linux-kernel - Re: [PATCH 2/2] chardev: showing minor range for chardev in the output of /proc/devices

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Date:   Tue, 12 Feb 2019 23:41:35 +0800
From:   cgxu519 <cgxu519@....com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 2/2] chardev: showing minor range for chardev in the
 output of /proc/devices

On 2/12/19 11:20 PM, Greg KH wrote:
> On Tue, Feb 12, 2019 at 11:18:22PM +0800, cgxu519 wrote:
>> On 2/12/19 5:02 PM, Greg KH wrote:
>>> On Tue, Feb 12, 2019 at 04:47:39PM +0800, Chengguang Xu wrote:
>>>> Currently chardev allows to share major, showing
>>>> major with minor range for chardev will be more
>>>> helpful.
>>>>
>>>> Signed-off-by: Chengguang Xu <cgxu519@....com>
>>>> ---
>>>>    fs/char_dev.c | 4 +++-
>>>>    1 file changed, 3 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/fs/char_dev.c b/fs/char_dev.c
>>>> index b25b1da097d5..6f00acdeb308 100644
>>>> --- a/fs/char_dev.c
>>>> +++ b/fs/char_dev.c
>>>> @@ -55,7 +55,9 @@ void chrdev_show(struct seq_file *f, off_t offset)
>>>>    	mutex_lock(&chrdevs_lock);
>>>>    	for (cd = chrdevs[major_to_index(offset)]; cd; cd = cd->next) {
>>>>    		if (cd->major == offset)
>>>> -			seq_printf(f, "%3d %s\n", cd->major, cd->name);
>>>> +			seq_printf(f, "%3d %s (%u-%u)\n", cd->major, cd->name,
>>>> +				   cd->baseminor,
>>>> +				   cd->baseminor + cd->minorct - 1);
>>> You are changing the format of a userspace file, what tools are going to
>>> break when you do this?
>> I'll remove this part in V2. Do you have any idea how to get the minor
>> range info for particular major? Or adding a similar file to somewhere
>> under /sys is acceptable?
> Why do you need to know the minor range?  What can userspace do with
> this that actually matters?

Assume that when we try to load a driver module and fail with -EBUSY
because of minor range overlapping, then what can we do for this case?
we even don't know what range has occupied and what range is available.

Also, I think we can obviously notice range overlapping bugs by showing
all registered minor ranges.

Thanks,
Chengguang.