machine-id: be nice and generate compliant v4 UUIDs

Newly generated machine IDs now qualify as randomized v4 UUIds. This is
trivial to do and hopefully increases adoption of the ID for various
purposes.

1 files changed, 30 insertions, 8 deletions

diff --git a/src/machine-id-setup.c b/src/machine-id-setup.c
index 98e288e1b5..be51d0dec7 100644
--- a/src/machine-id-setup.c
+++ b/src/machine-id-setup.c
@@ -32,16 +32,28 @@
 #include "util.h"
 #include "log.h"
 
+static void make_v4_uuid(unsigned char *id) {
+        /* Stolen from generate_random_uuid() of drivers/char/random.c
+         * in the kernel sources */
+
+        /* Set UUID version to 4 --- truly random generation */
+        id[6] = (id[6] & 0x0F) | 0x40;
+
+        /* Set the UUID variant to DCE */
+        id[8] = (id[8] & 0x3F) | 0x80;
+}
+
 static int generate(char id[34]) {
         int fd;
-        char buf[16];
-        char *p, *q;
+        unsigned char buf[16], *p;
+        char *q;
         ssize_t k;
 
         assert(id);
 
         /* First, try reading the D-Bus machine id, unless it is a symlink */
-        if ((fd = open("/var/lib/dbus/machine-id", O_RDONLY|O_CLOEXEC|O_NOCTTY|O_NOFOLLOW)) >= 0) {
+        fd = open("/var/lib/dbus/machine-id", O_RDONLY|O_CLOEXEC|O_NOCTTY|O_NOFOLLOW);
+        if (fd >= 0) {
 
                 k = loop_read(fd, id, 33, false);
                 close_nointr_nofail(fd);
@@ -56,7 +68,8 @@ static int generate(char id[34]) {
         }
 
         /* If that didn't work, generate a random machine id */
-        if ((fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY)) < 0) {
+        fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
+        if (fd < 0) {
                 log_error("Failed to open /dev/urandom: %m");
                 return -errno;
         }
@@ -69,6 +82,11 @@ static int generate(char id[34]) {
                 return k < 0 ? (int) k : -EIO;
         }
 
+        /* Turn this into a valid v4 UUID, to be nice. Note that we
+         * only guarantee this for newly generated UUIDs, not for
+         * pre-existing ones.*/
+        make_v4_uuid(buf);
+
         for (p = buf, q = id; p < buf + sizeof(buf); p++, q += 2) {
                 q[0] = hexchar(*p >> 4);
                 q[1] = hexchar(*p & 15);
@@ -96,10 +114,12 @@ int machine_id_setup(void) {
          * will be owned by root it doesn't matter much, but maybe
          * people look. */
 
-        if ((fd = open("/etc/machine-id", O_RDWR|O_CREAT|O_CLOEXEC|O_NOCTTY, 0444)) >= 0)
+        fd = open("/etc/machine-id", O_RDWR|O_CREAT|O_CLOEXEC|O_NOCTTY, 0444);
+        if (fd >= 0)
                 writable = true;
         else {
-                if ((fd = open("/etc/machine-id", O_RDONLY|O_CLOEXEC|O_NOCTTY)) < 0) {
+                fd = open("/etc/machine-id", O_RDONLY|O_CLOEXEC|O_NOCTTY);
+                if (fd < 0) {
                         umask(m);
                         log_error("Cannot open /etc/machine-id: %m");
                         return -errno;
@@ -126,7 +146,8 @@ int machine_id_setup(void) {
         /* Hmm, so, the id currently stored is not useful, then let's
          * generate one */
 
-        if ((r = generate(id)) < 0)
+        r = generate(id);
+        if (r < 0)
                 goto finish;
 
         if (S_ISREG(st.st_mode) && writable) {
@@ -146,7 +167,8 @@ int machine_id_setup(void) {
 
         mkdir_p("/run/systemd", 0755);
 
-        if ((r = write_one_line_file("/run/systemd/machine-id", id)) < 0) {
+        r = write_one_line_file("/run/systemd/machine-id", id);
+        if (r < 0) {
                 log_error("Cannot write /run/systemd/machine-id: %s", strerror(-r));
 
                 unlink("/run/systemd/machine-id");