linux-kernel - Re: [PATCH] char: misc: Use __register_chrdev() to register the actual minor counts

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Message-ID: <2025032054-stylus-chemist-1cd4@gregkh>
Date: Thu, 20 Mar 2025 21:44:54 -0700
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: Hou Wenlong <houwenlong.hwl@...group.com>
Cc: linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH] char: misc: Use __register_chrdev() to register the
 actual minor counts

On Fri, Mar 21, 2025 at 11:54:42AM +0800, Hou Wenlong wrote:
> In commit ab760791c0cf ("char: misc: Increase the maximum number of
> dynamic misc devices to 1048448"), the minor number of misc char device
> is increased to MINORMASK + 1. However, it still uses register_chrdev(),
> which defaults to registering the misc char device with only 256 minor
> numbers.  Therefore, when a misc device is given a minor number greater
> than 255, the device file fails to open in kobj_lookup(). As a solution,
> use __register_chrdev() to register the actual minor counts to
> accommodate this increase.
> 
> Signed-off-by: Hou Wenlong <houwenlong.hwl@...group.com>
> ---
> In our environment, the per-cpu coresight_tmc driver occupies too many
> minor numbers, which ultimately allocates minor number 256 for our misc
> device, resulting in a failure to open the device file in /dev. I'm not
> sure if this is a bug or if userspace can only use fewer than 256 minor
> numbers. Therefore, I didn't add a fix tag here.
> ---
>  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 f7dd455dd0dd..dda466f9181a 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;
>  
> -- 
> 2.31.1
> 

I think this is the same change as this one:
	https://lore.kernel.org/r/20250317-misc-chrdev-v1-1-6cd05da11aef@igalia.com

right?

If so, can you provide a reviewed-by or tested-by on that one?  I
haven't picked it up yet, sorry.

greg k-h