2 files changed, 26 insertions, 0 deletions
diff --git a/src/exit/atexit.c b/src/exit/atexit.c
index 854e9fdd..92c91c9d 100644
--- a/src/exit/atexit.c
+++ b/src/exit/atexit.c
@@ -19,6 +19,7 @@ static struct fl
 	void *a[COUNT];
 } builtin, *head;
 
+static int finished_atexit;
 static int slot;
 static volatile int lock[1];
 volatile int *const __atexit_lockptr = lock;
@@ -34,6 +35,10 @@ void __funcs_on_exit()
 		func(arg);
 		LOCK(lock);
 	}
+	/* Unlock to prevent deadlock if a global dtor
+	 * attempts to call atexit. */
+	finished_atexit = 1;
+	UNLOCK(lock);
 }
 
 void __cxa_finalize(void *dso)
@@ -44,6 +49,13 @@ int __cxa_atexit(void (*func)(void *), void *arg, void *dso)
 {
 	LOCK(lock);
 
+	/* Prevent dtors from registering further atexit
+	 * handlers that would never be run. */
+	if (finished_atexit) {
+		UNLOCK(lock);
+		return -1;
+	}
+
 	/* Defer initialization of head so it can be in BSS */
 	if (!head) head = &builtin;
 
diff --git a/src/exit/exit.c b/src/exit/exit.c
index a6869b37..17b33cc6 100644
--- a/src/exit/exit.c
+++ b/src/exit/exit.c
@@ -1,6 +1,9 @@
 #include <stdlib.h>
 #include <stdint.h>
 #include "libc.h"
+#include "pthread_impl.h"
+#include "atomic.h"
+#include "syscall.h"
 
 static void dummy()
 {
@@ -26,6 +29,17 @@ weak_alias(libc_exit_fini, __libc_exit_fini);
 
 _Noreturn void exit(int code)
 {
+	/* Handle potentially concurrent or recursive calls to exit,
+	 * whose behaviors have traditionally been undefined by the
+	 * standards. Using a custom lock here avoids pulling in lock
+	 * machinery and lets us trap recursive calls while supporting
+	 * multiple threads contending to be the one to exit(). */
+	static volatile int exit_lock[1];
+	int tid =  __pthread_self()->tid;
+	int prev = a_cas(exit_lock, 0, tid);
+	if (prev == tid) a_crash();
+	else if (prev) for (;;) __sys_pause();
+
 	__funcs_on_exit();
 	__libc_exit_fini();
 	__stdio_exit();