linux-kernel - [PATCH] char: misc: register chrdev region with all possible minors

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Message-Id: <20250317-misc-chrdev-v1-1-6cd05da11aef@igalia.com>
Date: Mon, 17 Mar 2025 10:59:55 -0300
From: Thadeu Lima de Souza Cascardo <cascardo@...lia.com>
To: Arnd Bergmann <arnd@...db.de>, 
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
 D Scott Phillips <scott@...amperecomputing.com>
Cc: linux-kernel@...r.kernel.org, kernel-dev@...lia.com, 
 kernel test robot <oliver.sang@...el.com>, 
 Thadeu Lima de Souza Cascardo <cascardo@...lia.com>
Subject: [PATCH] char: misc: register chrdev region with all possible
 minors

register_chrdev will only register the first 256 minors of a major chrdev.
That means that dynamically allocated misc devices with minor above 255
will fail to open with -ENXIO.

This was found by kernel test robot when testing a different change that
makes all dynamically allocated minors be above 255. This has, however,
been separately tested by creating 256 serio_raw devices with the help of
userio driver.

Ever since allowing misc devices with minors above 128, this has been
possible.

Fix it by registering all minor numbers from 0 to MINORMASK + 1 for
MISC_MAJOR.

Reported-by: kernel test robot <oliver.sang@...el.com>
Closes: https://lore.kernel.org/oe-lkp/202503171507.6c8093d0-lkp@intel.com
Fixes: ab760791c0cf ("char: misc: Increase the maximum number of dynamic misc devices to 1048448")
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@...lia.com>
---
 drivers/char/misc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/misc.c b/drivers/char/misc.c
index f7dd455dd0dd3c7c9956e772b5ab8bd83a67a4a6..dda466f9181acf76564b5e41ed6e858928e56182 100644
--- a/drivers/char/misc.c
+++ b/drivers/char/misc.c
@@ -315,7 +315,7 @@ static int __init misc_init(void)
 		goto fail_remove;
 
 	err = -EIO;
-	if (register_chrdev(MISC_MAJOR, "misc", &misc_fops))
+	if (__register_chrdev(MISC_MAJOR, 0, MINORMASK + 1, "misc", &misc_fops))
 		goto fail_printk;
 	return 0;
 

---
base-commit: 2dc25093218f5d42391549de6fe45e1aa9325676
change-id: 20250317-misc-chrdev-b26b572a47fc

Best regards,
-- 
Thadeu Lima de Souza Cascardo <cascardo@...lia.com>