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Date: Mon, 12 Feb 2007 22:35:52 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Alexey Dobriyan <adobriyan@openvz.org>, viro@ftp.linux.org.uk,
       linux-kernel@vger.kernel.org, duncan.sands@math.u-psud.fr
Subject: Re: [PATCH v4] Fix rmmod/read/write races in /proc entries
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On Sun, 11 Feb 2007 23:23:30 +0300 Alexey Dobriyan <adobriyan@gmail.com> wrote:

> [PATCH v4] Fix rmmod/read/write races in /proc entries

This:

static ssize_t
proc_file_write(struct file *file, const char __user *buffer,
		size_t count, loff_t *ppos)
{
	struct inode *inode = file->f_path.dentry->d_inode;
	struct proc_dir_entry * dp;
	ssize_t rv = -EIO;
	
	dp = PDE(inode);

	if (!dp->write_proc)
		goto out;

	spin_lock(&dp->pde_unload_lock);
	if (!dp->proc_fops)
		/*
		 * remove_proc_entry() marked PDE as "going away".
		 * No new writers allowed.
		 */
		goto out_unlock;

versus

		spin_lock(&de->pde_unload_lock);
		/*
		 * Stop accepting new readers/writers. If you're dynamically
		 * allocating ->proc_fops, save a pointer somewhere.
		 */
		de->proc_fops = NULL;
		/* Wait until all existing readers/writers are done. */
		if (de->pde_users > 0) {
			struct completion c;

			init_completion(&c);
			if (!de->pde_unload_completion)
				de->pde_unload_completion = &c;

			spin_unlock(&de->pde_unload_lock);
			spin_unlock(&proc_subdir_lock);

			wait_for_completion(de->pde_unload_completion);

			spin_lock(&proc_subdir_lock);
			goto continue_removing;
		}
		spin_unlock(&de->pde_unload_lock);
  <here>
	...
	<free de>

What prevents proc_file_write() from looking up and playing with this de in
<here>?


Also... lockdep wants this:

--- a/fs/proc/generic.c~fix-rmmod-read-write-races-in-proc-entries-fix
+++ a/fs/proc/generic.c
@@ -790,9 +790,8 @@ void remove_proc_entry(const char *name,
 		de->proc_fops = NULL;
 		/* Wait until all existing readers/writers are done. */
 		if (de->pde_users > 0) {
-			struct completion c;
+			DECLARE_COMPLETION_ONSTACK(c);
 
-			init_completion(&c);
 			if (!de->pde_unload_completion)
 				de->pde_unload_completion = &c;
 
_