firstboot: process the root account after sysusers created it

We would create root account from sysusers or from firstboot, depending on
which one ran earlier. Since firstboot offers more options, in particular can
set the root password, we needed to order it earlier. This created an ugly
ordering requirement:

systemd-sysusers.service > systemd-firstboot.service > ... >
  systemd-remount-fs.service > systemd-tmpfiles-setup-dev.service >
  systemd-sysusers.service

We want sysusers.service to create basic users, so we can create nodes in dev,
so we can operate on block devices and such, so that we can resize and remount
things. But at the same time, systemd-firstboot.service can only work if it is
run early, before systemd-sysusers.service has created /etc/passwd. We can't
have it both ways: the units that want to have a fully writable root file
system cannot be ordered before units which are required to do file system
preparation.

Instead of trying to order firstboot very early, let's let it do its thing even
if it is started later. Instead of refusing to create to the root account if
/etc/passwd and /etc/shadow exist, actually check if the account is configured.
Now sysusers writes root account with password PASSWORD_UNPROVISIONED
("!unprovisioned"), and then firstboot checks for this, and will configure root
in this case.

This allows sysusers to be executed earlier (or accounts to be set up earlier
in another way).

This effectively reverts b825ab1a99.

src/basic/user-util.h

@@ -150,3 +150,8 @@ static inline bool hashed_password_is_locked_or_invalid(const char *password) {
 
 /* A password indicating "hey, no password required for login" */
 #define PASSWORD_NONE ""
+
+/* Used by sysusers to indicate that the password should be filled in by firstboot.
+ * Also see https://github.com/systemd/systemd/pull/24680#pullrequestreview-1439464325.
+ */
+#define PASSWORD_UNPROVISIONED "!unprovisioned"

src/firstboot/firstboot.c

@@ -234,17 +234,72 @@ static int prompt_loop(const char *text, char **l, unsigned percentage, bool (*i
 }
 
 static int should_configure(int dir_fd, const char *filename) {
+        _cleanup_fclose_ FILE *passwd = NULL, *shadow = NULL;
+        int r;
+
         assert(dir_fd >= 0);
         assert(filename);
 
-        if (faccessat(dir_fd, filename, F_OK, AT_SYMLINK_NOFOLLOW) < 0) {
-                if (errno != ENOENT)
-                        return log_error_errno(errno, "Failed to access %s: %m", filename);
+        if (streq(filename, "passwd") && !arg_force)
+                /* We may need to do additional checks, so open the file. */
+                r = xfopenat(dir_fd, filename, "re", O_NOFOLLOW, &passwd);
+        else
+                r = RET_NERRNO(faccessat(dir_fd, filename, F_OK, AT_SYMLINK_NOFOLLOW));
 
+        if (r == -ENOENT)
                 return true; /* missing */
+        if (r < 0)
+                return log_error_errno(r, "Failed to access %s: %m", filename);
+        if (arg_force)
+                return true; /* exists, but if --force was given we should still configure the file. */
+
+        if (!passwd)
+                return false;
+
+        /* In case of /etc/passwd, do an additional check for the root password field.
+         * We first check that passwd redirects to shadow, and then we check shadow.
+         */
+        struct passwd *i;
+        while ((r = fgetpwent_sane(passwd, &i)) > 0) {
+                if (!streq(i->pw_name, "root"))
+                        continue;
+
+                if (streq_ptr(i->pw_passwd, PASSWORD_SEE_SHADOW))
+                        break;
+                log_debug("passwd: root account with non-shadow password found, treating root as configured");
+                return false;
+        }
+        if (r < 0)
+                return log_error_errno(r, "Failed to read %s: %m", filename);
+        if (r == 0) {
+                log_debug("No root account found in %s, assuming root is not configured.", filename);
+                return true;
         }
 
-        return arg_force; /* exists, but if --force was given we should still configure the file. */
+        r = xfopenat(dir_fd, "shadow", "re", O_NOFOLLOW, &shadow);
+        if (r == -ENOENT) {
+                log_debug("No shadow file found, assuming root is not configured.");
+                return true; /* missing */
+        }
+        if (r < 0)
+                return log_error_errno(r, "Failed to access shadow: %m");
+
+        struct spwd *j;
+        while ((r = fgetspent_sane(shadow, &j)) > 0) {
+                if (!streq(j->sp_namp, "root"))
+                        continue;
+
+                bool unprovisioned = streq_ptr(j->sp_pwdp, PASSWORD_UNPROVISIONED);
+                log_debug("Root account found, %s.",
+                          unprovisioned ? "with unprovisioned password, treating root as not configured" :
+                                          "treating root as configured");
+                return unprovisioned;
+        }
+        if (r < 0)
+                return log_error_errno(r, "Failed to read shadow: %m");
+        assert(r == 0);
+        log_debug("No root account found in shadow, assuming root is not configured.");
+        return true;
 }
 
 static bool locale_is_installed_bool(const char *name) {

src/sysusers/sysusers.c

@@ -618,7 +618,6 @@ static int write_temporary_shadow(const char *shadow_path, FILE **ret_tmpfile, c
 
                 struct spwd n = {
                         .sp_namp = i->name,
-                        .sp_pwdp = (char*) PASSWORD_LOCKED_AND_INVALID,
                         .sp_lstchg = lstchg,
                         .sp_min = -1,
                         .sp_max = -1,
@@ -641,6 +640,11 @@ static int write_temporary_shadow(const char *shadow_path, FILE **ret_tmpfile, c
 
                 if (creds_password)
                         n.sp_pwdp = creds_password;
+                else if (streq(i->name, "root"))
+                        /* Let firstboot set the password later */
+                        n.sp_pwdp = (char*) PASSWORD_UNPROVISIONED;
+                else
+                        n.sp_pwdp = (char*) PASSWORD_LOCKED_AND_INVALID;
 
                 r = putspent_sane(&n, shadow);
                 if (r < 0)

units/systemd-firstboot.service

@@ -16,7 +16,7 @@ ConditionFirstBoot=yes
 
 DefaultDependencies=no
 After=systemd-remount-fs.service
-Before=systemd-sysusers.service systemd-vconsole-setup.service sysinit.target first-boot-complete.target
+Before=systemd-vconsole-setup.service sysinit.target first-boot-complete.target
 Wants=first-boot-complete.target
 Conflicts=shutdown.target
 Before=shutdown.target