block: Fix the maximum minor value is blk_alloc_ext_minor()

ida_alloc_range(..., min, max, ...) returns values from min to max,
inclusive.

So, NR_EXT_DEVT is a valid idx returned by blk_alloc_ext_minor().

This is an issue because in device_add_disk(), this value is used in:
   ddev->devt = MKDEV(disk->major, disk->first_minor);
and NR_EXT_DEVT is '(1 << MINORBITS)'.

So, should 'disk->first_minor' be NR_EXT_DEVT, it would overflow.

Fixes: 22ae8ce8b892 ("block: simplify bdev/disk lookup in blkdev_get")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/cc17199798312406b90834e433d2cefe8266823d.1648306232.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/genhd.c b/block/genhd.c
index 8244f09..19b8a6e 100644 (file)
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -335,7 +335,7 @@ int blk_alloc_ext_minor(void)
 {
        int idx;
 
-       idx = ida_alloc_range(&ext_devt_ida, 0, NR_EXT_DEVT, GFP_KERNEL);
+       idx = ida_alloc_range(&ext_devt_ida, 0, NR_EXT_DEVT - 1, GFP_KERNEL);
        if (idx == -ENOSPC)
                return -EBUSY;
        return idx;