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

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Message-ID: <ZhS/M6pa9AHyvb0y@yilunxu-OptiPlex-7050>
Date: Tue, 9 Apr 2024 12:08:19 +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, 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.

Thanks,
Yilun

> 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
>