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

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Message-ID: <20250321055548.GA106825@k08j02272.eu95sqa>
Date: Fri, 21 Mar 2025 13:55:48 +0800
From: "Hou Wenlong" <houwenlong.hwl@...group.com>
To: Thadeu Lima de Souza Cascardo <cascardo@...lia.com>
Cc: Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	D Scott Phillips <scott@...amperecomputing.com>,
	linux-kernel@...r.kernel.org, kernel-dev@...lia.com,
	kernel test robot <oliver.sang@...el.com>
Subject: Re: [PATCH] char: misc: register chrdev region with all possible
 minors

On Mon, Mar 17, 2025 at 10:59:55AM -0300, Thadeu Lima de Souza Cascardo wrote:
> 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>
> 
>
Thank you for the fixes; we encountered the same problem in our
environment as well.

Tested-by: Hou Wenlong <houwenlong.hwl@...group.com>