mtd: mtdchar: Do not corrupt backing device of device node inode

We cannot modify file->f_mapping->backing_dev_info, because it will corrupt
backing device of device node inode, since file->f_mapping is equal to
inode->i_mapping (see __dentry_open() in fs/open.c).

Let's introduce separate inode for MTD device with appropriate backing
device.

[dwmw2: Refactor to keep it all entirely within mtdchar.c; use iget_locked()]

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

drivers/mtd/mtdchar.c

@@ -15,19 +15,23 @@
 #include <linux/smp_lock.h>
 #include <linux/backing-dev.h>
 #include <linux/compat.h>
+#include <linux/mount.h>
 
 #include <linux/mtd/mtd.h>
 #include <linux/mtd/compatmac.h>
 
 #include <asm/uaccess.h>
 
+#define MTD_INODE_FS_MAGIC 0x11307854
+static struct vfsmount *mtd_inode_mnt __read_mostly;
 
 /*
  * Data structure to hold the pointer to the mtd device as well
  * as mode information ofr various use cases.
  */
 struct mtd_file_info {
 	struct mtd_info *mtd;
+	struct inode *ino;
 	enum mtd_file_modes mode;
 };
 
@@ -64,6 +68,7 @@ static int mtd_open(struct inode *inode, struct file *file)
 	int ret = 0;
 	struct mtd_info *mtd;
 	struct mtd_file_info *mfi;
+	struct inode *mtd_ino;
 
 	DEBUG(MTD_DEBUG_LEVEL0, "MTD_open\n");
 
@@ -85,22 +90,36 @@ static int mtd_open(struct inode *inode, struct file *file)
 		goto out;
 	}
 
-	if (mtd->backing_dev_info)
-		file->f_mapping->backing_dev_info = mtd->backing_dev_info;
+	mtd_ino = iget_locked(mtd_inode_mnt->mnt_sb, devnum);
+	if (!mtd_ino) {
+		put_mtd_device(mtd);
+		ret = -ENOMEM;
+		goto out;
+	}
+	if (mtd_ino->i_state & I_NEW) {
+		mtd_ino->i_private = mtd;
+		mtd_ino->i_mode = S_IFCHR;
+		mtd_ino->i_data.backing_dev_info = mtd->backing_dev_info;
+		unlock_new_inode(mtd_ino);
+	}
+	file->f_mapping = mtd_ino->i_mapping;
 
 	/* You can't open it RW if it's not a writeable device */
 	if ((file->f_mode & FMODE_WRITE) && !(mtd->flags & MTD_WRITEABLE)) {
+		iput(mtd_ino);
 		put_mtd_device(mtd);
 		ret = -EACCES;
 		goto out;
 	}
 
 	mfi = kzalloc(sizeof(*mfi), GFP_KERNEL);
 	if (!mfi) {
+		iput(mtd_ino);
 		put_mtd_device(mtd);
 		ret = -ENOMEM;
 		goto out;
 	}
+	mfi->ino = mtd_ino;
 	mfi->mtd = mtd;
 	file->private_data = mfi;
 
@@ -122,6 +141,8 @@ static int mtd_close(struct inode *inode, struct file *file)
 	if ((file->f_mode & FMODE_WRITE) && mtd->sync)
 		mtd->sync(mtd);
 
+	iput(mfi->ino);
+
 	put_mtd_device(mtd);
 	file->private_data = NULL;
 	kfree(mfi);
@@ -951,22 +972,80 @@ static const struct file_operations mtd_fops = {
 #endif
 };
 
+static int mtd_inodefs_get_sb(struct file_system_type *fs_type, int flags,
+                               const char *dev_name, void *data,
+                               struct vfsmount *mnt)
+{
+        return get_sb_pseudo(fs_type, "mtd_inode:", NULL, MTD_INODE_FS_MAGIC,
+                             mnt);
+}
+
+static struct file_system_type mtd_inodefs_type = {
+       .name = "mtd_inodefs",
+       .get_sb = mtd_inodefs_get_sb,
+       .kill_sb = kill_anon_super,
+};
+
+static void mtdchar_notify_add(struct mtd_info *mtd)
+{
+}
+
+static void mtdchar_notify_remove(struct mtd_info *mtd)
+{
+	struct inode *mtd_ino = ilookup(mtd_inode_mnt->mnt_sb, mtd->index);
+
+	if (mtd_ino) {
+		/* Destroy the inode if it exists */
+		mtd_ino->i_nlink = 0;
+		iput(mtd_ino);
+	}
+}
+
+static struct mtd_notifier mtdchar_notifier = {
+	.add = mtdchar_notify_add,
+	.remove = mtdchar_notify_remove,
+};
+
 static int __init init_mtdchar(void)
 {
-	int status;
+	int ret;
 
-	status = __register_chrdev(MTD_CHAR_MAJOR, 0, 1 << MINORBITS,
+	ret = __register_chrdev(MTD_CHAR_MAJOR, 0, 1 << MINORBITS,
 				   "mtd", &mtd_fops);
-	if (status < 0) {
-		printk(KERN_NOTICE "Can't allocate major number %d for Memory Technology Devices.\n",
-		       MTD_CHAR_MAJOR);
+	if (ret < 0) {
+		pr_notice("Can't allocate major number %d for "
+				"Memory Technology Devices.\n", MTD_CHAR_MAJOR);
+		return ret;
 	}
 
-	return status;
+	ret = register_filesystem(&mtd_inodefs_type);
+	if (ret) {
+		pr_notice("Can't register mtd_inodefs filesystem: %d\n", ret);
+		goto err_unregister_chdev;
+	}
+
+	mtd_inode_mnt = kern_mount(&mtd_inodefs_type);
+	if (IS_ERR(mtd_inode_mnt)) {
+		ret = PTR_ERR(mtd_inode_mnt);
+		pr_notice("Error mounting mtd_inodefs filesystem: %d\n", ret);
+		goto err_unregister_filesystem;
+	}
+	register_mtd_user(&mtdchar_notifier);
+
+	return ret;
+
+err_unregister_filesystem:
+	unregister_filesystem(&mtd_inodefs_type);
+err_unregister_chdev:
+	__unregister_chrdev(MTD_CHAR_MAJOR, 0, 1 << MINORBITS, "mtd");
+	return ret;
 }
 
 static void __exit cleanup_mtdchar(void)
 {
+	unregister_mtd_user(&mtdchar_notifier);
+	mntput(mtd_inode_mnt);
+	unregister_filesystem(&mtd_inodefs_type);
 	__unregister_chrdev(MTD_CHAR_MAJOR, 0, 1 << MINORBITS, "mtd");
 }