[RFC PATCH 4/4] livepatch: Implement livepatch hybrid mode

[Yafang Shao <laoar.shao@gmail.com>]

Livepatching allows for rapid experimentation with new kernel features
without interrupting production workloads. However, static livepatches lack
the flexibility required to tune features based on task-specific attributes,
such as cgroup membership, which is critical in multi-tenant k8s
environments. Furthermore, hardcoding logic into a livepatch prevents
dynamic adjustments based on the runtime environment.

To address this, we propose a hybrid approach using BPF. Our production use
case involves:

1. Deploying a Livepatch function to serve as a stable BPF hook.

2. Utilizing bpf_override_return() to dynamically modify the return value
   of that hook based on the current task's context.

A significant challenge arises when atomic-replace is enabled. In this
mode, deploying a new livepatch changes the target function's address,
forcing a re-attachment of the BPF program. This re-attachment latency is
unacceptable in critical paths, such as those handling networking policies.

To solve this, we introduce a hybrid livepatch mode that allows specific
patches to remain non-replaceable, ensuring the function address remains
stable and the BPF program stays attached.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
---
 kernel/livepatch/core.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/livepatch/core.c b/kernel/livepatch/core.c
index 04f9e84f114f..5a44154131c8 100644
--- a/kernel/livepatch/core.c
+++ b/kernel/livepatch/core.c
@@ -665,6 +665,8 @@ static int klp_add_nops(struct klp_patch *patch)
 		klp_for_each_object(old_patch, old_obj) {
 			int err;
 
+			if (!old_patch->replaceable)
+				continue;
 			err = klp_add_object_nops(patch, old_obj);
 			if (err)
 				return err;
@@ -837,6 +839,8 @@ void klp_free_replaced_patches_async(struct klp_patch *new_patch)
 	klp_for_each_patch_safe(old_patch, tmp_patch) {
 		if (old_patch == new_patch)
 			return;
+		if (!old_patch->replaceable)
+			continue;
 		klp_free_patch_async(old_patch);
 	}
 }
@@ -1239,6 +1243,8 @@ void klp_unpatch_replaced_patches(struct klp_patch *new_patch)
 		if (old_patch == new_patch)
 			return;
 
+		if (!old_patch->replaceable)
+			continue;
 		old_patch->enabled = false;
 		klp_unpatch_objects(old_patch);
 	}
-- 
2.47.3