From f0fdbdb003cb3e8d8188c1c01701b510505b5b8b Mon Sep 17 00:00:00 2001
From: Lennart Poettering <lennart@poettering.net>
Date: Wed, 27 Aug 2014 21:42:20 +0200
Subject: [PATCH] util: fix minimal race where we might miss SIGTERMs when
 forking off an agent

Before forking, block all signals, and unblock them afterwards. This way
the child will have them blocked, and we won't lose them.

(cherry picked from commit 8a7c93d858c342744adf481565d8bb03b9713dcf)

Related: #1134818
---
 src/shared/util.c | 35 +++++++++++++++++++++++------------
 1 file changed, 23 insertions(+), 12 deletions(-)

diff --git a/src/shared/util.c b/src/shared/util.c
index e4448ff..9f25b15 100644
--- a/src/shared/util.c
+++ b/src/shared/util.c
@@ -5002,9 +5002,9 @@ int fd_inc_rcvbuf(int fd, size_t n) {
 }
 
 int fork_agent(pid_t *pid, const int except[], unsigned n_except, const char *path, ...) {
-        pid_t parent_pid, agent_pid;
-        int fd;
         bool stdout_is_tty, stderr_is_tty;
+        pid_t parent_pid, agent_pid;
+        sigset_t ss, saved_ss;
         unsigned n, i;
         va_list ap;
         char **l;
@@ -5012,16 +5012,25 @@ int fork_agent(pid_t *pid, const int except[], unsigned n_except, const char *pa
         assert(pid);
         assert(path);
 
-        parent_pid = getpid();
-
         /* Spawns a temporary TTY agent, making sure it goes away when
          * we go away */
 
+        parent_pid = getpid();
+
+        /* First we temporarily block all signals, so that the new
+         * child has them blocked initially. This way, we can be sure
+         * that SIGTERMs are not lost we might send to the agent. */
+        assert_se(sigfillset(&ss) >= 0);
+        assert_se(sigprocmask(SIG_SETMASK, &ss, &saved_ss) >= 0);
+
         agent_pid = fork();
-        if (agent_pid < 0)
+        if (agent_pid < 0) {
+                assert_se(sigprocmask(SIG_SETMASK, &saved_ss, NULL) >= 0);
                 return -errno;
+        }
 
         if (agent_pid != 0) {
+                assert_se(sigprocmask(SIG_SETMASK, &saved_ss, NULL) >= 0);
                 *pid = agent_pid;
                 return 0;
         }
@@ -5032,24 +5041,26 @@ int fork_agent(pid_t *pid, const int except[], unsigned n_except, const char *pa
         if (prctl(PR_SET_PDEATHSIG, SIGTERM) < 0)
                 _exit(EXIT_FAILURE);
 
+        /* Make sure we actually can kill the agent, if we need to, in
+         * case somebody invoked us from a shell script that trapped
+         * SIGTERM or so... */
+        reset_all_signal_handlers();
+        reset_signal_mask();
+
         /* Check whether our parent died before we were able
-         * to set the death signal */
+         * to set the death signal and unblock the signals */
         if (getppid() != parent_pid)
                 _exit(EXIT_SUCCESS);
 
         /* Don't leak fds to the agent */
         close_all_fds(except, n_except);
 
-        /* Make sure we actually can kill the agent, if we need to, in
-         * case somebody invoked us from a shell script that trapped
-         * SIGTERM or so... */
-        reset_all_signal_handlers();
-        reset_signal_mask();
-
         stdout_is_tty = isatty(STDOUT_FILENO);
         stderr_is_tty = isatty(STDERR_FILENO);
 
         if (!stdout_is_tty || !stderr_is_tty) {
+                int fd;
+
                 /* Detach from stdout/stderr. and reopen
                  * /dev/tty for them. This is important to
                  * ensure that when systemctl is started via