proc: fix NULL ->i_fop oops

proc_kill_inodes() can clear ->i_fop in the middle of vfs_readdir resulting in
NULL dereference during "file->f_op->readdir(file, buf, filler)".

The solution is to remove proc_kill_inodes() completely:

a) we don't have tricky modules implementing their tricky readdir hooks which
   could keeping this revoke from hell.

b) In a situation when module is gone but PDE still alive, standard
   readdir will return only "." and "..", because pde->next was cleared by
   remove_proc_entry().

c) the race proc_kill_inode() destined to prevent is not completely
   fixed, just race window made smaller, because vfs_readdir() is run
   without sb_lock held and without file_list_lock held.  Effectively,
   ->i_fop is cleared at random moment, which can't fix properly anything.

BUG: unable to handle kernel NULL pointer dereference at virtual address 00000018
printing eip: c1061205 *pdpt = 0000000005b22001 *pde = 0000000000000000
Oops: 0000 [#1] PREEMPT SMP
Modules linked in: foo af_packet ipv6 cpufreq_ondemand loop serio_raw sr_mod k8temp cdrom hwmon amd_rng
Pid: 2033, comm: find Not tainted (2.6.24-rc1-b1d08ac0 #2)
EIP: 0060:[<c1061205>] EFLAGS: 00010246 CPU: 0
EIP is at vfs_readdir+0x47/0x74
EAX: c6b6a780 EBX: 00000000 ECX: c1061040 EDX: c5decf94
ESI: c6b6a780 EDI: fffffffe EBP: c9797c54 ESP: c5decf78
 DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
Process find (pid: 2033, ti=c5dec000 task=c64bba90 task.ti=c5dec000)
Stack: c5decf94 c1061040 fffffff7 0805ffbc 00000000 c6b6a780 c1061295 0805ffbc
       00000000 00000400 00000000 00000004 0805ffbc 4588eff4 c5dec000 c10026ba
       00000004 0805ffbc 00000400 0805ffbc 4588eff4 bfdc6c70 000000dc 0000007b
Call Trace:
 [<c1061040>] filldir64+0x0/0xc5
 [<c1061295>] sys_getdents64+0x63/0xa5
 [<c10026ba>] sysenter_past_esp+0x5f/0x85
 =======================
Code: 49 83 78 18 00 74 43 8d 6b 74 bf fe ff ff ff 89 e8 e8 b8 c0 12 00 f6 83 2c 01 00 00 10 75 22 8b 5e 10 8b 4c 24 04 89 f0 8b 14 24 <ff> 53 18 f6 46 1a 04 89 c7 75 0b 8b 56 0c 8b 46 08 e8 c8 66 00
EIP: [<c1061205>] vfs_readdir+0x47/0x74 SS:ESP 0068:c5decf78

hch: "Nice, getting rid of this is a very good step formwards.
      Unfortunately we have another copy of this junk in
      security/selinux/selinuxfs.c:sel_remove_entries() which would need the
      same treatment."
Signed-off-by: Alexey Dobriyan <adobriyan@sw.ru>
Acked-by: Christoph Hellwig <hch@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: James Morris <jmorris@namei.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/generic.c

@@ -555,41 +555,6 @@ static int proc_register(struct proc_dir_entry * dir, struct proc_dir_entry * dp
 	return 0;
 }
 
-/*
- * Kill an inode that got unregistered..
- */
-static void proc_kill_inodes(struct proc_dir_entry *de)
-{
-	struct list_head *p;
-	struct super_block *sb;
-
-	/*
-	 * Actually it's a partial revoke().
-	 */
-	spin_lock(&sb_lock);
-	list_for_each_entry(sb, &proc_fs_type.fs_supers, s_instances) {
-		file_list_lock();
-		list_for_each(p, &sb->s_files) {
-			struct file *filp = list_entry(p, struct file,
-							f_u.fu_list);
-			struct dentry *dentry = filp->f_path.dentry;
-			struct inode *inode;
-			const struct file_operations *fops;
-
-			if (dentry->d_op != &proc_dentry_operations)
-				continue;
-			inode = dentry->d_inode;
-			if (PDE(inode) != de)
-				continue;
-			fops = filp->f_op;
-			filp->f_op = NULL;
-			fops_put(fops);
-		}
-		file_list_unlock();
-	}
-	spin_unlock(&sb_lock);
-}
-
 static struct proc_dir_entry *proc_create(struct proc_dir_entry **parent,
 					  const char *name,
 					  mode_t mode,
@@ -764,8 +729,6 @@ void remove_proc_entry(const char *name, struct proc_dir_entry *parent)
 continue_removing:
 		if (S_ISDIR(de->mode))
 			parent->nlink--;
-		if (!S_ISREG(de->mode))
-			proc_kill_inodes(de);
 		de->nlink = 0;
 		WARN_ON(de->subdir);
 		if (!atomic_read(&de->count))

fs/proc/internal.h

@@ -78,5 +78,3 @@ static inline int proc_fd(struct inode *inode)
 {
 	return PROC_I(inode)->fd;
 }
-
-extern struct file_system_type proc_fs_type;

fs/proc/root.c

@@ -98,7 +98,7 @@ static void proc_kill_sb(struct super_block *sb)
 	put_pid_ns(ns);
 }
 
-struct file_system_type proc_fs_type = {
+static struct file_system_type proc_fs_type = {
 	.name		= "proc",
 	.get_sb		= proc_get_sb,
 	.kill_sb	= proc_kill_sb,