[PATCH 0/3] fix side effects of loop reuse

[PATCH 0/3] fix side effects of loop reuse

[Stanislav Brabec]

Detailed testing discovered some regressions in the loop reuse code:


1) mount -oloop=/dev/loop1 changed its behavior

Note that the proposed solution keeps overlay checks active. There is
another possible solution: disable all checks:

--- a/libmount/src/context_loopdev.c
+++ b/libmount/src/context_loopdev.c
@@ -215,7 +215,7 @@ int mnt_context_setup_loopdev(struct libmnt_context *cxt)
 	 * mechanism to detect it. To prevent data corruption, the same loop
 	 * device has to be recycled.
 	*/
-	if (backing_file) {
+	if (backing_file && !loopval) {
 		rc = loopcxt_init(&lc, 0);
 		if (rc)
 			goto done_no_deinit;

There is still a small regression that could be fixed in the future:
If both the existing and the requested devices are read-only, it is
safe to not reuse existing loop device.

So we can think about allowing of:
losetup --read-only /dev/loop0 file.img
mount -oro,loop=/dev/loop1 file.img /mnt

2) If mount re-uses an existing loop device, then the loop device is
detached, even if autoclean flag is off.

Note that I was not able to find a mechanism, how mount gets the "loop"
in the output, so I did the change in mount, and not umount.


-- 
Best Regards / S pozdravem,

Stanislav Brabec
software developer
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Re: [PATCH 0/3] fix side effects of loop reuse

[Karel Zak]

On Thu, Jan 12, 2017 at 03:45:07PM +0100, Stanislav Brabec wrote:
> -	if (backing_file) {
> +	if (backing_file && !loopval) {

This is not so bad solution, if you use -oloop=/dev/loopN you probably
know what are you doing :-)

Anyway, all the three patches applied. Thanks (and extra thank for the
tests).

    Karel


-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com