polkit: temporarily spawn of a polkit agent in terminals for possibly authenticated operations

src/shared/util.c

@@ -6035,3 +6035,88 @@ int fd_inc_rcvbuf(int fd, size_t n) {
 
         return 1;
 }
+
+int fork_agent(pid_t *pid, const char *path, ...) {
+        pid_t parent_pid, agent_pid;
+        int fd;
+        bool stdout_is_tty, stderr_is_tty;
+        unsigned n, i;
+        va_list ap;
+        char **l;
+
+        assert(pid);
+        assert(path);
+
+        parent_pid = getpid();
+
+        /* Spawns a temporary TTY agent, making sure it goes away when
+         * we go away */
+
+        agent_pid = fork();
+        if (agent_pid < 0)
+                return -errno;
+
+        if (agent_pid != 0) {
+                *pid = agent_pid;
+                return 0;
+        }
+
+        /* In the child:
+         *
+         * Make sure the agent goes away when the parent dies */
+        if (prctl(PR_SET_PDEATHSIG, SIGTERM) < 0)
+                _exit(EXIT_FAILURE);
+
+        /* Check whether our parent died before we were able
+         * to set the death signal */
+        if (getppid() != parent_pid)
+                _exit(EXIT_SUCCESS);
+
+        /* Don't leak fds to the agent */
+        close_all_fds(NULL, 0);
+
+        stdout_is_tty = isatty(STDOUT_FILENO);
+        stderr_is_tty = isatty(STDERR_FILENO);
+
+        if (!stdout_is_tty || !stderr_is_tty) {
+                /* Detach from stdout/stderr. and reopen
+                 * /dev/tty for them. This is important to
+                 * ensure that when systemctl is started via
+                 * popen() or a similar call that expects to
+                 * read EOF we actually do generate EOF and
+                 * not delay this indefinitely by because we
+                 * keep an unused copy of stdin around. */
+                fd = open("/dev/tty", O_WRONLY);
+                if (fd < 0) {
+                        log_error("Failed to open /dev/tty: %m");
+                        _exit(EXIT_FAILURE);
+                }
+
+                if (!stdout_is_tty)
+                        dup2(fd, STDOUT_FILENO);
+
+                if (!stderr_is_tty)
+                        dup2(fd, STDERR_FILENO);
+
+                if (fd > 2)
+                        close(fd);
+        }
+
+        /* Count arguments */
+        va_start(ap, path);
+        for (n = 0; va_arg(ap, char*); n++)
+                ;
+        va_end(ap);
+
+        /* Allocate strv */
+        l = alloca(sizeof(char *) * (n + 1));
+
+        /* Fill in arguments */
+        va_start(ap, path);
+        for (i = 0; i <= n; i++)
+                l[i] = va_arg(ap, char*);
+        va_end(ap);
+
+        execv(path, l);
+        _exit(EXIT_FAILURE);
+}

src/shared/util.h

@@ -528,4 +528,7 @@ int is_kernel_thread(pid_t pid);
 
 int fd_inc_sndbuf(int fd, size_t n);
 int fd_inc_rcvbuf(int fd, size_t n);
+
+int fork_agent(pid_t *pid, const char *path, ...);
+
 #endif