);			/* the address of errno */
+
+		if (i == 0) {
+			struct sockaddr_storage ss;
+			struct statvfs stvfs;
+			struct termios tios;
+			struct statfs stfs;
+			socklen_t ssl;
+			off_t off;
+
+			/*
+			 * Prime-sized mappings encourage fragmentation;
+			 * thus exposing some address entropy.
+			 */
+			struct mm {
+				size_t	npg;
+				void	*p;
+			} mm[] = {
+			    { 17, MAP_FAILED }, { 3, MAP_FAILED },
+			    { 11, MAP_FAILED }, { 2, MAP_FAILED },
+			    { 5, MAP_FAILED }, { 3, MAP_FAILED },
+			    { 7, MAP_FAILED }, { 1, MAP_FAILED },
+			    { 57, MAP_FAILED }, { 3, MAP_FAILED },
+			    { 131, MAP_FAILED }, { 1, MAP_FAILED },
+			};
+
+			for (m = 0; m < sizeof mm/sizeof(mm[0]); m++) {
+				HASHX(mm[m].p = mmap(NULL, mm[m].npg * pgsiz,
+				    PROT_READ|PROT_WRITE,
+				    MAP_PRIVATE|MAP_ANON, -1, (off_t)0), p);
+				if (mm[m].p != MAP_FAILED) {
+					char *mp;
+
+					/* Touch some memory... */
+					mp = mm[m].p;
+					mp[counter % (mm[m].npg * pgsiz - 1)] = 1;
+					counter += (int)((long)(mm[m].p) / pgsiz);
+				}
+
+				/* Check counters and times... */
+				for (ii = 0; ii < sizeof(cl)/sizeof(cl[0]);
+				    ii++) {
+					HASHX((e = clock_gettime(cl[ii], &ts)) == -1,
+					    ts);
+					if (e != -1)
+						counter += (int)ts.tv_nsec;
+				}
+
+				HASHX((e = getrusage(RUSAGE_SELF, &ru)) == -1, ru);
+				if (e != -1) {
+					counter += (int)ru.ru_utime.tv_sec;
+					counter += (int)ru.ru_utime.tv_usec;
+				}
+			}
+
+			for (m = 0; m < sizeof mm/sizeof(mm[0]); m++) {
+				if (mm[m].p != MAP_FAILED)
+					munmap(mm[m].p, mm[m].npg * pgsiz);
+				mm[m].p = MAP_FAILED;
+			}
+
+			HASHX(stat(".", &st) == -1, st);
+			HASHX(statvfs(".", &stvfs) == -1, stvfs);
+			HASHX(statfs(".", &stfs) == -1, stfs);
+
+			HASHX(stat("/", &st) == -1, st);
+			HASHX(statvfs("/", &stvfs) == -1, stvfs);
+			HASHX(statfs("/", &stfs) == -1, stfs);
+
+			HASHX((e = fstat(0, &st)) == -1, st);
+			if (e == -1) {
+				if (S_ISREG(st.st_mode) || S_ISFIFO(st.st_mode) ||
+				    S_ISSOCK(st.st_mode)) {
+					HASHX(fstatvfs(0, &stvfs) == -1, stvfs);
+					HASHX(fstatfs(0, &stfs) == -1, stfs);
+					HASHX((off = lseek(0, (off_t)0,
+					    SEEK_CUR)) < 0, off);
+				}
+				if (S_ISCHR(st.st_mode)) {
+					HASHX(tcgetattr(0, &tios) == -1, tios);
+				} else if (S_ISSOCK(st.st_mode)) {
+					memset(&ss, 0, sizeof ss);
+					ssl = sizeof(ss);
+					HASHX(getpeername(0, (void *)&ss,
+					    &ssl) == -1, ss);
+				}
+			}
+
+			HASHX((e = getrusage(RUSAGE_CHILDREN, &ru)) == -1, ru);
+			if (e != -1) {
+				counter += (int)ru.ru_utime.tv_sec;
+				counter += (int)ru.ru_utime.tv_usec;
+			}
+		} else {
+			/* Subsequent hashes absorb previous result */
+			HASHD(results);
+		}
+
+		HASHX((e = gettimeofday(&tv, NULL)) == -1, tv);
+		if (e != -1) {
+			counter += (int)tv.tv_sec;
+			counter += (int)tv.tv_usec;
+		}
+
+		HASHD(counter);
+
+		} /* repeat */
+		SHA512_Final(results, &ctx);
+		memcpy(buf + i, results, min(sizeof(results), len - i));
