proc 1/2: do PDE usecounting even for ->read_proc, ->write_proc

struct proc_dir_entry::owner is going to be removed. Now it's only necessary
to protect PDEs which are using ->read_proc, ->write_proc hooks.

However, ->owner assignments are racy and make it very easy for someone to switch
->owner on live PDE (as some subsystems do) without fixing refcounts and so on.

http://bugzilla.kernel.org/show_bug.cgi?id=12454

So, ->owner is on death row.

Proxy file operations exist already (proc_file_operations), just bump usecount
when necessary.
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>

fs/proc/generic.c

@@ -37,7 +37,7 @@ static int proc_match(int len, const char *name, struct proc_dir_entry *de)
 #define PROC_BLOCK_SIZE	(PAGE_SIZE - 1024)
 
 static ssize_t
-proc_file_read(struct file *file, char __user *buf, size_t nbytes,
+__proc_file_read(struct file *file, char __user *buf, size_t nbytes,
 	       loff_t *ppos)
 {
 	struct inode * inode = file->f_path.dentry->d_inode;
@@ -182,20 +182,48 @@ proc_file_read(struct file *file, char __user *buf, size_t nbytes,
 	return retval;
 }
 
+static ssize_t
+proc_file_read(struct file *file, char __user *buf, size_t nbytes,
+	       loff_t *ppos)
+{
+	struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
+	ssize_t rv = -EIO;
+
+	spin_lock(&pde->pde_unload_lock);
+	if (!pde->proc_fops) {
+		spin_unlock(&pde->pde_unload_lock);
+		return rv;
+	}
+	pde->pde_users++;
+	spin_unlock(&pde->pde_unload_lock);
+
+	rv = __proc_file_read(file, buf, nbytes, ppos);
+
+	pde_users_dec(pde);
+	return rv;
+}
+
 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;
-	
-	dp = PDE(inode);
-
-	if (!dp->write_proc)
-		return -EIO;
+	struct proc_dir_entry *pde = PDE(file->f_path.dentry->d_inode);
+	ssize_t rv = -EIO;
+
+	if (pde->write_proc) {
+		spin_lock(&pde->pde_unload_lock);
+		if (!pde->proc_fops) {
+			spin_unlock(&pde->pde_unload_lock);
+			return rv;
+		}
+		pde->pde_users++;
+		spin_unlock(&pde->pde_unload_lock);
 
-	/* FIXME: does this routine need ppos?  probably... */
-	return dp->write_proc(file, buffer, count, dp->data);
+		/* FIXME: does this routine need ppos?  probably... */
+		rv = pde->write_proc(file, buffer, count, pde->data);
+		pde_users_dec(pde);
+	}
+	return rv;
 }
 
 

fs/proc/inode.c

@@ -127,7 +127,7 @@ static void __pde_users_dec(struct proc_dir_entry *pde)
 		complete(pde->pde_unload_completion);
 }
 
-static void pde_users_dec(struct proc_dir_entry *pde)
+void pde_users_dec(struct proc_dir_entry *pde)
 {
 	spin_lock(&pde->pde_unload_lock);
 	__pde_users_dec(pde);

fs/proc/internal.h

@@ -91,3 +91,4 @@ struct pde_opener {
 	int (*release)(struct inode *, struct file *);
 	struct list_head lh;
 };
+void pde_users_dec(struct proc_dir_entry *pde);