[PATCH] ptrace: allow to ptrace /sbin/init

[Oleg Nesterov <oleg@tv-sign.ru>]

Afaics, currently there are no kernel problems with ptracing init, it can't
lose SIGNAL_UNKILLABLE flag and be killed/stopped by accident.

The ability to strace/debug init can be very useful if you try to figure
out why it does not work as expected.

However, admin should know what he does, "gdb /sbin/init 1" stops init, it
can't reap orphaned zombies or take care of /etc/inittab until continued.
It is even possible to crash init (and thus the whole system) if you wish,
ptracer has full control.

See also the long discussion: http://marc.info/?t=120628018600001

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- 25/kernel/ptrace.c~INIT_PTRACEABLE	2008-03-26 21:31:39.000000000 +0300
+++ 25/kernel/ptrace.c	2008-03-26 21:33:35.000000000 +0300
@@ -168,8 +168,6 @@ int ptrace_attach(struct task_struct *ta
 	audit_ptrace(task);
 
 	retval = -EPERM;
-	if (task->pid <= 1)
-		goto out;
 	if (same_thread_group(task, current))
 		goto out;
 
@@ -517,12 +515,6 @@ struct task_struct *ptrace_get_task_stru
 {
 	struct task_struct *child;
 
-	/*
-	 * Tracing init is not allowed.
-	 */
-	if (pid == 1)
-		return ERR_PTR(-EPERM);
-
 	read_lock(&tasklist_lock);
 	child = find_task_by_vpid(pid);
 	if (child)