Ignore SIGTTOU during flush_stdin()

fixes #3500

1 files changed, 20 insertions, 9 deletions

diff --git a/src/firejail/util.c b/src/firejail/util.c
index 6bfc80903..3aa0584d6 100644
--- a/src/firejail/util.c
+++ b/src/firejail/util.c
@@ -957,16 +957,27 @@ int remove_overlay_directory(void) {
         return 0;
 }
 
+// flush stdin if it is connected to a tty and has input
 void flush_stdin(void) {
-        if (isatty(STDIN_FILENO)) {
-                int cnt = 0;
-                int rv = ioctl(STDIN_FILENO, FIONREAD, &cnt);
-                if (rv == 0 && cnt) {
-                        fwarning("removing %d bytes from stdin\n", cnt);
-                        rv = ioctl(STDIN_FILENO, TCFLSH, TCIFLUSH);
-                        (void) rv;
-                }
-        }
+        if (!isatty(STDIN_FILENO))
+                return;
+
+        int cnt = 0;
+        int rv = ioctl(STDIN_FILENO, FIONREAD, &cnt);
+        if (rv != 0 || cnt == 0)
+                return;
+
+        fwarning("removing %d bytes from stdin\n", cnt);
+
+        // If this process is backgrounded, below ioctl() will trigger
+        // SIGTTOU and stop us. We avoid this by ignoring SIGTTOU for
+        // the duration of the ioctl.
+        sighandler_t hdlr = signal(SIGTTOU, SIG_IGN);
+        rv = ioctl(STDIN_FILENO, TCFLSH, TCIFLUSH);
+        signal(SIGTTOU, hdlr);
+
+        if (rv)
+                fwarning("Flushing stdin failed: %s\n", strerror(errno));
 }
 
 // return 1 if new directory was created, else return 0