From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [PATCH] fs: fix i_writecount on shmem and friends
Date: Thu, 13 Mar 2014 04:29:34 +0000
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 <20140313150800.07878c80@notabene.brown>
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Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	David Herrmann <dh.herrmann@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Howells <dhowells@redhat.com>,
	Oleg Nesterov <oleg@redhat.com>
To: NeilBrown <neilb@suse.de>
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On Thu, Mar 13, 2014 at 03:08:00PM +1100, NeilBrown wrote:
> +		inode = mddev->bitmap_info.file->f_mapping->host;
> +		if (!S_ISREG(inode->i_mode)) {
> +			printk(KERN_ERR "%s: error: bitmap file must be a regular file\n",
> +			       mdname(mddev));
> +			fput(mddev->bitmap_info.file);
> +			mddev->bitmap_info.file = NULL;
> +			return -EBADF;
> +		}
> +		if (atomic_read(&inode->i_writecount) != 1) {

Umm...  I think you ought to check more than that.  At the very least you
want to check that you have it opened for write - you don't want e.g.
a filesystem containing that puppy remounted r/o under you.  Another thing
is, what happens if it's not a buffer cache backed one?  Hell, what happens
if it's a file on NFS?  You are relying on bmap() working, right?  So it
looks like you ought to check if ->bmap() is there.  And I really wonder
how well does it play with journalling fs...