[PATCH V3] brd: check and limit max_part par

[Zhiqiang Liu <liuzhiqiang26@xxxxxxxxxx>]

In brd_init func, rd_nr num of brd_device are firstly allocated
and add in brd_devices, then brd_devices are traversed to add each
brd_device by calling add_disk func. When allocating brd_device,
the disk->first_minor is set to i * max_part, if rd_nr * max_part
is larger than MINORMASK, two different brd_device may have the same
devt, then only one of them can be successfully added.
when rmmod brd.ko, it will cause oops when calling brd_exit.

Follow those steps:
  # modprobe brd rd_nr=3 rd_size=102400 max_part=1048576
  # rmmod brd
then, the oops will appear.

Oops log:
[  726.613722] Call trace:
[  726.614175]  kernfs_find_ns+0x24/0x130
[  726.614852]  kernfs_find_and_get_ns+0x44/0x68
[  726.615749]  sysfs_remove_group+0x38/0xb0
[  726.616520]  blk_trace_remove_sysfs+0x1c/0x28
[  726.617320]  blk_unregister_queue+0x98/0x100
[  726.618105]  del_gendisk+0x144/0x2b8
[  726.618759]  brd_exit+0x68/0x560 [brd]
[  726.619501]  __arm64_sys_delete_module+0x19c/0x2a0
[  726.620384]  el0_svc_common+0x78/0x130
[  726.621057]  el0_svc_handler+0x38/0x78
[  726.621738]  el0_svc+0x8/0xc
[  726.622259] Code: aa0203f6 aa0103f7 aa1e03e0 d503201f (7940e260)

Here, we add brd_check_and_reset_par func to check and limit max_part par.

--
V2->V3: (suggested by Ming Lei)
 - clear .minors when running out of consecutive minor space in brd_alloc
 - remove limit of rd_nr

V1->V2: add more checks in brd_check_par_valid as suggested by Ming Lei.

Signed-off-by: Zhiqiang Liu <liuzhiqiang26@xxxxxxxxxx>
---
 drivers/block/brd.c | 35 ++++++++++++++++++++++++++++-------
 1 file changed, 28 insertions(+), 7 deletions(-)

diff --git a/drivers/block/brd.c b/drivers/block/brd.c
index df8103dd40ac..2295a0bafb5e 100644
--- a/drivers/block/brd.c
+++ b/drivers/block/brd.c
@@ -330,16 +330,16 @@ static const struct block_device_operations brd_fops = {
 /*
  * And now the modules code and kernel interface.
  */
-static int rd_nr = CONFIG_BLK_DEV_RAM_COUNT;
-module_param(rd_nr, int, 0444);
+static unsigned int rd_nr = CONFIG_BLK_DEV_RAM_COUNT;
+module_param(rd_nr, uint, 0444);
 MODULE_PARM_DESC(rd_nr, "Maximum number of brd devices");

 unsigned long rd_size = CONFIG_BLK_DEV_RAM_SIZE;
 module_param(rd_size, ulong, 0444);
 MODULE_PARM_DESC(rd_size, "Size of each RAM disk in kbytes.");

-static int max_part = 1;
-module_param(max_part, int, 0444);
+static unsigned int max_part = 1;
+module_param(max_part, uint, 0444);
 MODULE_PARM_DESC(max_part, "Num Minors to reserve between devices");

 MODULE_LICENSE("GPL");
@@ -393,7 +393,14 @@ static struct brd_device *brd_alloc(int i)
 	if (!disk)
 		goto out_free_queue;
 	disk->major		= RAMDISK_MAJOR;
-	disk->first_minor	= i * max_part;
+	/*
+	 * Clear .minors when running out of consecutive minor space since
+	 * GENHD_FL_EXT_DEVT is set, and we can allocate from extended devt.
+	 */
+	if ((i * disk->minors) & ~MINORMASK)
+		disk->minors = 0;
+	else
+		disk->first_minor = i * disk->minors;
 	disk->fops		= &brd_fops;
 	disk->private_data	= brd;
 	disk->queue		= brd->brd_queue;
@@ -468,6 +475,21 @@ static struct kobject *brd_probe(dev_t dev, int *part, void *data)
 	return kobj;
 }

+static inline void brd_check_and_reset_par(void)
+{
+	if (unlikely(!rd_nr))
+		rd_nr = 1;
+
+	if (unlikely(!max_part))
+		max_part = 1;
+
+	if (max_part > DISK_MAX_PARTS) {
+		pr_info("brd: max_part can't be larger than %d, reset max_part = %d.\n",
+			DISK_MAX_PARTS, DISK_MAX_PARTS);
+		max_part = DISK_MAX_PARTS;
+	}
+}
+
 static int __init brd_init(void)
 {
 	struct brd_device *brd, *next;
@@ -491,8 +513,7 @@ static int __init brd_init(void)
 	if (register_blkdev(RAMDISK_MAJOR, "ramdisk"))
 		return -EIO;

-	if (unlikely(!max_part))
-		max_part = 1;
+	brd_check_and_reset_par();

 	for (i = 0; i < rd_nr; i++) {
 		brd = brd_alloc(i);
-- 
2.19.1