linux-kernel - Re: [PATCH] drm: fix i_mapping and f_mapping initialization in drm

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Date:	Sun, 31 Mar 2013 12:34:18 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Ilija Hadzic <ilijahadzic@...il.com>
Cc:	dri-devel@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>,
	Thomas Hellstrom <thellstrom@...are.com>,
	Marco Munderloh <munderl@....uni-hannover.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drm: fix i_mapping and f_mapping initialization in
 drm_open in error path

On Sat 30-03-13 18:26:53, Ilija Hadzic wrote:
> This looks a bit like a hack and it doesn't look right,
> conceptually. If the call fails, it should restore things as if
> nothing has ever happened and overwriting old_mapping is not going to
> do the trick.

OK, I thought this is what the patch does as it falls back to
&inode->i_data which is the default mapping for all inodes or it uses
what used to be in device mapping.

I am obviously not familiar with the drm code but it feels a bit strange
that the device mapping can be different than inode's resp. file's one
and even more confusing that inode and file are saved separately.

> I think the right way to fix it would be to separately store the
> original mapping for filp->f_mapping and inode->i_mapping and restore
> it from their respective temporary variables if drm_open_helper or
> drm_setup fail. Attached is a quick patch to show you
[...]
> @@ -137,6 +139,8 @@ int drm_open(struct inode *inode, struct file *filp)
>  	if (!dev->open_count++)
>  		need_setup = 1;
>  	mutex_lock(&dev->struct_mutex);
> +	old_fmapping = filp->f_mapping;
> +	old_imapping = inode->i_mapping;

How can file and inode mappings be different?

>  	old_mapping = dev->dev_mapping;
>  	if (old_mapping == NULL)
>  		dev->dev_mapping = &inode->i_data;
> @@ -159,8 +163,8 @@ int drm_open(struct inode *inode, struct file *filp)
>  
>  err_undo:
>  	mutex_lock(&dev->struct_mutex);
> -	filp->f_mapping = old_mapping;
> -	inode->i_mapping = old_mapping;
> +	filp->f_mapping = old_fmapping;
> +	inode->i_mapping = old_imapping;
>  	iput(container_of(dev->dev_mapping, struct inode, i_data));
>  	dev->dev_mapping = old_mapping;
>  	mutex_unlock(&dev->struct_mutex);
-- 
1.8.1.5

-- 
Michal Hocko
SUSE Labs
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