lxc_setup_tios(): Ignore SIGTTOU and SIGTTIN signals

Prevent an endless loop while executing lxc-attach in the background: The kernel might fire SIGTTOU while an ioctl() in tcsetattr() is executed. When the ioctl() is resumed and retries, the signal handler interrupts it again. We can't configure the TTY to stop sending the signals in the first place since that is a modification/write to the TTY already. Still we clear the TOSTOP flag to prevent further signals. Command to reproduce the hang: ---------------------------- cat > lxc_hang.sh << EOF /usr/bin/timeout 5s /usr/bin/lxc-attach -n SOMECONTAINER -- /bin/true EOF sh lxc_hang.sh # hangs ---------------------------- Signed-off-by: Thomas Jarosch <thomas.jarosch@intra2net.com>

lxc_setup_tios(): Ignore SIGTTOU and SIGTTIN signals
4dc96430 · Thomas Jarosch · Christian Brauner · 920da314 · 4dc96430
Commit 4dc96430 authored Feb 02, 2017 by Thomas Jarosch Committed by Christian Brauner Feb 08, 2017
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console.c src/lxc/console.c +9 -1

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--- a/src/lxc/console.c
+++ b/src/lxc/console.c
@@ -257,6 +257,14 @@ int lxc_setup_tios(int fd, struct termios *oldtios)
 		return -1;
 	}
+	/* ensure we don't end up in an endless loop:
+	 * The kernel might fire SIGTTOU while an
+	 * ioctl() in tcsetattr() is executed. When the ioctl()
+	 * is resumed and retries, the signal handler interrupts it again.
+	 */
+	signal (SIGTTIN, SIG_IGN);
+	signal (SIGTTOU, SIG_IGN);
 	newtios = *oldtios;
 	/* We use the same settings that ssh does. */
@@ -265,7 +273,7 @@ int lxc_setup_tios(int fd, struct termios *oldtios)
 #ifdef IUCLC
 	newtios.c_iflag &= ~IUCLC;
 #endif
-	newtios.c_lflag &= ~(ISIG | ICANON | ECHO | ECHOE | ECHOK | ECHONL);
+	newtios.c_lflag &= ~(TOSTOP | ISIG | ICANON | ECHO | ECHOE | ECHOK | ECHONL);
 #ifdef IEXTEN
 	newtios.c_lflag &= ~IEXTEN;
 #endif