On Mon 2018-03-05 13:54:38, Miroslav Benes wrote:
> On Mon, 5 Mar 2018, Evgenii Shatokhin wrote:
>
> > Hi,
>
> Hi,
>
> > > The atomic replace allows to create cumulative patches. They
> > > are useful when you maintain many livepatches and want to remove
> > > one that is lower on the stack. In addition it is very useful when
> > > more patches touch the same function and there are dependencies
> > > between them.
> >
> > I have experimented with this updated patchset a bit.
> > It looks like replace operation fails if there is a loaded but disabled patch.
> >
> > Suppose there are 2 binary patches changing the same kernel functions. Load
> > patch1, then disable it (echo 0 > /sys/kernel/livepatch/patch1/enabled), then
> > try load patch2 with atomic replace. I get "Device or resource busy" error at
> > this point.
> >
> > It seems, __klp_enable_patch() returns -EBUSY for some reason. I haven't dug
> > deeper into that code yet.
>
> Yes, it is "enforce stacking" check in __klp_enable_patch():
>
> /* enforce stacking: only the first disabled patch can be enabled */
> if (patch->list.prev != &klp_patches &&
> !list_prev_entry(patch, list)->enabled)
> return -EBUSY;
>
> So not connected to this patch set. We've had the behaviour for quite some
> time.
We actually wanted to enabled this for the patches with the replace
flag. It is even described in
Documentation/livepatch/cumulative-patches.txt.
Of course, you could always unregister all the disabled patches that
block the new one. But it might be inconvenient.
I do not see any big reason why this should be disabled:
+ klp_add_nops() takes into account even disabled patches.
In the worst case, we might enable some NOPs that are
not really needed.
+ klp_throw_away_replaced_patches() removes all patches down
the stack.
I am going to enable this in a separate patch in v10.
Best Regards,
Petr
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