linux-kernel - Re: [PATCH v3] Check for immutable/append flag in fallocate path

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Date:	Tue, 8 Mar 2011 16:11:21 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Marco Stornelli <marco.stornelli@...il.com>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v3] Check for immutable/append flag in fallocate path

On Sat, Mar 05, 2011 at 10:37:45AM +0100, Marco Stornelli wrote:
> From: Marco Stornelli <marco.stornelli@...il.com>
> 
> In the fallocate path the kernel doesn't check for the immutable/append
> flag. It's possible to have a race condition in this scenario: an
> application open a file in read/write and it does something, meanwhile
> root set the immutable flag on the file, the application at that point
> can call fallocate with success. In addition, we don't allow to do any
> unreserve operation on an append only file but only the reserve one.
> 
> Signed-off-by: Marco Stornelli <marco.stornelli@...il.com>
> ---
> Patch is against 2.6.38-rc7
> 
> ChangeLog:
> v3: Modified do_fallocate instead of every single fs
> v2: Added the check for append-only file for XFS
> v1: First draft
> 
> --- open.c.orig	2011-03-01 22:55:12.000000000 +0100
> +++ open.c	2011-03-04 15:28:43.000000000 +0100
> @@ -233,6 +233,14 @@ int do_fallocate(struct file *file, int
>  
>  	if (!(file->f_mode & FMODE_WRITE))
>  		return -EBADF;
> +
> +	/* It's not possible punch hole on append only file */
> +	if (mode & FALLOC_FL_PUNCH_HOLE && IS_APPEND(inode))
> +		return -EPERM;

Seeing as I didn't get an answer in before you reposted, I still
think punching an append-only file is a valid thing to want to do.

I've seen this done in the past for application-level transaction
journal files. The journal file is append only so new transactions
can only be written at the end of the file i.e. you cannot overwrite
(and therefore corrupt) existing transactions. However, once a
transaction is complete and the changes flushed to disk, the
transaction is punched out of the file to zero the range so it
doesn't get replayed during recovery after a system crash.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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