From: Oleg Nesterov As Eric pointed out, there is no problem with init starting with sid == pgid == 0, and this was historical linux behavior changed in 2.6.18. Remove kernel_init()->__set_special_pids(), this is unneeded and complicates the rules for sys_setsid(). This change and the previous change in daemonize() mean that /sbin/init does not need the special "session != 1" hack in sys_setsid() any longer. We can't remove this check yet, we should cleanup copy_process(CLONE_NEWPID) first, so update the comment only. Signed-off-by: Oleg Nesterov Acked-by: "Eric W. Biederman" Cc: Pavel Emelyanov Signed-off-by: Andrew Morton --- init/main.c | 1 - kernel/sys.c | 9 ++++----- 2 files changed, 4 insertions(+), 6 deletions(-) diff -puN init/main.c~start-the-global-sbin-init-with-00-special-pids init/main.c --- a/init/main.c~start-the-global-sbin-init-with-00-special-pids +++ a/init/main.c @@ -835,7 +835,6 @@ static int __init kernel_init(void * unu */ init_pid_ns.child_reaper = current; - __set_special_pids(task_pid(current)); cad_pid = task_pid(current); smp_prepare_cpus(setup_max_cpus); diff -puN kernel/sys.c~start-the-global-sbin-init-with-00-special-pids kernel/sys.c --- a/kernel/sys.c~start-the-global-sbin-init-with-00-special-pids +++ a/kernel/sys.c @@ -1054,12 +1054,11 @@ asmlinkage long sys_setsid(void) if (group_leader->signal->leader) goto out; - /* Fail if a process group id already exists that equals the proposed - * session id. + /* Fail if a process group id already exists that equals the + * proposed session id. * - * Don't check if session == 1 because kernel threads and CLONE_NEWPID - * tasks use this session id and so the check will always fail and make - * it so init cannot successfully call setsid. + * Don't check if session == 1, clone(CLONE_NEWPID) creates + * this group/session beforehand. */ if (session != 1 && pid_task(sid, PIDTYPE_PGID)) goto out; _