Re: BUG can fill process table

[Andrew Morton]

André Cruz wrote:
> 
> When a process (say dhcpcd) changes the IFF_UP flag to TRUE on an
> interface (say eth0) to bring it up, a new process is created named
> after the interface (eth0 in this case) and it's PPID is dhcpcd.
> 
> If dhcpcd later changes the IFF_UP flag to FALSE to bring the interface
> down, the eth0 process dies but stays as a zombie. The problem is that
> dhcpcd never receives a sigchld (suposedly eth0 is it's child) and even
> if it executes a wait() no process is reaped and wait() returns -1.
> 
> The worse part of this is that when dhcpcd wants to bring up the
> interface again a NEW eth0 process is created and so this starts to fill
> up the process table.
> 
> I see two solutions for this: either the interface process start with a
> PPID of 1 (I noticed that init has no problems dealing with them when
> they die) or dhcpcd should receive a sigchld and be able to reap them.
> 
> Btw why are these process even created? 2.2 didn't do it I think.
> 
> I don't know where the problem lies so if someone could tell me who to
> contact about this that would be great. Or maybe if this is already
> known?

Oh.  That one again.

Could you please try the below patch?

We really need to fix this - we shouldn't be giving random
userspace tasks surprise children just because some syscall decided
to create a kernel thread.

This patch _should_ fix it, as well as the problem reported in
April at http://uwsg.iu.edu/hypermail/linux/kernel/0104.1/0763.html
But the patch may be incomplete - I did it back in April and got
distracted partway through testing.

It reparents to init OK, but it's only a partial solution - the other
problem we have is that these kernel threads will inherit things
like UIDs, scheduling priority, scheduling policy, etc from the
parent.  I haven't seen any bug reports attributable to this, but....

I think we should have a standalone function which performs this
reparenting and also initialises siginficant values in *current to
useful values.

--- linux-2.4.4-pre3/kernel/sched.c	Sun Apr 15 15:34:25 2001
+++ linux-akpm/kernel/sched.c	Mon Apr 16 20:40:47 2001
@@ -32,6 +32,7 @@
 extern void timer_bh(void);
 extern void tqueue_bh(void);
 extern void immediate_bh(void);
+extern struct task_struct *child_reaper;
 
 /*
  * scheduler variables
@@ -1260,32 +1261,53 @@
 /*
  *	Put all the gunge required to become a kernel thread without
  *	attached user resources in one place where it belongs.
+ *
+ * 	Kernel 2.4.4-pre3, [e-mail]: reparent the caller
+ *	to init and set the exit signal to SIGCHLD so the thread
+ *	will be properly reaped if it exist.
  */
 
 void daemonize(void)
 {
 	struct fs_struct *fs;
-
+	struct task_struct *this_task = current;
 
 	/*
 	 * If we were started as result of loading a module, close all of the
 	 * user space pages.  We don't need them, and if we didn't close them
 	 * they would be locked into memory.
 	 */
-	exit_mm(current);
+	exit_mm(this_task);
 
-	current->session = 1;
-	current->pgrp = 1;
+	this_task->session = 1;
+	this_task->pgrp = 1;
 
 	/* Become as one with the init task */
 
-	exit_fs(current);	/* current->fs->count--; */
+	exit_fs(this_task);		/* this_task->fs->count--; */
 	fs = init_task.fs;
-	current->fs = fs;
+	this_task->fs = fs;
 	atomic_inc(&fs->count);
- 	exit_files(current);
-	current->files = init_task.files;
-	atomic_inc(&current->files->count);
+ 	exit_files(this_task);		/* this_task->files->count-- */
+	this_task->files = init_task.files;
+	atomic_inc(&this_task->files->count);
+
+	write_lock_irq(&tasklist_lock);
+
+	/* Reparent to init */
+	REMOVE_LINKS(this_task);
+	this_task->p_pptr = child_reaper;
+	this_task->p_opptr = child_reaper;
+	SET_LINKS(this_task);
+
+	/* Set the exit signal to SIGCHLD so we signal init on exit */
+	if (this_task->exit_signal != 0) {
+		printk(KERN_ERR "task `%s' exit_signal %d in daemonize()\n",
+			this_task->comm, this_task->exit_signal);
+	}
+	this_task->exit_signal = SIGCHLD;
+
+	write_unlock_irq(&tasklist_lock);
 }
 
 void __init init_idle(void)