livepatch: Add per-function applied/reverted state tracking marker

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
LIVEPATCH_PAYLOAD_VERSION up to 2.

[And also update the top of the design document]

Signed-off-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
Reviewed-by: Andra-Irina Paraschiv <andraprs@amazon.com>
Reviewed-by: Bjoern Doebel <doebel@amazon.de>
Reviewed-by: Martin Pohlack <mpohlack@amazon.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>

docs/misc/livepatch.pandoc

@@ -1,4 +1,4 @@
-# Xen Live Patching Design v1
+# Xen Live Patching Design v2
 
 ## Rationale
 
@@ -297,10 +297,14 @@ which describe the functions to be patched:
         uint32_t old_size;
         uint8_t version;
         uint8_t opaque[31];
+        /* Added to livepatch payload version 2: */
+        uint8_t applied;
+        uint8_t _pad[7];
     };
 
 The size of the structure is 64 bytes on 64-bit hypervisors. It will be
 52 on 32-bit hypervisors.
+The version 2 of the payload adds additional 8 bytes to the structure size.
 
  * `name` is the symbol name of the old function. Only used if `old_addr` is
    zero, otherwise will be used during dynamic linking (when hypervisor loads
@@ -324,9 +328,15 @@ The size of the structure is 64 bytes on 64-bit hypervisors. It will be
    * If the value of `new_addr` is zero then `new_size` determines how many
     instruction bytes to NOP (up to opaque size modulo smallest platform
     instruction - 1 byte x86 and 4 bytes on ARM).
- * `version` is to be one.
+ * `version` indicates version of the generated payload.
  * `opaque` **MUST** be zero.
 
+The version 2 of the payload adds the following fields to the structure:
+
+  * `applied` tracks function's applied/reverted state. It has a boolean type
+    either LIVEPATCH_FUNC_NOT_APPLIED or LIVEPATCH_FUNC_APPLIED.
+  * `_pad[7]` adds padding to align to 8 bytes.
+
 The size of the `livepatch_func` array is determined from the ELF section
 size.
 
@@ -378,6 +388,9 @@ A simple example of what a payload file can be:
         uint32_t old_size;
         uint8_t version;
         uint8_t pad[31];
+        /* Added to livepatch payload version 2: */
+        uint8_t applied;
+        uint8_t _pad[7];
     };
 
     /* Our replacement function for xen_extra_version. */

xen/common/livepatch.c

@@ -1182,7 +1182,7 @@ static int apply_payload(struct payload *data)
     ASSERT(!local_irq_is_enabled());
 
     for ( i = 0; i < data->nfuncs; i++ )
-        arch_livepatch_apply(&data->funcs[i]);
+        common_livepatch_apply(&data->funcs[i]);
 
     arch_livepatch_revive();
 
@@ -1216,7 +1216,7 @@ static int revert_payload(struct payload *data)
     }
 
     for ( i = 0; i < data->nfuncs; i++ )
-        arch_livepatch_revert(&data->funcs[i]);
+        common_livepatch_revert(&data->funcs[i]);
 
     /*
      * Since we are running with IRQs disabled and the hooks may call common
@@ -1248,6 +1248,29 @@ static inline void revert_payload_tail(struct payload *data)
     data->state = LIVEPATCH_STATE_CHECKED;
 }
 
+/*
+ * Check if an action has applied the same state to all payload's functions consistently.
+ */
+static inline bool was_action_consistent(const struct payload *data, livepatch_func_state_t expected_state)
+{
+    int i;
+
+    for ( i = 0; i < data->nfuncs; i++ )
+    {
+        struct livepatch_func *f = &(data->funcs[i]);
+
+        if ( f->applied != expected_state )
+        {
+            printk(XENLOG_ERR LIVEPATCH "%s: Payload has a function: '%s' with inconsistent applied state.\n",
+                   data->name, f->name ?: "noname");
+
+            return false;
+        }
+    }
+
+    return true;
+}
+
 /*
  * This function is executed having all other CPUs with no deep stack (we may
  * have cpu_idle on it) and IRQs disabled.
@@ -1274,6 +1297,9 @@ static void livepatch_do_action(void)
         else
             rc = apply_payload(data);
 
+        if ( !was_action_consistent(data, rc ? LIVEPATCH_FUNC_NOT_APPLIED : LIVEPATCH_FUNC_APPLIED) )
+            panic("livepatch: partially applied payload '%s'!\n", data->name);
+
         if ( rc == 0 )
             apply_payload_tail(data);
         break;
@@ -1288,6 +1314,9 @@ static void livepatch_do_action(void)
         else
             rc = revert_payload(data);
 
+        if ( !was_action_consistent(data, rc ? LIVEPATCH_FUNC_APPLIED : LIVEPATCH_FUNC_NOT_APPLIED) )
+            panic("livepatch: partially reverted payload '%s'!\n", data->name);
+
         if ( rc == 0 )
             revert_payload_tail(data);
         break;
@@ -1309,6 +1338,9 @@ static void livepatch_do_action(void)
             else
                 other->rc = revert_payload(other);
 
