Re: [PATCH 01/13] video: fb_defio: preserve user fb_ops

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Nov 27, 2019 at 06:31:57PM +0200, Jani Nikula wrote:
> Modifying fb_ops directly to override fb_mmap with fb_deferred_io_mmap
> and then resetting it to NULL afterwards causes problems all over the
> place. First, it prevents making the fbops member of struct fb_info a
> const pointer, which means we can't make struct fb_ops const
> anywhere. Second, a few places have to go out of their way to restore
> the original fb_mmap pointer that gets reset to NULL.
> 
> Preserve the passed in fb_ops by making a copy of it and modifying that
> instead. Add a deferred_io_private member to struct fb_info to store the
> pointer to the old fb_ops, and restore that at cleanup.

Quite the horror show. I wonder how hard it would be to just have each
driver that can use defio provide a mmap hook that either dispatches
to the defio one or the native one depending if defio is used or not...

A few drivers at least seem to always use defio so those should be
trivial. Some others seem to make the decision dynamically, which
would require custom .mmap(). Though I suppose all of them could just
be of the form
foo_mmap_wrap{)
{
	if (info->defio)
		defio_mmap()
	else
		foo_mmap();
}

Hmm. Actually is .fb_mmap() called from anywhere but fb_mmap()? If not
we could just shove the defio check there? Or if it's called from
several places we could try to wrap all calls in _fb_mmap() or somesuch.

> 
> Cc: Jaya Kumar <jayalk@intworks.biz>
> Cc: linux-fbdev@vger.kernel.org
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
> 
> ---
> 
> Note: If the approach is acceptable, we'll also need to handle the error
> returns on memory allocation failures at fb_deferred_io_init() call
> sites. There are 13.
> ---
>  drivers/video/fbdev/core/fb_defio.c | 25 ++++++++++++++++++++++---
>  include/linux/fb.h                  |  3 ++-
>  2 files changed, 24 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
> index 82c20c6047b0..36697844c1e0 100644
> --- a/drivers/video/fbdev/core/fb_defio.c
> +++ b/drivers/video/fbdev/core/fb_defio.c
> @@ -200,13 +200,23 @@ static void fb_deferred_io_work(struct work_struct *work)
>  	mutex_unlock(&fbdefio->lock);
>  }
>  
> -void fb_deferred_io_init(struct fb_info *info)
> +int fb_deferred_io_init(struct fb_info *info)
>  {
>  	struct fb_deferred_io *fbdefio = info->fbdefio;
> +	struct fb_ops *fbops;
>  
>  	BUG_ON(!fbdefio);
> +
> +	fbops = kmemdup(info->fbops, sizeof(*fbops), GFP_KERNEL);
> +	if (!fbops)
> +		return -ENOMEM;
> +
> +	fbops->fb_mmap = fb_deferred_io_mmap;
> +	info->deferred_io_private = info->fbops;
> +	info->fbops = fbops;
> +
>  	mutex_init(&fbdefio->lock);
> -	info->fbops->fb_mmap = fb_deferred_io_mmap;
> +
>  	INIT_DELAYED_WORK(&info->deferred_work, fb_deferred_io_work);
>  	INIT_LIST_HEAD(&fbdefio->pagelist);
>  	if (fbdefio->delay = 0) /* set a default of 1 s */
> @@ -229,6 +239,12 @@ void fb_deferred_io_cleanup(struct fb_info *info)
>  	int i;
>  
>  	BUG_ON(!fbdefio);
> +
> +	/* sanity check against misuse */
> +	if (WARN_ON(!info->deferred_io_private ||
> +		    info->fbops->fb_mmap != fb_deferred_io_mmap))
> +		return;
> +
>  	cancel_delayed_work_sync(&info->deferred_work);
>  
>  	/* clear out the mapping that we setup */
> @@ -237,7 +253,10 @@ void fb_deferred_io_cleanup(struct fb_info *info)
>  		page->mapping = NULL;
>  	}
>  
> -	info->fbops->fb_mmap = NULL;
> +	kfree(info->fbops);
> +	info->fbops = info->deferred_io_private;
> +	info->deferred_io_private = NULL;
> +
>  	mutex_destroy(&fbdefio->lock);
>  }
>  EXPORT_SYMBOL_GPL(fb_deferred_io_cleanup);
> diff --git a/include/linux/fb.h b/include/linux/fb.h
> index a6ad528990de..65f2abd47745 100644
> --- a/include/linux/fb.h
> +++ b/include/linux/fb.h
> @@ -470,6 +470,7 @@ struct fb_info {
>  #ifdef CONFIG_FB_DEFERRED_IO
>  	struct delayed_work deferred_work;
>  	struct fb_deferred_io *fbdefio;
> +	void *deferred_io_private;
>  #endif
>  
>  	struct fb_ops *fbops;
> @@ -658,7 +659,7 @@ static inline void __fb_pad_aligned_buffer(u8 *dst, u32 d_pitch,
>  
>  /* drivers/video/fb_defio.c */
>  int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma);
> -extern void fb_deferred_io_init(struct fb_info *info);
> +extern int fb_deferred_io_init(struct fb_info *info);
>  extern void fb_deferred_io_open(struct fb_info *info,
>  				struct inode *inode,
>  				struct file *file);
> -- 
> 2.20.1

-- 
Ville Syrjälä
Intel