[PATCH] block: z2ram: fix dangling gendisk pointer on add_disk() failure

[Muhammad Bilal <meatuni001@gmail.com>]

z2ram_register_disk() stores the freshly allocated gendisk in
z2ram_gendisk[minor] before calling add_disk(). If add_disk() fails,
it drops the reference with put_disk() but leaves the array entry
pointing at the now-freed gendisk instead of clearing it.

z2_init() only aborts and unwinds when minor 0 fails; a later minor
failing just leaves that dangling entry in place and the module still
reports successful load. z2_exit() then unconditionally calls
del_gendisk()/put_disk() on every entry in z2ram_gendisk[], so module
removal walks into the stale pointer left behind by the earlier
failure.

Clear the array entry when add_disk() fails, and skip NULL entries in
z2_exit(), matching how a minor that never registered is represented.

Fixes: 76487f024142 ("z2ram: use separate gendisk for the different modes")
Signed-off-by: Muhammad Bilal <meatuni001@gmail.com>
---
 drivers/block/z2ram.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/block/z2ram.c b/drivers/block/z2ram.c
index b216cba1c3bf..e7fabaa53ba1 100644
--- a/drivers/block/z2ram.c
+++ b/drivers/block/z2ram.c
@@ -333,8 +333,10 @@ static int z2ram_register_disk(int minor)
 
 	z2ram_gendisk[minor] = disk;
 	err = add_disk(disk);
-	if (err)
+	if (err) {
+		z2ram_gendisk[minor] = NULL;
 		put_disk(disk);
+	}
 	return err;
 }
 
@@ -379,6 +381,8 @@ static void __exit z2_exit(void)
 	unregister_blkdev(Z2RAM_MAJOR, DEVICE_NAME);
 
 	for (i = 0; i < Z2MINOR_COUNT; i++) {
+		if (!z2ram_gendisk[i])
+			continue;
 		del_gendisk(z2ram_gendisk[i]);
 		put_disk(z2ram_gendisk[i]);
 	}
-- 
2.55.0