+            if ( !was_action_consistent(other, rc ? LIVEPATCH_FUNC_APPLIED : LIVEPATCH_FUNC_NOT_APPLIED) )
+                panic("livepatch: partially reverted payload '%s'!\n", other->name);
+
             if ( other->rc == 0 )
                 revert_payload_tail(other);
             else
@@ -1329,6 +1361,9 @@ static void livepatch_do_action(void)
             else
                 rc = apply_payload(data);
 
+            if ( !was_action_consistent(data, rc ? LIVEPATCH_FUNC_NOT_APPLIED : LIVEPATCH_FUNC_APPLIED) )
+                panic("livepatch: partially applied payload '%s'!\n", data->name);
+
             if ( rc == 0 )
                 apply_payload_tail(data);
         }

xen/include/public/sysctl.h

@@ -832,7 +832,7 @@ struct xen_sysctl_cpu_featureset {
  *     If zero exit with success.
  */
 
-#define LIVEPATCH_PAYLOAD_VERSION 1
+#define LIVEPATCH_PAYLOAD_VERSION 2
 /*
  * .livepatch.funcs structure layout defined in the `Payload format`
  * section in the Live Patch design document.
@@ -840,6 +840,11 @@ struct xen_sysctl_cpu_featureset {
  * We guard this with __XEN__ as toolstacks SHOULD not use it.
  */
 #ifdef __XEN__
+typedef enum livepatch_func_state {
+    LIVEPATCH_FUNC_NOT_APPLIED,
+    LIVEPATCH_FUNC_APPLIED
+} livepatch_func_state_t;
+
 struct livepatch_func {
     const char *name;       /* Name of function to be patched. */
     void *new_addr;
@@ -848,6 +853,8 @@ struct livepatch_func {
     uint32_t old_size;
     uint8_t version;        /* MUST be LIVEPATCH_PAYLOAD_VERSION. */
     uint8_t opaque[31];
+    uint8_t applied;
+    uint8_t _pad[7];
 };
 typedef struct livepatch_func livepatch_func_t;
 #endif

xen/include/xen/livepatch.h

@@ -123,6 +123,34 @@ void arch_livepatch_post_action(void);
 
 void arch_livepatch_mask(void);
 void arch_livepatch_unmask(void);
+
+static inline void common_livepatch_apply(struct livepatch_func *func)
+{
+    /* If the action has been already executed on this function, do nothing. */
+    if ( func->applied == LIVEPATCH_FUNC_APPLIED )
+    {
+        printk(XENLOG_WARNING LIVEPATCH "%s: %s has been already applied before\n",
+                __func__, func->name);
+        return;
+    }
+
+    arch_livepatch_apply(func);
+    func->applied = LIVEPATCH_FUNC_APPLIED;
+}
+
+static inline void common_livepatch_revert(struct livepatch_func *func)
+{
+    /* If the apply action hasn't been executed on this function, do nothing. */
+    if ( !func->old_addr || func->applied == LIVEPATCH_FUNC_NOT_APPLIED )
+    {
+        printk(XENLOG_WARNING LIVEPATCH "%s: %s has not been applied before\n",
+                __func__, func->name);
+        return;
+    }
+
+    arch_livepatch_revert(func);
+    func->applied = LIVEPATCH_FUNC_NOT_APPLIED;
+}
 #else
 
 /*

xen/test/livepatch/Makefile

@@ -24,6 +24,9 @@ LIVEPATCH_PREPOST_HOOKS := xen_prepost_hooks.livepatch
 LIVEPATCH_PREPOST_HOOKS_FAIL := xen_prepost_hooks_fail.livepatch
 LIVEPATCH_ACTION_HOOKS := xen_action_hooks.livepatch
 LIVEPATCH_ACTION_HOOKS_NOFUNC := xen_action_hooks_nofunc.livepatch
+LIVEPATCH_ACTION_HOOKS_MARKER:= xen_action_hooks_marker.livepatch
+LIVEPATCH_ACTION_HOOKS_NOAPPLY:= xen_action_hooks_noapply.livepatch
+LIVEPATCH_ACTION_HOOKS_NOREVERT:= xen_action_hooks_norevert.livepatch
 
 LIVEPATCHES += $(LIVEPATCH)
 LIVEPATCHES += $(LIVEPATCH_BYE)
@@ -34,6 +37,9 @@ LIVEPATCHES += $(LIVEPATCH_PREPOST_HOOKS)
 LIVEPATCHES += $(LIVEPATCH_PREPOST_HOOKS_FAIL)
 LIVEPATCHES += $(LIVEPATCH_ACTION_HOOKS)
 LIVEPATCHES += $(LIVEPATCH_ACTION_HOOKS_NOFUNC)
+LIVEPATCHES += $(LIVEPATCH_ACTION_HOOKS_MARKER)
+LIVEPATCHES += $(LIVEPATCH_ACTION_HOOKS_NOAPPLY)
+LIVEPATCHES += $(LIVEPATCH_ACTION_HOOKS_NOREVERT)
 
