block: check bdev_read_only() from blkdev_get()

bdev read-only status can be queried using bdev_read_only() and may
change while the device is being opened.  Enforce it by checking it
from blkdev_get() after open succeeds.

This makes bdev_read_only() check in open_bdev_exclusive() and
fsg_lun_open() unnecessary.  Drop them.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Cc: linux-usb@vger.kernel.org

drivers/usb/gadget/storage_common.c

@@ -543,7 +543,7 @@ static int fsg_lun_open(struct fsg_lun *curlun, const char *filename)
 	ro = curlun->initially_ro;
 	if (!ro) {
 		filp = filp_open(filename, O_RDWR | O_LARGEFILE, 0);
-		if (-EROFS == PTR_ERR(filp))
+		if (PTR_ERR(filp) == -EROFS || PTR_ERR(filp) == -EACCES)
 			ro = 1;
 	}
 	if (ro)
@@ -558,10 +558,7 @@ static int fsg_lun_open(struct fsg_lun *curlun, const char *filename)
 
 	if (filp->f_path.dentry)
 		inode = filp->f_path.dentry->d_inode;
-	if (inode && S_ISBLK(inode->i_mode)) {
-		if (bdev_read_only(inode->i_bdev))
-			ro = 1;
-	} else if (!inode || !S_ISREG(inode->i_mode)) {
+	if (!inode || (!S_ISREG(inode->i_mode) && !S_ISBLK(inode->i_mode))) {
 		LINFO(curlun, "invalid file type: %s\n", filename);
 		goto out;
 	}

fs/block_dev.c

@@ -1149,6 +1149,12 @@ int blkdev_get(struct block_device *bdev, fmode_t mode, void *holder)
 
 	res = __blkdev_get(bdev, mode, 0);
 
+	/* __blkdev_get() may alter read only status, check it afterwards */
+	if (!res && (mode & FMODE_WRITE) && bdev_read_only(bdev)) {
+		__blkdev_put(bdev, mode, 0);
+		res = -EACCES;
+	}
+
 	if (whole) {
 		/* finish claiming */
 		spin_lock(&bdev_lock);
@@ -1453,11 +1459,6 @@ struct block_device *open_bdev_exclusive(const char *path, fmode_t mode, void *h
 	if (error)
 		return ERR_PTR(error);
 
-	if ((mode & FMODE_WRITE) && bdev_read_only(bdev)) {
-		blkdev_put(bdev, mode);
-		return ERR_PTR(-EACCES);
-	}
-
 	return bdev;
 }