start.c: handle potential signal flood

Signalfd does not guarantee that we'll get an event for every signal. So if 3 tasks exit at the same time, we may get only one sigchld event. Therefore, in signal_handler(), always check whether init has exited. Do with with WNOWAIT so that we can still wait4 to cleanup the init after lxc_poll() exists (rather than complicating the code). Note - there is still a race in the kernel which can cause the container init to become a defunct child of the host init (!). This doesn't solve that, but is a potential (if very unlikely) race which apw pointed out while we were trying to create a reproducer for the kernel bug. Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com> Acked-by: Stéphane Graber <stgraber@ubuntu.com>

start.c: handle potential signal flood
80507ee8 · Serge Hallyn · 48c63f8d · 80507ee8
Commit 80507ee8 authored Aug 29, 2013 by Serge Hallyn
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Showing with 12 additions and 3 deletions

start.c src/lxc/start.c +12 -3

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--- a/src/lxc/start.c
+++ b/src/lxc/start.c
@@ -162,8 +162,10 @@ static int signal_handler(int fd, void *data,
 			   struct lxc_epoll_descr *descr)
 {
 	struct signalfd_siginfo siginfo;
+	siginfo_t info;
 	int ret;
 	pid_t *pid = data;
+	bool init_died = false;

 	ret = read(fd, &siginfo, sizeof(siginfo));
 	if (ret < 0) {
@@ -176,16 +178,23 @@ static int signal_handler(int fd, void *data,
 		return -1;
 	}

+	// check whether init is running
+	info.si_pid = 0;
+	ret = waitid(P_PID, *pid, &info, WEXITED | WNOWAIT | WNOHANG);
+	if (ret == 0 && info.si_pid == *pid) {
+		init_died = true;
+	}
+
 	if (siginfo.ssi_signo != SIGCHLD) {
 		kill(*pid, siginfo.ssi_signo);
 		INFO("forwarded signal %d to pid %d", siginfo.ssi_signo, *pid);
-		return 0;
+		return init_died ? 1 : 0;
 	}

 	if (siginfo.ssi_code == CLD_STOPPED ||
 	    siginfo.ssi_code == CLD_CONTINUED) {
 		INFO("container init process was stopped/continued");
-		return 0;
+		return init_died ? 1 : 0;
 	}

 	/* more robustness, protect ourself from a SIGCHLD sent
@@ -193,7 +202,7 @@ static int signal_handler(int fd, void *data,
 	 */
 	if (siginfo.ssi_pid != *pid) {
 		WARN("invalid pid for SIGCHLD");
-		return 0;
+		return init_died ? 1 : 0;
 	}

 	DEBUG("container init process exited");