Fix rmmod/read/write races in /proc entries

Fix following races:
===========================================
1. Write via ->write_proc sleeps in copy_from_user(). Module disappears
   meanwhile. Or, more generically, system call done on /proc file, method
   supplied by module is called, module dissapeares meanwhile.

   pde = create_proc_entry()
   if (!pde)
	return -ENOMEM;
   pde->write_proc = ...
				open
				write
				copy_from_user
   pde = create_proc_entry();
   if (!pde) {
	remove_proc_entry();
	return -ENOMEM;
	/* module unloaded */
   }
				*boom*
==========================================
2. bogo-revoke aka proc_kill_inodes()

  remove_proc_entry		vfs_read
  proc_kill_inodes		[check ->f_op validness]
				[check ->f_op->read validness]
				[verify_area, security permissions checks]
	->f_op = NULL;
				if (file->f_op->read)
					/* ->f_op dereference, boom */

NOTE, NOTE, NOTE: file_operations are proxied for regular files only. Let's
see how this scheme behaves, then extend if needed for directories.
Directories creators in /proc only set ->owner for them, so proxying for
directories may be unneeded.

NOTE, NOTE, NOTE: methods being proxied are ->llseek, ->read, ->write,
->poll, ->unlocked_ioctl, ->ioctl, ->compat_ioctl, ->open, ->release.
If your in-tree module uses something else, yell on me. Full audit pending.

[akpm@linux-foundation.org: build fix]
Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/proc_fs.h

@@ -7,6 +7,8 @@
 #include <linux/magic.h>
 #include <asm/atomic.h>
 
+struct completion;
+
 /*
  * The proc filesystem constants/structures
  */
@@ -56,6 +58,14 @@ struct proc_dir_entry {
 	gid_t gid;
 	loff_t size;
 	const struct inode_operations *proc_iops;
+	/*
+	 * NULL ->proc_fops means "PDE is going away RSN" or
+	 * "PDE is just created". In either case, e.g. ->read_proc won't be
+	 * called because it's too late or too early, respectively.
+	 *
+	 * If you're allocating ->proc_fops dynamically, save a pointer
+	 * somewhere.
+	 */
 	const struct file_operations *proc_fops;
 	get_info_t *get_info;
 	struct module *owner;
@@ -66,6 +76,9 @@ struct proc_dir_entry {
 	atomic_t count;		/* use count */
 	int deleted;		/* delete flag */
 	void *set;
+	int pde_users;	/* number of callers into module in progress */
+	spinlock_t pde_unload_lock; /* proc_fops checks and pde_users bumps */
+	struct completion *pde_unload_completion;
 };
 
 struct kcore_list {