Simply writing /var/lib/systemd/random-seed back to /dev/(u)random does not
increase the entropy bit count.  Therefore use the RNDADDENTROPY ioctl to
write back the bytes and increase the entropy bit counter.

Related to bnc#892096

---
 src/random-seed/random-seed.c |   65 +++++++++++++++++++++++++++++++++++++++---
 1 file changed, 61 insertions(+), 4 deletions(-)

Index: systemd-218/src/random-seed/random-seed.c
===================================================================
--- systemd-218.orig/src/random-seed/random-seed.c
+++ systemd-218/src/random-seed/random-seed.c
@@ -22,7 +22,9 @@
 #include <unistd.h>
 #include <fcntl.h>
 #include <errno.h>
+#include <linux/random.h>
 #include <string.h>
+#include <sys/ioctl.h>
 #include <sys/stat.h>
 
 #include "log.h"
@@ -32,8 +34,9 @@
 #define POOL_SIZE_MIN 512
 
 int main(int argc, char *argv[]) {
-        _cleanup_close_ int seed_fd = -1, random_fd = -1;
+        _cleanup_close_ int seed_fd = -1, random_fd = -1, entropy_fd = -1;
         _cleanup_free_ void* buf = NULL;
+        size_t entropy_count = 0;
         size_t buf_size = 0;
         ssize_t k;
         int r;
@@ -82,6 +85,23 @@ int main(int argc, char *argv[]) {
 
         if (streq(argv[1], "load")) {
 
+                entropy_fd = open(RANDOM_SEED_DIR "entropy_count", O_RDONLY|O_CLOEXEC|O_NOCTTY, 0600);
+                if (entropy_fd < 0) {
+                         entropy_count = 0;
+                         if (errno != ENOENT) {
+                                log_error("Failed to open " RANDOM_SEED "/entropy_count: %m");
+                                r = -errno;
+                                goto finish;
+                         }
+                } else {
+                        r = read(entropy_fd, &entropy_count, sizeof(entropy_count));
+                        if (r < 0) {
+                                log_error("Failed to read entropy count file: %m");
+                                r = -errno;
+                                goto finish;
+                        }
+                }
+
                 seed_fd = open(RANDOM_SEED, O_RDWR|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600);
                 if (seed_fd < 0) {
                         seed_fd = open(RANDOM_SEED, O_RDONLY|O_CLOEXEC|O_NOCTTY);
@@ -113,12 +133,34 @@ int main(int argc, char *argv[]) {
                 } else {
                         lseek(seed_fd, 0, SEEK_SET);
 
-                        r = loop_write(random_fd, buf, (size_t) k, false);
-                        if (r < 0)
-                                log_error_errno(r, "Failed to write seed to /dev/urandom: %m");
+                        if (entropy_count && (size_t) k == buf_size) {
+                                struct rand_pool_info entropy = {
+                                        .entropy_count = entropy_count,
+                                        .buf_size = buf_size,
+                                };
+                                entropy.buf[0] = ((__u32*)buf)[0];
+                                r = ioctl(random_fd, RNDADDENTROPY, &entropy);
+                                if (r < 0) {
+                                        log_error("Failed to write seed to /dev/urandom: %m");
+                                        r = -errno;
+                                }
+                        } else {
+                                k = loop_write(random_fd, buf, (size_t) k, false);
+                                if (k <= 0) {
+                                        log_error("Failed to write seed to /dev/urandom: %s", r < 0 ? strerror(-r) : "short write");
+                                        r = k == 0 ? -EIO : (int) k;
+                                }
+			}
                 }
 
         } else if (streq(argv[1], "save")) {
+                /* Read available entropy count, if possible */
+                f = fopen("/proc/sys/kernel/random/entropy_avail", "re");
+                if (f) {
+                        if (fscanf(f, "%zu", &entropy_count) < 0)
+                                entropy_count = 0;
+                        fclose(f);
+                }
 
                 seed_fd = open(RANDOM_SEED, O_WRONLY|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600);
                 if (seed_fd < 0) {
@@ -134,6 +176,21 @@ int main(int argc, char *argv[]) {
                         goto finish;
                 }
 
+                if (entropy_count) {
+                        entropy_fd = open(RANDOM_SEED_DIR "entropy_count", O_WRONLY|O_CLOEXEC|O_NOCTTY|O_CREAT, 0600);
+                        if (seed_fd < 0) {
+                                log_error("Failed to open " RANDOM_SEED_DIR "entropy_count: %m");
+                                r = -errno;
+                                goto finish;
+                        }
+                        r = write(entropy_fd, &entropy_count, sizeof(entropy_count));
+                        if (r < 0) {
+                                log_error("Failed to write entropy count file: %m");
+                                r = -errno;
+                                goto finish;
+                        }
+                }
+
         } else {
                 log_error("Unknown verb %s.", argv[1]);
                 r = -EINVAL;