linux-kernel - Re: [PATCH 1/5] debugfs: Prevent NULL dereference reading from string property

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Message-ID: <1efdb84f-5a84-4c1e-8d0c-bb516c0aebf7@opensource.cirrus.com>
Date:   Tue, 16 May 2023 19:04:42 +0100
From:   Richard Fitzgerald <rf@...nsource.cirrus.com>
To:     Greg KH <gregkh@...uxfoundation.org>
CC:     <rafael@...nel.org>, <linux-kernel@...r.kernel.org>,
        <patches@...nsource.cirrus.com>
Subject: Re: [PATCH 1/5] debugfs: Prevent NULL dereference reading from string
 property

On 16/5/23 18:43, Greg KH wrote:
> On Tue, May 16, 2023 at 06:29:52PM +0100, Richard Fitzgerald wrote:
>> On 16/5/23 17:33, Greg KH wrote:
>>> On Tue, May 16, 2023 at 05:07:49PM +0100, Richard Fitzgerald wrote:
>>>> Check in debugfs_read_file_str() if the string pointer is NULL.
>>>>
>>>> It is perfectly reasonable that a driver may wish to export a string
>>>> to debugfs that can have the value NULL to indicate empty/unused/ignore.
>>>
>>> Does any in-kernel driver do this today?
>>
>> I don't know. The history here is that I was using debugfs_create_str()
>> to add a debugfs to a driver and made these improvements along the way.
>> Ultimately I had a reason to use a custom reader implementation.
>> But as I'd already written these patches I thought I'd send them.
>>
>>>
>>> If not, why not fix up the driver instead?
>>>
>>
>> Well... could do. Though it seems a bit odd to me that a driver
>> design should be forced by the debugfs API, instead of the debugfs API
>> fitting normal code design. It's pretty standard and idiomatic for code
>> to use if (!str) { /* bail */ } type logic, so why shouldn't the debugfs
>> API handle that?
>>
>>>>
>>>> Signed-off-by: Richard Fitzgerald <rf@...nsource.cirrus.com>
>>>> ---
>>>>    fs/debugfs/file.c | 3 +++
>>>>    1 file changed, 3 insertions(+)
>>>>
>>>> diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
>>>> index 1f971c880dde..2c085ab4e800 100644
>>>> --- a/fs/debugfs/file.c
>>>> +++ b/fs/debugfs/file.c
>>>> @@ -878,6 +878,9 @@ ssize_t debugfs_read_file_str(struct file *file, char __user *user_buf,
>>>>    		return ret;
>>>>    	str = *(char **)file->private_data;
>>>> +	if (!str)
>>>> +		return simple_read_from_buffer(user_buf, count, ppos, "\n", 1);
>>>
>>> Why not print "(NULL)"?
>>>
>>
>> Again, could do. My thought here is that a debugfs can be piped into
>> tools and having to insert a catch for "(NULL)" in the pipeline is a
>> nuisance. This is a bit different from a dmesg print, which is less
>> likely to be used this way or to guarantee machine-parsing.
>> However, I don't mind changing to "(NULL)" if you prefer.
> 
> If a driver wants an "empty" string, they should provide an empty
> string.  We don't do empty values for any other type of pointer, right?
> 
> Actually we really should just bail out with an error if this is NULL,
> let's not paper over bad drivers like this.
> 

I don't understand this comment.
I think you'll find there is a very large amount of kernel code that
uses a NULL value in a pointer to mean ignore/unspecified in
some way. This has always been accepted C coding style.

The whole idea that a driver is "bad" for signalling some state
by a pointer being NULL makes no sense.

Please ignore this patch chain. I really don't feel like writing
non-idiomatic C code just to work around badly designed debugfs APIs.
Better to write a custom read().

> thanks,
> 
> greg k-h