[LEDE-DEV] [PATCH libubox 1/2] uloop: Fix race condition in SIGCHLD handling

[Michal Sojka]

When uloop_process_add() is called outside of uloop_run(), i.e. not
from a callback (which is the case of at least utrace and ujail),
child events can be missed. The reason is that when SIGCHILD handler
is installed in uloop_run(), after the uloop_process_add() is called,
then an initial signal could be missed.

Commit 4e3a47a ("uloop: use a waker for notifying sigchld and loop
cancel events", 2016-06-09) solved a similar problem and introduced
uloop_init() but forgot to move a call to uloop_setup_signals() there.
This is what this commit does.

Now, uloop_process_add() can be called any time after uloop_init()
without missing any event.

Signed-off-by: Michal Sojka <sojkam1 at fel.cvut.cz>
---
 uloop.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/uloop.c b/uloop.c
index d2f41bb..3813e18 100644
--- a/uloop.c
+++ b/uloop.c
@@ -116,6 +116,8 @@ static int waker_init(void)
 	return 0;
 }
 
+static void uloop_setup_signals(bool add);
+
 int uloop_init(void)
 {
 	if (uloop_init_pollfd() < 0)
@@ -126,6 +128,8 @@ int uloop_init(void)
 		return -1;
 	}
 
+	uloop_setup_signals(true);
+
 	return 0;
 }
 
@@ -525,12 +529,7 @@ int uloop_run_timeout(int timeout)
 	int next_time = 0;
 	struct timeval tv;
 
-	/*
-	 * Handlers are only updated for the first call to uloop_run() (and restored
-	 * when this call is done).
-	 */
-	if (!uloop_run_depth++)
-		uloop_setup_signals(true);
+	uloop_run_depth++;
 
 	uloop_status = 0;
 	uloop_cancelled = false;
@@ -553,14 +552,15 @@ int uloop_run_timeout(int timeout)
 		uloop_run_events(next_time);
 	}
 
-	if (!--uloop_run_depth)
-		uloop_setup_signals(false);
+	--uloop_run_depth;
 
 	return uloop_status;
 }
 
 void uloop_done(void)
 {
+	uloop_setup_signals(false);
+
 	if (poll_fd >= 0) {
 		close(poll_fd);
 		poll_fd = -1;
-- 
2.14.1