Quoting Serge E. Hallyn (serue@us.ibm.com):
> >From 945fe66259cd0cfdc2fe846287b7821e329a558c Mon Sep 17 00:00:00 2001
> From: sergeh@us.ibm.com <hallyn@kernel.(none)>
> Date: Tue, 9 Oct 2007 08:30:30 -0700
> Subject: [PATCH] namespaces: introduce sys_hijack (v4)
> 
> Move most of do_fork() into a new do_fork_task() which acts on
> a new argument, task, rather than on current.  do_fork() becomes
> a call to do_fork_task(current, ...).
> 
> Introduce sys_hijack (for x86 only so far).  It is like clone, but
>  in place of a stack pointer (which is assumed null) it accepts a
> pid.  The process identified by that pid is the one which is
> actually cloned.  Some state - include the file table, the signals
> and sighand (and hence tty), and the ->parent are taken from the
> calling process.
> 
> The effect is a sort of namespace enter.  The following program
> uses sys_hijack to 'enter' all namespaces of the specified pid.
> For instance in one terminal, do
> 
> 	mount -t cgroup -ons /cgroup
> 	hostname
> 	  qemu
> 	ns_exec -u /bin/sh
> 	  hostname serge
>           echo $$
>             1073
> 	  cat /proc/$$/cgroup
> 	    ns:/node_1073
> 
> In another terminal then do
> 
> 	hostname
> 	  qemu
> 	cat /proc/$$/cgroup
> 	  ns:/
> 	hijack 1073
> 	  hostname
> 	    serge
> 	  cat /proc/$$/cgroup
> 	    ns:/node_1073
> 
> sys_hijack is arch-dependent and is only implemented for i386 so far.
> 
> Changelog:
> 	Aug 23: send a stop signal to the hijacked process
> 		(like ptrace does).
> 	Oct 09: Update for 2.6.23-rc8-mm2 (mainly pidns)
> 		Don't take task_lock under rcu_read_lock
> 		Send hijacked process to cgroup_fork() as
> 		the first argument.
> 		Removed some unneeded task_locks.

Thanks to Cedric for finding an oops when using pid namespaces.  The
following patch fixes the problem.

In addition, to hijack a process in another pid namespace, the
hijack.c test program needs to be updated to do waitpid as
	while(waitpid(-1, &status, __WALL) != -1)
		;

as shown below:

==============================================================
hijack.c
==============================================================

int do_clone_task(void)
{
	execl("/bin/sh", "/bin/sh", NULL);
}

int main(int argc, char *argv[])
{
	int pid;
	int ret;
	int status;

	if (argc < 2)
		return 1;
	pid = atoi(argv[1]);

	ret = syscall(327, SIGCHLD, pid, NULL, NULL);

	if  (ret == 0) {
		return do_clone_task();
	} else if (ret < 0) {
		perror("sys_hijack");
	} else {
		printf("waiting on cloned process %d\n", ret);
		while(waitpid(-1, &status, __WALL) != -1)
			;
		printf("cloned process %d exited with %d\n", ret, status);
	}

	return ret;
}
==============================================================

>From f1d9621e8325471e3ccde7f5fc2ed5a7be582524 Mon Sep 17 00:00:00 2001
From: sergeh@us.ibm.com <hallyn@kernel.(none)>
Date: Thu, 11 Oct 2007 14:26:05 -0700
Subject: [PATCH 2/2] hijack: pidns bugfix

My change to alloc_pid was bogus and introduced a pidns bug.  Fix.

Signed-off-by: sergeh@us.ibm.com <hallyn@kernel.(none)>
---
 include/linux/pid.h |    2 +-
 kernel/fork.c       |    2 +-
 kernel/pid.c        |    5 ++---
 3 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 145dce7..e29a900 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -119,7 +119,7 @@ extern struct pid *find_pid(int nr);
 extern struct pid *find_get_pid(int nr);
 extern struct pid *find_ge_pid(int nr, struct pid_namespace *);
 
-extern struct pid *alloc_pid(struct task_struct *task);
+extern struct pid *alloc_pid(struct pid_namespace *ns);
 extern void FASTCALL(free_pid(struct pid *pid));
 extern void zap_pid_ns_processes(struct pid_namespace *pid_ns);
 
diff --git a/kernel/fork.c b/kernel/fork.c
index ac73f3e..c1d4672 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1154,7 +1154,7 @@ static struct task_struct *copy_process(struct task_struct *task,
 
 	if (pid != &init_struct_pid) {
 		retval = -ENOMEM;
-		pid = alloc_pid(task);
+		pid = alloc_pid(task_active_pid_ns(p));
 		if (!pid)
 			goto bad_fork_cleanup_namespaces;
 
diff --git a/kernel/pid.c b/kernel/pid.c
index b887a6a..d7388d7 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -238,15 +238,14 @@ fastcall void free_pid(struct pid *pid)
 	call_rcu(&pid->rcu, delayed_put_pid);
 }
 
-struct pid *alloc_pid(struct task_struct *srctsk)
+struct pid *alloc_pid(struct pid_namespace *ns)
 {
 	struct pid *pid;
 	enum pid_type type;
 	int i, nr;
-	struct pid_namespace *tmp, *ns;
+	struct pid_namespace *tmp;
 	struct upid *upid;
 
-	ns = task_active_pid_ns(srctsk);
 	pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
 	if (!pid)
 		goto out;
-- 
1.5.1

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