from https://www.mail-archive.com/skaware@list.skarnet.org/msg01006.html

diff --git a/src/supervision/s6-supervise.c
b/src/supervision/s6-supervise.c index 2e8fa38..7605a82 100644
--- a/src/supervision/s6-supervise.c
+++ b/src/supervision/s6-supervise.c
@@ -9,6 +9,9 @@
 #include <errno.h>
 #include <fcntl.h>
 #include <signal.h>
+#ifdef WANT_CLONE_NEWPID
+#  include <sched.h>
+#endif
 #include <skalibs/allreadwrite.h>
 #include <skalibs/bytestr.h>
 #include <skalibs/uint.h>
@@ -203,6 +206,67 @@ static int maybesetsid (void)
   return 1 ;
 }
 
+ static void exec_run(int p[2], int notifyp[2], int fd) gccattr_noreturn ;
+ static void exec_run(int p[2], int notifyp[2], int fd)
+ {
+  char const *cargv[2] = { "run", 0 } ;
+  PROG = "s6-supervise (child)" ;
+  selfpipe_finish() ;
+  if (notifyp[0] >= 0) close(notifyp[0]) ;
+  close(p[0]) ;
+  if (notifyp[1] >= 0 && fd_move(fd, notifyp[1]) < 0)
+  {
+    failcoe(p[1]) ;
+    strerr_diefu1sys(127, "move notification descriptor") ;
+  }
+  if (!maybesetsid())
+  {
+    failcoe(p[1]) ;
+    strerr_diefu1sys(127, "access ./nosetsid") ;
+  }
+  execve("./run", (char *const *)cargv, (char *const *)environ) ;
+  failcoe(p[1]) ;
+  strerr_dieexec(127, "run") ;
+}
+
+static pid_t spawn_run_fork(int p[2], int notifyp[2], int fd)
+{
+  pid_t pid = fork() ;
+  if (!pid) exec_run(p, notifyp, fd) ;
+  return pid ;
+}
+
+#ifdef WANT_CLONE_NEWPID
+typedef struct
+{
+  int p[2] ;
+  int notifyp[2] ;
+  int fd ;
+} exec_run_t ;
+
+static int exec_run_shim(void *ctx) gccattr_noreturn ;
+static int exec_run_shim(void *ctx)
+{
+  exec_run_t *er = (exec_run_t *) ctx ;
+  exec_run(er->p, er->notifyp, er->fd) ;
+}
+
+static pid_t spawn_run(int p[2], int notifyp[2], int fd)
+{
+  exec_run_t arg = { { p[0], p[1] }, { notifyp[0], notifyp[1] }, fd } ;
+  char child_stack[SIGSTKSZ] ;
+  if (access("clone-newpid", F_OK) < 0 && errno == ENOENT)
+    return spawn_run_fork(p, notifyp, fd) ;
+  return (pid_t) clone(&exec_run_shim, child_stack + sizeof(child_stack),
+      CLONE_NEWPID | SIGCHLD, &arg) ;
+}
+#else /* if !defined(WANT_CLONE_NEWPID) */
+static pid_t spawn_run(int p[2], int notifyp[2], int fd)
+{
+  return spawn_run_fork(p, notifyp, fd) ;
+}
+#endif /* defined(WANT_CLONE_NEWPID) */
+
 static void trystart (void)
 {
   int p[2] ;
@@ -222,7 +286,7 @@ static void trystart (void)
     fd_close(p[1]) ; fd_close(p[0]) ;
     return ;
   }
-  pid = fork() ;
+  pid = spawn_run(p, notifyp, (int)fd) ;
   if (pid < 0)
   {
     settimeout(60) ;
@@ -232,27 +296,6 @@ static void trystart (void)
     fd_close(p[1]) ; fd_close(p[0]) ;
     return ;
   }
-  else if (!pid)
-  {
-    char const *cargv[2] = { "run", 0 } ;
-    PROG = "s6-supervise (child)" ;
-    selfpipe_finish() ;
-    if (notifyp[0] >= 0) close(notifyp[0]) ;
-    close(p[0]) ;
-    if (notifyp[1] >= 0 && fd_move((int)fd, notifyp[1]) < 0)
-    {
-      failcoe(p[1]) ;
-      strerr_diefu1sys(127, "move notification descriptor") ;
-    }
-    if (!maybesetsid())
-    {
-      failcoe(p[1]) ;
-      strerr_diefu1sys(127, "access ./nosetsid") ;
-    }
-    execve("./run", (char *const *)cargv, (char *const *)environ) ;
-    failcoe(p[1]) ;
-    strerr_dieexec(127, "run") ;
-  }
   if (notifyp[1] >= 0) fd_close(notifyp[1]) ;
   fd_close(p[1]) ;
   {