linux-kernel - Re: [PATCH v2] fpga: region: add owner module and take its refcount

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Message-ID: <9d016f83-8e7f-4bdf-8610-e3d0b49f7097@redhat.com>
Date: Wed, 10 Apr 2024 11:42:23 +0200
From: Marco Pagani <marpagan@...hat.com>
To: Xu Yilun <yilun.xu@...ux.intel.com>
Cc: Moritz Fischer <mdf@...nel.org>, Wu Hao <hao.wu@...el.com>,
 Xu Yilun <yilun.xu@...el.com>, Tom Rix <trix@...hat.com>,
 Jonathan Corbet <corbet@....net>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Alan Tull <atull@...nsource.altera.com>, linux-fpga@...r.kernel.org,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] fpga: region: add owner module and take its refcount



On 2024-04-09 06:08, Xu Yilun wrote:
> On Wed, Apr 03, 2024 at 03:34:22PM +0200, Marco Pagani wrote:
>>
>>
>> On 2024-04-01 11:34, Xu Yilun wrote:
>>> On Wed, Mar 27, 2024 at 05:00:20PM +0100, Marco Pagani wrote:
>>>> The current implementation of the fpga region assumes that the low-level
>>>> module registers a driver for the parent device and uses its owner pointer
>>>> to take the module's refcount. This approach is problematic since it can
>>>> lead to a null pointer dereference while attempting to get the region
>>>> during programming if the parent device does not have a driver.
>>>>
>>>> To address this problem, add a module owner pointer to the fpga_region
>>>> struct and use it to take the module's refcount. Modify the functions for
>>>> registering a region to take an additional owner module parameter and
>>>> rename them to avoid conflicts. Use the old function names for helper
>>>> macros that automatically set the module that registers the region as the
>>>> owner. This ensures compatibility with existing low-level control modules
>>>> and reduces the chances of registering a region without setting the owner.
>>>>
>>>> Also, update the documentation to keep it consistent with the new interface
>>>> for registering an fpga region.
>>>>
>>>> Other changes: unlock the mutex before calling put_device() in
>>>> fpga_region_put() to avoid potential use after release issues.
>>>
>>> Please try not to mix different changes in one patch, especially for
>>> a "bug fix" as you said.
>>
>> You are right. I'll split out the change and eventually send it as a
>> separate patch.
>>
>>> And I do have concern about the fix, see below.
>>>
>>> [...]
>>>
>>>> @@ -53,7 +53,7 @@ static struct fpga_region *fpga_region_get(struct fpga_region *region)
>>>>  	}
>>>>  
>>>>  	get_device(dev);
>>>> -	if (!try_module_get(dev->parent->driver->owner)) {
>>>> +	if (!try_module_get(region->br_owner)) {
>>>>  		put_device(dev);
>>>>  		mutex_unlock(&region->mutex);
>>>>  		return ERR_PTR(-ENODEV);
>>>> @@ -75,9 +75,9 @@ static void fpga_region_put(struct fpga_region *region)
>>>>  
>>>>  	dev_dbg(dev, "put\n");
>>>>  
>>>> -	module_put(dev->parent->driver->owner);
>>>> -	put_device(dev);
>>>> +	module_put(region->br_owner);
>>>>  	mutex_unlock(&region->mutex);
>>>
>>> If there is concern the region would be freed after put_device(), then
>>> why still keep the sequence in fpga_region_get()?
>>
>> Ouch, sorry, I forgot to make the change also in fpga_region_get().
>>
>>> And is it possible region is freed before get_device() in
>>> fpga_region_get()?
>>
>> If the user follows the usual pattern (i.e., waiting for
> 
> I can see the only safe way is fpga_region_program_fpga() or fpga_region_get()
> should be included in:
> 
>   region = fpga_region_class_find();
>   ...
>   put_device(&region->dev);
> 
> That is to say, fpga_region_get() should not be called when there is no
> region dev reference hold beforehand. In this case, no use after release
> risk. That's why I was thinking about some documentation.
> 
> Another concern is we'd better keep the get/put operations symmetrical
> for easy maintaining, as long as it doesn't cause problem.

Now I see your point. So, you suggest changing only the docs to clarify
that the region must be taken with fpga_region_class_find() before
programming it with fpga_region_program_fpga()?

That's fine by me. However, this made me wonder why we need to take the
region dev with get_device() in fpga_region_program_fpga()->fpga_region_get().
If we assume that the user must always call fpga_region_class_find()
before programming with fpga_region_program_fpga(), why do we need the
double get?

Thanks,
Marco
 
>> fpga_region_program_fpga() to complete before calling
>> fpga_region_unregister()) there should be no problem. However, I think
>> releasing the device before unlocking the mutex contained in the context
>> associated with the device makes the code brittle and more prone to
>> problems.
>>
>>> Or we should clearly document how/when to use these functions?
>>  
>> I think it is not necessary to change the documentation since the
>> in-kernel programming API will not be affected by the change.
>>
[...]