[PATCH] rfkill: allocate static minor

[PATCH] rfkill: allocate static minor

[Marcel Holtmann]

udev has a feature of creating /dev/<node> device-nodes if it finds
a devnode:<node> modalias. This allows for auto-loading of modules that
provide the node. This requires to use a statically allocated minor
number for misc character devices.

However, rfkill uses dynamic minor numbers and prevents auto-loading
of the module. So allocate the next static misc minor number and use
it for rfkill.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
---
 include/linux/miscdevice.h | 1 +
 net/rfkill/core.c          | 9 +++++++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/include/linux/miscdevice.h b/include/linux/miscdevice.h
index 3247a3dc7934..b06b75776a32 100644
--- a/include/linux/miscdevice.h
+++ b/include/linux/miscdevice.h
@@ -57,6 +57,7 @@
 #define UHID_MINOR		239
 #define USERIO_MINOR		240
 #define VHOST_VSOCK_MINOR	241
+#define RFKILL_MINOR		242
 #define MISC_DYNAMIC_MINOR	255
 
 struct device;
diff --git a/net/rfkill/core.c b/net/rfkill/core.c
index f9b08a6d8dbe..0bf9bf1ceb8f 100644
--- a/net/rfkill/core.c
+++ b/net/rfkill/core.c
@@ -1316,10 +1316,12 @@ static const struct file_operations rfkill_fops = {
 	.llseek		= no_llseek,
 };
 
+#define RFKILL_NAME "rfkill"
+
 static struct miscdevice rfkill_miscdev = {
-	.name	= "rfkill",
 	.fops	= &rfkill_fops,
-	.minor	= MISC_DYNAMIC_MINOR,
+	.name	= RFKILL_NAME,
+	.minor	= RFKILL_MINOR,
 };
 
 static int __init rfkill_init(void)
@@ -1371,3 +1373,6 @@ static void __exit rfkill_exit(void)
 	class_unregister(&rfkill_class);
 }
 module_exit(rfkill_exit);
+
+MODULE_ALIAS_MISCDEV(RFKILL_MINOR);
+MODULE_ALIAS("devname:" RFKILL_NAME);
-- 
2.21.0

Re: [PATCH] rfkill: allocate static minor

[Greg KH]

On Thu, Oct 24, 2019 at 07:40:42PM +0200, Marcel Holtmann wrote:
> udev has a feature of creating /dev/<node> device-nodes if it finds
> a devnode:<node> modalias. This allows for auto-loading of modules that
> provide the node. This requires to use a statically allocated minor
> number for misc character devices.
> 
> However, rfkill uses dynamic minor numbers and prevents auto-loading
> of the module. So allocate the next static misc minor number and use
> it for rfkill.

As rfkill has been around for a long time, what new use case is needing
to auto-load this based on a major number?

thanks,

greg k-h

Re: [PATCH] rfkill: allocate static minor

[Marcel Holtmann]

Hi Greg,

>> udev has a feature of creating /dev/<node> device-nodes if it finds
>> a devnode:<node> modalias. This allows for auto-loading of modules that
>> provide the node. This requires to use a statically allocated minor
>> number for misc character devices.
>> 
>> However, rfkill uses dynamic minor numbers and prevents auto-loading
>> of the module. So allocate the next static misc minor number and use
>> it for rfkill.
> 
> As rfkill has been around for a long time, what new use case is needing
> to auto-load this based on a major number?

we have bug reports from iwd users where it fails opening /dev/rfkill. Since iwd can be actually started before the WiFi hardware is fully probed and all its drivers are loaded, we have a race-condition here if rfkill is not capable of auto-loading.

The difference is really that iwd is a fully self-contained WiFi daemon compared to wpa_supplicant which is just some sort of helper. iwd is fully hot plug capable as well compared to wpa_supplicant. It looks like this is exposing the race condition for our users. Frankly, we should have fixed rfkill a long time ago when we fixed uinput, uhid etc, but seems we forgot it. I assume mainly because it magically got loaded in time by some module dependencies.

Regards

Marcel

Re: [PATCH] rfkill: allocate static minor

[Greg KH]

On Thu, Oct 24, 2019 at 10:23:57PM +0200, Marcel Holtmann wrote:
> Hi Greg,
> 
> >> udev has a feature of creating /dev/<node> device-nodes if it finds
> >> a devnode:<node> modalias. This allows for auto-loading of modules that
> >> provide the node. This requires to use a statically allocated minor
> >> number for misc character devices.
> >> 
> >> However, rfkill uses dynamic minor numbers and prevents auto-loading
> >> of the module. So allocate the next static misc minor number and use
> >> it for rfkill.
> > 
> > As rfkill has been around for a long time, what new use case is needing
> > to auto-load this based on a major number?
> 
> we have bug reports from iwd users where it fails opening /dev/rfkill. Since iwd can be actually started before the WiFi hardware is fully probed and all its drivers are loaded, we have a race-condition here if rfkill is not capable of auto-loading.
> 
> The difference is really that iwd is a fully self-contained WiFi daemon compared to wpa_supplicant which is just some sort of helper. iwd is fully hot plug capable as well compared to wpa_supplicant. It looks like this is exposing the race condition for our users. Frankly, we should have fixed rfkill a long time ago when we fixed uinput, uhid etc, but seems we forgot it. I assume mainly because it magically got loaded in time by some module dependencies.

You need a better email client, one with \n characters...

Anyway, this sounds reasonable, I'll go queue this up for 5.5.

thanks,

greg k-h