[PATCH] mtd: Do not corrupt backing device for inode

["Kirill A. Shutemov" <kirill@shutemov.name>]

We cannot modify file->f_mapping->backing_dev_info directly, because it
will corrupt backing device for inode, since inode->i_mapping ==
file->f_mapping (see __dentry_open() in fs/open.c).

So we have to copy f_mapping first.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
---
 drivers/mtd/mtdchar.c |   22 +++++++++++++++++++++-
 1 files changed, 21 insertions(+), 1 deletions(-)

diff --git a/drivers/mtd/mtdchar.c b/drivers/mtd/mtdchar.c
index 5b081cb..d85be4d 100644
--- a/drivers/mtd/mtdchar.c
+++ b/drivers/mtd/mtdchar.c
@@ -88,8 +88,23 @@ static int mtd_open(struct inode *inode, struct file *file)
 		goto out;
 	}
 
-	if (mtd->backing_dev_info)
+	if (mtd->backing_dev_info) {
+		/*
+		 * We assume that file->f_mapping is equal to inode->i_mapping.
+		 */
+		BUG_ON(file->f_mapping != inode->i_mapping);
+
+		/*
+		 * We cannot modify file->f_mapping->backing_dev_info directly,
+		 * because it will corrupt backing device for inode, since
+		 * inode->i_mapping is equal to file->f_mapping. So we have to
+		 * copy f_mapping first.
+		 */
+		file->f_mapping = kmalloc(sizeof(*file->f_mapping), GFP_KERNEL);
+		memcpy(file->f_mapping, inode->i_mapping,
+				sizeof(*file->f_mapping));
 		file->f_mapping->backing_dev_info = mtd->backing_dev_info;
+	}
 
 	/* You can't open it RW if it's not a writeable device */
 	if ((file->f_mode & FMODE_WRITE) && !(mtd->flags & MTD_WRITEABLE)) {
@@ -129,6 +144,11 @@ static int mtd_close(struct inode *inode, struct file *file)
 	file->private_data = NULL;
 	kfree(mfi);
 
+	if (mtd->backing_dev_info) {
+		kfree(file->f_mapping);
+		file->f_mapping = inode->i_mapping;
+	}
+
 	return 0;
 } /* mtd_close */
 
-- 
1.7.0.4