[CRIU] [PATCH v3 4/5] restore: fix race with helpers' kids dying too early

[Tycho Andersen]

We masked off SIGCHLD in wait_on_helpers_zombies(), but in fact this is too
late: zombies can die any time after CR_STATE_RESTORE before this function
is called, which lead to us getting "unexpected" deaths. Instead, we should
mask off SIGCHLD before the helpers finish CR_STATE_RESTORE, since they're
explicitly going to wait on all their kids to make sure they don't die
anyway.

Signed-off-by: Tycho Andersen <tycho.andersen at canonical.com>
---
 criu/cr-restore.c | 24 ++++++++++--------------
 1 file changed, 10 insertions(+), 14 deletions(-)

diff --git a/criu/cr-restore.c b/criu/cr-restore.c
index b4b7038..977e049 100644
--- a/criu/cr-restore.c
+++ b/criu/cr-restore.c
@@ -749,15 +749,6 @@ static unsigned long task_entries_pos;
 static int wait_on_helpers_zombies(void)
 {
 	struct pstree_item *pi;
-	sigset_t blockmask, oldmask;
-
-	sigemptyset(&blockmask);
-	sigaddset(&blockmask, SIGCHLD);
-
-	if (sigprocmask(SIG_BLOCK, &blockmask, &oldmask) == -1) {
-		pr_perror("Can not set mask of blocked signals");
-		return -1;
-	}
 
 	list_for_each_entry(pi, &current->children, sibling) {
 		pid_t pid = pi->pid.virt;
@@ -779,11 +770,6 @@ static int wait_on_helpers_zombies(void)
 		}
 	}
 
-	if (sigprocmask(SIG_SETMASK, &oldmask, NULL) == -1) {
-		pr_perror("Can not unset mask of blocked signals");
-		BUG();
-	}
-
 	return 0;
 }
 
@@ -871,6 +857,16 @@ static int restore_one_task(int pid, CoreEntry *core)
 	else if (current->pid.state == TASK_DEAD)
 		ret = restore_one_zombie(core);
 	else if (current->pid.state == TASK_HELPER) {
+		sigset_t blockmask, oldmask;
+
+		sigemptyset(&blockmask);
+		sigaddset(&blockmask, SIGCHLD);
+
+		if (sigprocmask(SIG_BLOCK, &blockmask, &oldmask) == -1) {
+			pr_perror("Can not set mask of blocked signals");
+			return -1;
+		}
+
 		restore_finish_stage(CR_STATE_RESTORE);
 		if (rsti(current)->helper_cb)
 			rsti(current)->helper_cb();
-- 
2.7.4