linux-kernel - Re: [PATCH] loop: clear read-only flag in loop_clr

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Date:	Mon, 14 Feb 2011 12:47:48 +0100
From:	Milan Broz <mbroz@...hat.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Tao Ma <tm@....ma>, linux-kernel@...r.kernel.org,
	Jens Axboe <axboe@...nel.dk>,
	device-mapper development <dm-devel@...hat.com>
Subject: Re: [PATCH] loop: clear read-only flag in loop_clr_fd.


On 02/14/2011 11:30 AM, Tejun Heo wrote:
> Umm... This was reported some time ago and patches were already
> posted.  Milan, can you test whether the following two patches fix the
> problems you're seeing?  Jens, what's the status of these patches?
>
>   http://thread.gmane.org/gmane.linux.kernel/1090211/focus=1090666
>
With patch below (loop cannot be built as module) it fixes the loop problem.

But it doesn't fix the read-only snapshot issue and I guess there will be
the same problem with read-only MD code too.
(so the 2) issue here https://lkml.org/lkml/2011/2/12/209).

If the call is changed intentionally, we have to fix unconditional blkdev open
calls with read-write flag in this code.
Before doing that I would like to know if it was intentional change or not...

You can simple try this reproducer (works on older kernel, second readonly
snapshot create fails now with permission denied)
+ dmsetup create x --readonly --table '0 131072 snapshot /dev/loop0 /dev/loop1 p 8'
device-mapper: reload ioctl failed: Permission denied

#!/bin/bash -x
modprobe loop

dd if=/dev/zero of=/x.img bs=1M count=64
dd if=/dev/zero of=/xs.img bs=1M count=64
losetup /dev/loop0 /x.img
losetup /dev/loop1 /xs.img
sync
dmsetup create x --table "0 131072 snapshot /dev/loop0 /dev/loop1 p 8"
udevadm settle
dmsetup remove x

losetup -d /dev/loop0
losetup -d /dev/loop1
losetup -r /dev/loop0 /x.img
losetup -r /dev/loop1 /xs.img
dmsetup create x --readonly --table "0 131072 snapshot /dev/loop0 /dev/loop1 p 8"
dmsetup table

dmsetup remove x
losetup -d /dev/loop0
losetup -d /dev/loop1

Milan

--
Export bdgrap to allow loop module build 

Signed-off-by: Milan Broz <mbroz@...hat.com> 
diff --git a/fs/block_dev.c b/fs/block_dev.c
index 333a7bb..c9cf9f7 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -574,6 +574,7 @@ struct block_device *bdgrab(struct block_device *bdev)
 	ihold(bdev->bd_inode);
 	return bdev;
 }
+EXPORT_SYMBOL(bdgrab);
 
 long nr_blockdev_pages(void)
 {


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