sys_swapoff() symlink bug

sys_swapoff() symlink bug

[Michael Buesch]

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Hi,

The following script demonstrates a bug in the swapon/swapoff
syscalls in the latest 2.6 kernel (I did not check 2.4).

#/bin/sh
# 3:12 is hda12, which is a swap partition on my system.
SWAP_MAJOR=3
SWAP_MINOR=12
echo "running swaps:"
cat /proc/swaps
echo "--"
mkdir /testdev
mknod /testdev/testnode b $SWAP_MAJOR $SWAP_MINOR
ln -s /testdev/testnode /testdev/testlink
swapon /testdev/testlink
echo "swapon returned $?"
echo "running swaps:"
cat /proc/swaps
echo "--"
swapoff /testdev/testdev
echo "swapoff returned $?"
echo "running swaps:"
cat /proc/swaps
echo "--"
rm -Rf /testdev


The output of this script on my system is:

running swaps:
- --
swapon returned 0
running swaps:
Filename                                Type            Size    Used    Priority
/testdev/testnode                        partition      976712  0       -32
- --
swapoff returned 255
running swaps:
Filename                                Type            Size    Used    Priority
/testdev/testnode                        partition      976712  0       -32
- --


As you can see the swapoff fails, because it is called on
the node instead of the symlink that was used for swapon.
I think this should not fail.

The code that fails to detect, that the symlink _is_ actually
the same swapspace as the node itself, is mm/swapfile.c:1038

	for (type = swap_list.head; type >= 0; type = swap_info[type].next) {
		p = swap_info + type;
		if ((p->flags & SWP_ACTIVE) == SWP_ACTIVE) {

/* I'm not really sure about what we should check here, instead
 * of f_mapping to detect it.
 */
			if (p->swap_file->f_mapping == mapping)
				break;
		}
		prev = type;
	}

- -- 
Regards Michael Buesch  [ http://www.tuxsoft.de.vu ]

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