[PATCH] ptrace: it is fun to strace /sbin/init

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

(re-send with updated changelog)

Ptracing of /sbin/init is not allowed. Of course, this is dangerous, but may
be useful. Introduce the kernel boot parameter to allow this, so that we can't
surprise some special/secured systems.

Afaics, with the recent changes there is no kernel problems with ptracing init,
it can't lose SIGNAL_UNKILLABLE flag and be killed by accident. However, admin
should know what it does, "gdb /sbin/init 1" stops init, it can't reap 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.

The "if (pid == 1)" check in ptrace_get_task_struct() is killed, ptrace_attach
does the same check.

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

--- 25/kernel/ptrace.c~5_INIT_PTRACE	2008-03-16 17:22:04.000000000 +0300
+++ 25/kernel/ptrace.c	2008-03-16 18:33:02.000000000 +0300
@@ -160,6 +160,15 @@ int ptrace_may_attach(struct task_struct
 	return !err;
 }
 
+static int allow_ptrace_init;
+
+static int __init __allow_ptrace_init(char *str)
+{
+	allow_ptrace_init = 1;
+	return 1;
+}
+__setup("init_ptrace", __allow_ptrace_init);
+
 int ptrace_attach(struct task_struct *task)
 {
 	int retval;
@@ -168,7 +177,7 @@ int ptrace_attach(struct task_struct *ta
 	audit_ptrace(task);
 
 	retval = -EPERM;
-	if (task->pid <= 1)
+	if (unlikely(is_global_init(task)) && likely(!allow_ptrace_init))
 		goto out;
 	if (same_thread_group(task, current))
 		goto out;
@@ -518,12 +527,6 @@ 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)
--- 25/Documentation/kernel-parameters.txt~5_INIT_PTRACE	2008-02-15 16:58:12.000000000 +0300
+++ 25/Documentation/kernel-parameters.txt	2008-03-16 18:30:28.000000000 +0300
@@ -803,6 +803,8 @@ and is between 256 and 4096 characters. 
 			Run specified binary instead of /sbin/init as init
 			process.
 
+	init_ptrace	[KNL] Allows to ptrace init.
+
 	initcall_debug	[KNL] Trace initcalls as they are executed.  Useful
 			for working out where the kernel is dying during
 			startup.