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

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Date: Wed, 3 Apr 2024 15:34:22 +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 2024-04-01 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
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.

Thanks,
Marco