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Re: [Xen-devel] [PATCH 09/14] livepatch: Add per-function applied/reverted state tracking marker


On 22/08/2019 08:44, Wieczorkiewicz, Pawel wrote:

On 21. Aug 2019, at 23:34, Julien Grall <julien.grall@xxxxxxx <mailto:julien.grall@xxxxxxx>> wrote:
On 8/21/19 9:19 AM, Pawel Wieczorkiewicz wrote:
Livepatch only tracks an entire payload applied/reverted state. But,
with an option to supply the apply_payload() and/or revert_payload()
functions as optional hooks, it becomes possible to intermix the
execution of the original apply_payload()/revert_payload() functions
with their dynamically supplied counterparts.
It is important then to track the current state of every function
being patched and prevent situations of unintentional double-apply
or unapplied revert.
To support that, it is necessary to extend public interface of the
livepatch. The struct livepatch_func gets additional field holding
the applied/reverted state marker.
To reflect the livepatch payload ABI change, bump the version flag
The above solution only applies to x86 architecture for now.

Is this going to break livepatch on Arm? If so, do you have any plan to fix it?

No, I do not think it is. But, I am unable to test on Arm (No access to HW and 
so I took the conservative approach here.
Arm provides decent free model (see FoundationModel) that you can use for basic testing. Alternatively, you QEMU also support virtualization extension.

Let me have a look at the code (I will answer separately) to see if I can spot anything.


diff --git a/xen/include/xen/livepatch.h b/xen/include/xen/livepatch.h
index 2aec532ee2..a93126f631 100644
--- a/xen/include/xen/livepatch.h
+++ b/xen/include/xen/livepatch.h
@@ -117,7 +117,7 @@ int arch_livepatch_quiesce(void);
 void arch_livepatch_revive(void);
   void arch_livepatch_apply(struct livepatch_func *func);
-void arch_livepatch_revert(const struct livepatch_func *func);
+void arch_livepatch_revert(struct livepatch_func *func);

I doubt livepatch on Arm will compile after this change.

What would be you suggestion then?

Cross-compiler are nowadays widely available. So build testing your changes in common code would be the minimum.

In this case, as you dropped the const from the prototype, you will need to do the same in the declaration.

Shall I limit the change to X86 everywhere
  Or maybe drop the compilation flag completely?

I am a bit confused. Which compilation flag do you refer to?


Julien Grall

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