[v4] char: misc: Various cleanup for miscdevice

[PATCH v4 6/8] char: misc: Does not request module for miscdevice with dynamic minor

Posted by Zijun Hu 3 months ago

From: Zijun Hu <zijun.hu@oss.qualcomm.com>

misc_open() may request module for miscdevice with dynamic minor
which is meaningless since macro MODULE_ALIAS_MISCDEV() is not
applicable for dynamic minor.

Fix by only requesting module for miscdevice with fixed minor.

Signed-off-by: Zijun Hu <zijun.hu@oss.qualcomm.com>
---
 drivers/char/misc.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/char/misc.c b/drivers/char/misc.c
index 96ed343cf5c8509a09855020049a9af82a3ede95..a0aae0fc792666a7bdc0ba00da9dc02ff9cead42 100644
--- a/drivers/char/misc.c
+++ b/drivers/char/misc.c
@@ -132,7 +132,8 @@ static int misc_open(struct inode *inode, struct file *file)
 		break;
 	}
 
-	if (!new_fops) {
+	/* Only request module for fixed minor code */
+	if (!new_fops && minor < MISC_DYNAMIC_MINOR) {
 		mutex_unlock(&misc_mtx);
 		request_module("char-major-%d-%d", MISC_MAJOR, minor);
 		mutex_lock(&misc_mtx);
@@ -144,10 +145,11 @@ static int misc_open(struct inode *inode, struct file *file)
 			new_fops = fops_get(iter->fops);
 			break;
 		}
-		if (!new_fops)
-			goto fail;
 	}
 
+	if (!new_fops)
+		goto fail;
+
 	/*
 	 * Place the miscdevice in the file's
 	 * private_data so it can be used by the

-- 
2.34.1

Re: [PATCH v4 6/8] char: misc: Does not request module for miscdevice with dynamic minor

Posted by Greg Kroah-Hartman 3 months ago

On Fri, Jul 04, 2025 at 09:26:04PM +0800, Zijun Hu wrote:
> From: Zijun Hu <zijun.hu@oss.qualcomm.com>
> 
> misc_open() may request module for miscdevice with dynamic minor
> which is meaningless since macro MODULE_ALIAS_MISCDEV() is not
> applicable for dynamic minor.
> 
> Fix by only requesting module for miscdevice with fixed minor.
> 
> Signed-off-by: Zijun Hu <zijun.hu@oss.qualcomm.com>
> ---
>  drivers/char/misc.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/char/misc.c b/drivers/char/misc.c
> index 96ed343cf5c8509a09855020049a9af82a3ede95..a0aae0fc792666a7bdc0ba00da9dc02ff9cead42 100644
> --- a/drivers/char/misc.c
> +++ b/drivers/char/misc.c
> @@ -132,7 +132,8 @@ static int misc_open(struct inode *inode, struct file *file)
>  		break;
>  	}
>  
> -	if (!new_fops) {
> +	/* Only request module for fixed minor code */
> +	if (!new_fops && minor < MISC_DYNAMIC_MINOR) {
>  		mutex_unlock(&misc_mtx);
>  		request_module("char-major-%d-%d", MISC_MAJOR, minor);
>  		mutex_lock(&misc_mtx);
> @@ -144,10 +145,11 @@ static int misc_open(struct inode *inode, struct file *file)
>  			new_fops = fops_get(iter->fops);
>  			break;
>  		}
> -		if (!new_fops)
> -			goto fail;
>  	}
>  
> +	if (!new_fops)
> +		goto fail;
> +

This is fine, but is it going to break any existing users that happened
to rely on this behaviour?

thanks,

greg k-h

Re: [PATCH v4 6/8] char: misc: Does not request module for miscdevice with dynamic minor

Posted by Zijun Hu 3 months ago

On 2025/7/6 16:55, Greg Kroah-Hartman wrote:
>> -		if (!new_fops)
>> -			goto fail;
>>  	}
>>  
>> +	if (!new_fops)
>> +		goto fail;
>> +
> This is fine, but is it going to break any existing users that happened
> to rely on this behaviour?

no since there are no kernel users who register miscdevice with minor > 255.