Block SIGCHLD before booting LKL

src/lkl-wrap.c

@@ -1,5 +1,7 @@
 /* SPDX-License-Identifier: MIT */
 
+#include <pthread.h>
+#include <signal.h>
 #include <stdio.h>
 #include <string.h>
 
@@ -21,6 +23,14 @@ int kbox_boot_kernel(const char *cmdline)
     const char *effective = cmdline;
     char buf[512];
     int ret;
+    sigset_t mask;
+
+    /* Block SIGCHLD before booting LKL so that all kernel threads created
+     * by lkl_start_kernel inherit the blocked mask.
+     */
+    sigemptyset(&mask);
+    sigaddset(&mask, SIGCHLD);
+    pthread_sigmask(SIG_BLOCK, &mask, NULL);
 
     if (!cmdline || !*cmdline) {
         effective = "console=null";

src/seccomp-supervisor.c

@@ -492,16 +492,12 @@ int kbox_run_supervisor(const struct kbox_sysnrs *sysnrs,
     if (socketpair_create(sp) < 0)
         return -1;
 
-    /* Block SIGCHLD before fork so the parent cannot lose the signal
-     * in the window between fork and signalfd creation.  Save the
-     * caller's mask so both parent and child can restore it later.
+    /* Save the caller's mask so both parent and child can
+     * restore it later.
      */
     {
-        sigset_t chld_mask;
-        sigemptyset(&chld_mask);
-        sigaddset(&chld_mask, SIGCHLD);
-        if (sigprocmask(SIG_BLOCK, &chld_mask, &old_mask) < 0) {
-            fprintf(stderr, "sigprocmask(SIG_BLOCK): %s\n", strerror(errno));
+        if (sigprocmask(SIG_SETMASK, NULL, &old_mask) < 0) {
+            fprintf(stderr, "sigprocmask(SIG_SETMASK): %s\n", strerror(errno));
             close(sp[0]);
             close(sp[1]);
             return -1;