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

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Date: Mon, 1 Apr 2024 17:34:04 +0800
From: Xu Yilun <yilun.xu@...ux.intel.com>
To: Marco Pagani <marpagan@...hat.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 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.

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()?

And is it possible region is freed before get_device() in
fpga_region_get()?

Or we should clearly document how/when to use these functions?

Thanks,
Yilun

> +	put_device(dev);
>  }