+		i += min(sizeof(results), len - i);
+	}
+	memset(results, 0, sizeof results);
+	if (gotdata(buf, len) == 0) {
+		errno = save_errno;
+		return 0;		/* satisfied */
+	}
+	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 RANDOM_UUID
+	/*
+	 * 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. 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 /* RANDOM_UUID */
+
+	/*
+	 * 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_HARD_WHEN_LINUX_DEPRECATES_SYSCTL
+#ifdef FAIL_HARD_WHEN_LINUX_DEPRECATES_SYSCTL
+	raise(SIGKILL);
+#endif
+	ret = getentropy_fallback(buf, len);
+	if (ret != -1)
+		return (ret);
+
+	errno = EIO;
+	return (ret);
+}

src/lib/libcrypto/crypto/getentropy_linux.c

@@ -0,0 +1,439 @@
+/*	$OpenBSD: getentropy_linux.c,v 1.1 2014/06/20 19:08:11 beck Exp $	*/
+
+/*
+ * Copyright (c) 2014 Theo de Raadt <deraadt@openbsd.org>
+ * Copyright (c) 2014 Bob Beck <beck@obtuse.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#define	_POSIX_C_SOURCE 199309L
+#define	_GNU_SOURCE     1
+#include <sys/types.h>
+#include <sys/param.h>
+#include <sys/ioctl.h>
+#include <sys/resource.h>
+#include <sys/syscall.h>
+#include <sys/sysctl.h>
+#include <sys/statvfs.h>
+#include <sys/socket.h>
+#include <sys/mount.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/time.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <termios.h>
+#include <fcntl.h>
+#include <signal.h>
+#include <string.h>
+#include <errno.h>
+#include <unistd.h>
+#include <time.h>
+#include <openssl/sha.h>
+
+#include <linux/random.h>
+#include <sys/vfs.h>
+
+#define REPEAT 5
+#define min(a, b) (((a) < (b)) ? (a) : (b))
+
+#define HASHX(a, b) \
+	do { \
+		if ((a)) \
+			HASHD(errno); \
+		else \
+			HASHD(b); \
+	} while (0)
+
+#define HASHD(xxx)	(SHA512_Update(&ctx, (char *)&(xxx), sizeof (xxx)))
+
+int	getentropy(void *buf, size_t len);
+
+extern int main(int, char *argv[]);
+static int gotdata(char *buf, size_t len);
+
+/*
+ * XXX Should be replaced with a proper entropy measure.
+ */
+static int
+gotdata(char *buf, size_t len)
+{
+	char	any_set = 0;
+	size_t	i;
+
+	for (i = 0; i < len; ++i)
+		any_set |= buf[i];
+	if (any_set == 0)
+		return -1;
+	return 0;
+}
+
+static int
+getentropy_urandom(void *buf, size_t len)
+{
+	struct stat st;
+	size_t i;
+	int fd, cnt;
+	int save_errno = errno;
+
+start:
+#ifdef O_CLOEXEC
+	fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC, 0);
+	if (fd == -1) {
+		if (errno == EINTR)
+			goto start;
+		goto nodevrandom;
+	}
+#else
+	fd = open("/dev/urandom", O_RDONLY, 0);
+	if (fd == -1) {
+		if (errno == EINTR)
+			goto start;
+			goto nodevrandom;
+	}
+	fcntl(fd, F_SETFD, fcntl(fd, F_GETFD) | FD_CLOEXEC);
+#endif
+
+	/* Lightly verify that the device node looks sane */
+	if (fstat(fd, &st) == -1 || !S_ISCHR(st.st_mode)) {
+		close(fd);
+		goto nodevrandom;
+	}
+	if (ioctl(fd, RNDGETENTCNT, &cnt) == -1) {
+		close(fd);
+		goto nodevrandom;
+	}
+	for (i = 0; i < len; ) {
+		size_t wanted = len - i;
+		ssize_t ret = read(fd, buf + i, wanted);
+
