linux-kernel - Re: [PATCH] Reset file->f_op in snd_card_file

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Date:	Wed, 04 Oct 2006 16:18:08 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Karsten Wiese <annabellesgarden@...oo.de>
Cc:	linux-kernel@...r.kernel.org, alsa-devel@...ts.sourceforge.net,
	mingo@...e.hu
Subject: Re: [PATCH] Reset file->f_op in snd_card_file_remove(). Take 2

At Wed, 4 Oct 2006 12:47:19 +0200,
Karsten Wiese wrote:
> 
> Am Mittwoch, 4. Oktober 2006 11:22 schrieb Takashi Iwai:
> > 
> > It should call snd_card_free_when_closed() instead.
> > 
> IMHO, that would just make sure that the bug happens.
> Please see my annotations, starting with // in:
> 
> void fastcall __fput(struct file *file)
> {
> 	struct dentry *dentry = file->f_dentry;
> 	struct vfsmount *mnt = file->f_vfsmnt;
> 	struct inode *inode = dentry->d_inode;
> 
> 	might_sleep();
> 
> 	fsnotify_close(file);
> 	/*
> 	 * The function eventpoll_release() should be the first called
> 	 * in the file cleanup chain.
> 	 */
> 	eventpoll_release(file);
> 	locks_remove_flock(file);
> 
> 	if (file->f_op && file->f_op->release)
> 		file->f_op->release(inode, file);
> // Here snd_hwdep_release() is called.
> // snd_hwdep_release() calls snd_card_file_remove().
> // snd_card_file_remove() sees card->free_on_last_close ist set,
> // calls snd_card_do_free().
> // snd_card_do_free frees file->f_op but doesn't set it NULL.
> //
> 	security_file_free(file);
> 	if (unlikely(inode->i_cdev != NULL))
> 		cdev_put(inode->i_cdev);
> 	fops_put(file->f_op);
> // file->f_op has already been freeed!
> // fops_put(file->f_op) is likely to oops.

Yes, this looks like an invalid access.

The problem is that we use kmalloc for allocating a dummy f_op.
IMO, the simlest solution is to use a static dummy f_op.


Takashi
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