pids: init_struct_pid.tasks should never see the swapper process

"statically initialize struct pid for swapper" commit 820e45db says:

	Statically initialize a struct pid for the swapper process (pid_t == 0)
	and attach it to init_task.  This is needed so task_pid(), task_pgrp()
	and task_session() interfaces work on the swapper process also.

OK, but:

	- it doesn't make sense to add init_task.pids[].node into
	  init_struct_pid.tasks[], and in fact this just wrong.

	  idle threads are special, they shouldn't be visible on any
	  global list. In particular do_each_pid_task(init_struct_pid)
	  shouldn't see swapper.

	  This is the actual reason why kill(0, SIGKILL) from /sbin/init
	  (which starts with 0,0 special pids) crashes the kernel. The
	  signal sent to pgid/sid == 0 must never see idle threads, even
	  if the previous patch fixed the crash itself.

	- we have other idle threads running on the non-boot CPUs, see
	  the next patch.

Change INIT_STRUCT_PID/INIT_PID_LINK to create the empty/unhashed
hlist_head/hlist_node. Like any other idle thread swapper can never exit,
so detach_pid()->__hlist_del() is not possible, but we could change
INIT_PID_LINK() to set pprev = &next if needed.

All we need is the valid swapper->pids[].pid == &init_struct_pid.

Reported-by: Mathias Krause <mathias.krause@secunet.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Cedric Le Goater <clg@fr.ibm.com>
Cc: Dave Hansen <haveblue@us.ibm.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Herbert Poetzl <herbert@13thfloor.at>
Cc: Mathias Krause <Mathias.Krause@secunet.com>
Acked-by: Roland McGrath <roland@redhat.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Cc: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 4 deletions

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index 6deb1ba52fdb..94fecb748a28 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -45,9 +45,9 @@ extern struct group_info init_groups;
 #define INIT_STRUCT_PID {                                               \
         .count          = ATOMIC_INIT(1),                               \
         .tasks          = {                                             \
-                { .first = &init_task.pids[PIDTYPE_PID].node },         \
-                { .first = &init_task.pids[PIDTYPE_PGID].node },        \
-                { .first = &init_task.pids[PIDTYPE_SID].node },         \
+                { .first = NULL },                                      \
+                { .first = NULL },                                      \
+                { .first = NULL },                                      \
         },                                                              \
         .level          = 0,                                            \
         .numbers        = { {                                           \
@@ -61,7 +61,7 @@ extern struct group_info init_groups;
 {                                                               \
         .node = {                                               \
                 .next = NULL,                                   \
-                .pprev = &init_struct_pid.tasks[type].first,    \
+                .pprev = NULL,                                  \
         },                                                      \
         .pid = &init_struct_pid,                                \
 }