+		if (ret == -1) {
+			if (errno == EAGAIN || errno == EINTR)
+				continue;
+			close(fd);
+			goto nodevrandom;
+		}
+		i += ret;
+	}
+	close(fd);
+	if (gotdata(buf, len) == 0) {
+		errno = save_errno;
+		return 0;		/* satisfied */
+	}
+nodevrandom:
+	errno = EIO;
+	return -1;
+}
+
+#ifdef RANDOM_UUID
+static int
+getentropy_sysctl(void *buf, size_t len)
+{
+	static const int mib[] = { CTL_KERN, KERN_RANDOM, RANDOM_UUID };
+	size_t i, chunk;
+	int save_errno = errno;
+
+	for (i = 0; i < len; ) {
+		chunk = min(len - i, 16);
+
+		/* SYS__sysctl because some systems already removed sysctl() */
+		struct __sysctl_args args = {
+			.name = mib,
+			.nlen = 3,
+			.oldval = &buf[i],
+			.oldlenp = &chunk,
+		};
+		if (syscall(SYS__sysctl, &args) != 0)
+			goto sysctlfailed;
+		i += chunk;
+	}
+	if (gotdata(buf, len) == 0) {
+		errno = save_errno;
+		return (0);			/* satisfied */
+	}
+sysctlfailed:
+	errno = EIO;
+	return -1;
+}
+#endif /* RANDOM_UUID */
+
+static int cl[] = {
+	CLOCK_REALTIME,
+#ifdef CLOCK_MONOTONIC
+	CLOCK_MONOTONIC,
+#endif
+#ifdef CLOCK_MONOTONIC_RAW
+	CLOCK_MONOTONIC_RAW,
+#endif
+#ifdef CLOCK_TAI
+	CLOCK_TAI,
+#endif
+#ifdef CLOCK_VIRTUAL
+	CLOCK_VIRTUAL,
+#endif
+#ifdef CLOCK_UPTIME
+	CLOCK_UPTIME,
+#endif
+#ifdef CLOCK_PROCESS_CPUTIME_ID
+	CLOCK_PROCESS_CPUTIME_ID,
+#endif
+#ifdef CLOCK_THREAD_CPUTIME_ID
+	CLOCK_THREAD_CPUTIME_ID,
+#endif
+};
+
+static int
+getentropy_fallback(void *buf, size_t len)
+{
+	uint8_t results[SHA512_DIGEST_LENGTH];
+	int save_errno = errno, e, m, pgsiz = getpagesize(), repeat;
+	static int counter;
+	struct timespec ts;
+	struct timeval tv;
+	struct rusage ru;
+	sigset_t sigset;
+	struct stat st;
+	SHA512_CTX ctx;
+	pid_t pid;
+	size_t i, ii;
+	void *p;
+
+	for (i = 0; i < len; ) {
+		SHA512_Init(&ctx);
+		for (repeat = 0; repeat < REPEAT; repeat++) {
+
+		HASHX((e = gettimeofday(&tv, NULL)) == -1, tv);
+		if (e != -1) {
+			counter += (int)tv.tv_sec;
+			counter += (int)tv.tv_usec;
+		}
+
+		for (ii = 0; ii < sizeof(cl)/sizeof(cl[0]); ii++)
+			HASHX(clock_gettime(cl[ii], &ts) == -1, ts);
+
+		HASHX((pid = getpid()) == -1, pid);
+		HASHX((pid = getsid(pid)) == -1, pid);
+		HASHX((pid = getppid()) == -1, pid);
+		HASHX((pid = getpgid(0)) == -1, pid);
+		HASHX((m = getpriority(0, 0)) == -1, m);
+
+		ts.tv_sec = 0;
+		ts.tv_nsec = 1;
+		(void) nanosleep(&ts, NULL);
+
+		HASHX(sigpending(&sigset) == -1, sigset);
+		HASHX(sigprocmask(SIG_BLOCK, NULL, &sigset) == -1, sigset);
+
+		HASHD(main);		/* an address in the main program */
+		HASHD(getentropy);		/* an address in this library */
+		HASHD(printf);		/* an address in libc */
+		p = (void *)&p;
+		HASHD(p);			/* an address on stack */
+		p = (void *)&errno;
+		HASHD(p);			/* the address of errno */
+
+		if (i == 0) {
+			struct sockaddr_storage ss;
+			struct statvfs stvfs;
+			struct termios tios;
+			struct statfs stfs;
+			socklen_t ssl;
+			off_t off;
+
+			/*
+			 * Prime-sized mappings encourage fragmentation;
+			 * thus exposing some address entropy.
