firstboot: follow lock protocol when changing /etc/shadow

1 files changed, 40 insertions, 0 deletions

diff --git a/src/shared/util.c b/src/shared/util.c
index bef87304e6..88511b69c0 100644
--- a/src/shared/util.c
+++ b/src/shared/util.c
@@ -6823,3 +6823,43 @@ bool is_localhost(const char *hostname) {
                endswith(hostname, ".localdomain") ||
                endswith(hostname, ".localdomain.");
 }
+
+int take_password_lock(const char *root) {
+
+        struct flock flock = {
+                .l_type = F_WRLCK,
+                .l_whence = SEEK_SET,
+                .l_start = 0,
+                .l_len = 0,
+        };
+
+        const char *path;
+        int fd, r;
+
+        /* This is roughly the same as lckpwdf(), but not as awful. We
+         * don't want to use alarm() and signals, hence we implement
+         * our own trivial version of this.
+         *
+         * Note that shadow-utils also takes per-database locks in
+         * addition to lckpwdf(). However, we don't given that they
+         * are redundant as they they invoke lckpwdf() first and keep
+         * it during everything they do. The per-database locks are
+         * awfully racy, and thus we just won't do them. */
+
+        if (root)
+                path = strappenda(root, "/etc/.pwd.lock");
+        else
+                path = "/etc/.pwd.lock";
+
+        fd = open(path, O_WRONLY|O_CREAT|O_CLOEXEC|O_NOCTTY|O_NOFOLLOW, 0600);
+        if (fd < 0)
+                return -errno;
+
+        r = fcntl(fd, F_SETLKW, &flock);
+        if (r < 0) {
+                safe_close(fd);
+                return -errno;
+        }
+
+        return fd;
+}