Avoid using signal() to establish a signal handler

This is explicitly warned against by man:signal(2).  With glibc, its
behavior depends on compiler options.  Use sigaction(2) instead.

Author: DemiMarie

agent/qrexec-agent-data.c

@@ -74,8 +74,14 @@ static void sigusr1_handler(int __attribute__((__unused__))x)
 void prepare_child_env(void) {
     char pid_s[10];
 
-    signal(SIGCHLD, sigchld_handler);
-    signal(SIGUSR1, sigusr1_handler);
+    struct sigaction action = {
+        .sa_handler = sigchld_handler,
+        .sa_flags = 0,
+    };
+    sigemptyset(&action.sa_mask);
+    if (sigaction(SIGCHLD, &action, NULL)) abort();
+    action.sa_handler = sigusr1_handler;
+    if (sigaction(SIGUSR1, &action, NULL)) abort();
     int res = snprintf(pid_s, sizeof(pid_s), "%d", getpid());
     if (res < 0) abort();
     if (res >= (int)sizeof(pid_s)) abort();

agent/qrexec-agent.c

@@ -928,8 +928,14 @@ int main(int argc, char **argv)
     }
 
     init();
-    signal(SIGCHLD, sigchld_handler);
-    signal(SIGTERM, sigterm_handler);
+    struct sigaction action = {
+        .sa_handler = sigchld_handler,
+        .sa_flags = SA_RESTART,
+    };
+    sigemptyset(&action.sa_mask);
+    sigaction(SIGCHLD, &action, NULL);
+    action.sa_handler = sigterm_handler;
+    sigaction(SIGTERM, &action, NULL);
     signal(SIGPIPE, SIG_IGN);
 
     sigemptyset(&selectmask);

daemon/qrexec-daemon-common.c

@@ -219,7 +219,6 @@ int handle_agent_handshake(libvchan_t *vchan, bool remote_send_first)
 static void sigchld_handler(int x __attribute__((__unused__)))
 {
     sigchld = 1;
-    signal(SIGCHLD, sigchld_handler);
 }
 
 /* See also qrexec-agent.c:wait_for_session_maybe() */
@@ -271,7 +270,12 @@ int prepare_local_fds(struct qrexec_parsed_command *command, struct buffer *stdi
 {
     if (stdin_buffer == NULL)
         abort();
-    if (signal(SIGCHLD, sigchld_handler) == SIG_ERR)
+    struct sigaction action = {
+        .sa_handler = sigchld_handler,
+        .sa_flags = 0,
+    };
+    sigemptyset(&action.sa_mask);
+    if (sigaction(SIGCHLD, &action, NULL))
         return 126;
     return execute_parsed_qubes_rpc_command(command, &local_pid, &local_stdin_fd, &local_stdout_fd,
             NULL, stdin_buffer);