zram: refuse to use zero sized block device as backing device

Patch series "zram: fix backing device setup issue", v2.

This series fixes two bugs of backing device setting:

- ZRAM should reject using a zero sized (or the uninitialized ZRAM
  device itself) as the backing device.
- Fix backing device leaking when removing a uninitialized ZRAM
  device.


This patch (of 2):

Setting a zero sized block device as backing device is pointless, and one
can easily create a recursive loop by setting the uninitialized ZRAM
device itself as its own backing device by (zram0 is uninitialized):

    echo /dev/zram0 > /sys/block/zram0/backing_dev

It's definitely a wrong config, and the module will pin itself, kernel
should refuse doing so in the first place.

By refusing to use zero sized device we avoided misuse cases including
this one above.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 013bf95 ("zram: add interface to specif backing device")
Signed-off-by: Kairui Song <[email protected]>
Reported-by: Desheng Wu <[email protected]>
Reviewed-by: Sergey Senozhatsky <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: ryncsn

drivers/block/zram/zram_drv.c

@@ -614,6 +614,12 @@ static ssize_t backing_dev_store(struct device *dev,
 	}
 
 	nr_pages = i_size_read(inode) >> PAGE_SHIFT;
+	/* Refuse to use zero sized device (also prevents self reference) */
+	if (!nr_pages) {
+		err = -EINVAL;
+		goto out;
+	}
+
 	bitmap_sz = BITS_TO_LONGS(nr_pages) * sizeof(long);
 	bitmap = kvzalloc(bitmap_sz, GFP_KERNEL);
 	if (!bitmap) {