+			 */
+			struct mm {
+				size_t	npg;
+				void	*p;
+			} mm[] = {
+			    { 17, MAP_FAILED }, { 3, MAP_FAILED },
+			    { 11, MAP_FAILED }, { 2, MAP_FAILED },
+			    { 5, MAP_FAILED }, { 3, MAP_FAILED },
+			    { 7, MAP_FAILED }, { 1, MAP_FAILED },
+			    { 57, MAP_FAILED }, { 3, MAP_FAILED },
+			    { 131, MAP_FAILED }, { 1, MAP_FAILED },
+			};
+
+			for (m = 0; m < sizeof mm/sizeof(mm[0]); m++) {
+				HASHX(mm[m].p = mmap(NULL, mm[m].npg * pgsiz,
+				    PROT_READ|PROT_WRITE,
+				    MAP_PRIVATE|MAP_ANON, -1, (off_t)0), p);
+				if (mm[m].p != MAP_FAILED) {
+					char *mp;
+
+					/* Touch some memory... */
+					mp = mm[m].p;
+					mp[counter % (mm[m].npg * pgsiz - 1)] = 1;
+					counter += (int)((long)(mm[m].p) / pgsiz);
+				}
+
+				/* Check counters and times... */
+				for (ii = 0; ii < sizeof(cl)/sizeof(cl[0]);
+				    ii++) {
+					HASHX((e = clock_gettime(cl[ii], &ts)) == -1,
+					    ts);
+					if (e != -1)
+						counter += (int)ts.tv_nsec;
+				}
+
+				HASHX((e = getrusage(RUSAGE_SELF, &ru)) == -1, ru);
+				if (e != -1) {
+					counter += (int)ru.ru_utime.tv_sec;
+					counter += (int)ru.ru_utime.tv_usec;
+				}
+			}
+
+			for (m = 0; m < sizeof mm/sizeof(mm[0]); m++) {
+				if (mm[m].p != MAP_FAILED)
+					munmap(mm[m].p, mm[m].npg * pgsiz);
+				mm[m].p = MAP_FAILED;
+			}
+
+			HASHX(stat(".", &st) == -1, st);
+			HASHX(statvfs(".", &stvfs) == -1, stvfs);
+			HASHX(statfs(".", &stfs) == -1, stfs);
+
+			HASHX(stat("/", &st) == -1, st);
+			HASHX(statvfs("/", &stvfs) == -1, stvfs);
+			HASHX(statfs("/", &stfs) == -1, stfs);
+
+			HASHX((e = fstat(0, &st)) == -1, st);
+			if (e == -1) {
+				if (S_ISREG(st.st_mode) || S_ISFIFO(st.st_mode) ||
+				    S_ISSOCK(st.st_mode)) {
+					HASHX(fstatvfs(0, &stvfs) == -1, stvfs);
+					HASHX(fstatfs(0, &stfs) == -1, stfs);
+					HASHX((off = lseek(0, (off_t)0,
+					    SEEK_CUR)) < 0, off);
+				}
+				if (S_ISCHR(st.st_mode)) {
+					HASHX(tcgetattr(0, &tios) == -1, tios);
+				} else if (S_ISSOCK(st.st_mode)) {
+					memset(&ss, 0, sizeof ss);
+					ssl = sizeof(ss);
+					HASHX(getpeername(0, (void *)&ss,
+					    &ssl) == -1, ss);
+				}
+			}
+
+			HASHX((e = getrusage(RUSAGE_CHILDREN, &ru)) == -1, ru);
+			if (e != -1) {
+				counter += (int)ru.ru_utime.tv_sec;
+				counter += (int)ru.ru_utime.tv_usec;
+			}
+		} else {
+			/* Subsequent hashes absorb previous result */
+			HASHD(results);
+		}
+
+		HASHX((e = gettimeofday(&tv, NULL)) == -1, tv);
+		if (e != -1) {
+			counter += (int)tv.tv_sec;
+			counter += (int)tv.tv_usec;
+		}
+
+		HASHD(counter);
+
+		} /* repeat */
+		SHA512_Final(results, &ctx);
+		memcpy(buf + i, results, min(sizeof(results), len - i));
+		i += min(sizeof(results), len - i);
+	}
+	memset(results, 0, sizeof results);
+	if (gotdata(buf, len) == 0) {
+		errno = save_errno;
+		return 0;		/* satisfied */
+	}
+	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 RANDOM_UUID
+	/*
+	 * 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. 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 /* RANDOM_UUID */
+
+	/*
+	 * 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_HARD_WHEN_LINUX_DEPRECATES_SYSCTL
+#ifdef FAIL_HARD_WHEN_LINUX_DEPRECATES_SYSCTL
+	raise(SIGKILL);
+#endif
+	ret = getentropy_fallback(buf, len);
+	if (ret != -1)
+		return (ret);
+
+	errno = EIO;
+	return (ret);
+}