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SubjectRE: [dm-devel] [PATCH 3/4] dm: convert dm_dev_internal.count from atomic_t to refcount_t
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> On Fri, Nov 24, 2017 at 2:36 AM, Reshetova, Elena
> <elena.reshetova@intel.com> wrote:
> >> On Fri, Oct 20, 2017 at 10:37:38AM +0300, Elena Reshetova wrote:
> >> > } else if (dd->dm_dev->mode != (mode | dd->dm_dev->mode)) {
> >> > r = upgrade_mode(dd, mode, t->md);
> >> > if (r)
> >> > return r;
> >> > + refcount_inc(&dd->count);
> >> > }
> >>
> >> Missing here:
> >>
> >> else
> >> refcount_inc(&dd->count);
> >>
> >> ?
> >
> > Oh, yes, thanks for catching this! I think this got unnoticed so far and patch was
> merged, so I am going to send a followup patch now.
>
> I pushed this fix and will send to Linus next week:
> https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-
> dm.git/commit/?h=dm-4.15&id=d908af82d06cc420f9581c97c6db941cb87e4434


I guess you mean this commit:
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=for-next&id=c2318d07ead871f058dda62e942ed7b6b1c1cfcf

Unfortunately it is not correct:

diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
index 88130b5..f6d32ee 100644
--- a/drivers/md/dm-table.c
+++ b/drivers/md/dm-table.c
@@ -451,15 +451,15 @@ int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
return r;
}

- refcount_set(&dd->count, 1);
+ refcount_set(&dd->count, 0);
list_add(&dd->list, &t->devices);

} else if (dd->dm_dev->mode != (mode | dd->dm_dev->mode)) {
r = upgrade_mode(dd, mode, t->md);
if (r)
return r;
- refcount_inc(&dd->count);
}
+ refcount_inc(&dd->count);

Problem will be here if you hit this refcount_inc() after the refcount_set(&dd->count, 0) earlier.
refcount_inc() does not increment on zero value *ever* for security reasons and instead people
should initialize refcounters to 1 always and do increments from there if needed.
This was the reason for the initial change I did, my mistake was just to forget to increment it also
in case condition (dd->dm_dev->mode != (mode | dd->dm_dev->mode)) fails.

I have issues with my intel smpt server for sending patches (I will get it fixed tomorrow from internal network),
so I am attaching the patch I did end of last week to this thread instead (or alternatively can properly send it tomorrow after fix).
Sorry for the delay!

Best Regards,
Elena.

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