 LIVEPATCH_DEBUG_DIR ?= $(DEBUG_DIR)/xen-livepatch
 
@@ -158,7 +164,26 @@ $(LIVEPATCH_ACTION_HOOKS): xen_action_hooks.o xen_hello_world_func.o note.o xen_
 $(LIVEPATCH_ACTION_HOOKS_NOFUNC): xen_action_hooks_nofunc.o note.o xen_note.o
 	$(LD) $(LDFLAGS) $(build_id_linker) -r -o $(LIVEPATCH_ACTION_HOOKS_NOFUNC) $^
 
+xen_actions_hooks_marker.o: config.h
+
+.PHONY: $(LIVEPATCH_ACTION_HOOKS_MARKER)
+$(LIVEPATCH_ACTION_HOOKS_MARKER): xen_action_hooks_marker.o xen_hello_world_func.o note.o xen_note.o
+	$(LD) $(LDFLAGS) $(build_id_linker) -r -o $(LIVEPATCH_ACTION_HOOKS_MARKER) $^
+
+xen_actions_hooks_noapply.o: config.h
+
+.PHONY: $(LIVEPATCH_ACTION_HOOKS_NOAPPLY)
+$(LIVEPATCH_ACTION_HOOKS_NOAPPLY): xen_action_hooks_marker.o xen_hello_world_func.o note.o xen_note.o
+	$(LD) $(LDFLAGS) $(build_id_linker) -r -o $(LIVEPATCH_ACTION_HOOKS_NOAPPLY) $^
+
+xen_actions_hooks_norevert.o: config.h
+
+.PHONY: $(LIVEPATCH_ACTION_HOOKS_NOREVERT)
+$(LIVEPATCH_ACTION_HOOKS_NOREVERT): xen_action_hooks_marker.o xen_hello_world_func.o note.o xen_note.o
+	$(LD) $(LDFLAGS) $(build_id_linker) -r -o $(LIVEPATCH_ACTION_HOOKS_NOREVERT) $^
+
 .PHONY: livepatch
 livepatch: $(LIVEPATCH) $(LIVEPATCH_BYE) $(LIVEPATCH_REPLACE) $(LIVEPATCH_NOP) $(LIVEPATCH_NO_XEN_BUILDID) \
            $(LIVEPATCH_PREPOST_HOOKS) $(LIVEPATCH_PREPOST_HOOKS_FAIL) $(LIVEPATCH_ACTION_HOOKS) \
-           $(LIVEPATCH_ACTION_HOOKS_NOFUNC)
+           $(LIVEPATCH_ACTION_HOOKS_NOFUNC) $(LIVEPATCH_ACTION_HOOKS_MARKER) $(LIVEPATCH_ACTION_HOOKS_NOAPPLY) \
+           $(LIVEPATCH_ACTION_HOOKS_NOREVERT)

xen/test/livepatch/xen_action_hooks.c

@@ -28,6 +28,7 @@ static int apply_hook(livepatch_payload_t *payload)
     {
         struct livepatch_func *func = &payload->funcs[i];
 
+        func->applied = LIVEPATCH_FUNC_APPLIED;
         apply_cnt++;
 
         printk(KERN_DEBUG "%s: applying: %s\n", __func__, func->name);
@@ -48,6 +49,7 @@ static int revert_hook(livepatch_payload_t *payload)
     {
         struct livepatch_func *func = &payload->funcs[i];
 
+        func->applied = LIVEPATCH_FUNC_NOT_APPLIED;
         revert_cnt++;
 
         printk(KERN_DEBUG "%s: reverting: %s\n", __func__, func->name);

xen/test/livepatch/xen_action_hooks_marker.c

@@ -0,0 +1,112 @@
+/*
+ * Copyright (c) 2019 Amazon.com, Inc. or its affiliates. All rights reserved.
+ *
+ */
+
+#include "config.h"
+#include <xen/lib.h>
+#include <xen/types.h>
+#include <xen/version.h>
+#include <xen/livepatch.h>
+#include <xen/livepatch_payload.h>
+
+#include <public/sysctl.h>
+
+static const char hello_world_patch_this_fnc[] = "xen_extra_version";
+extern const char *xen_hello_world(void);
+
+static int pre_apply_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        BUG_ON(func->applied == LIVEPATCH_FUNC_APPLIED);
+        printk(KERN_DEBUG "%s: pre applied: %s\n", __func__, func->name);
+    }
+
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+
+    return 0;
+}
+
+static void post_apply_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        BUG_ON(func->applied != LIVEPATCH_FUNC_APPLIED);
+        printk(KERN_DEBUG "%s: post applied: %s\n", __func__, func->name);
+    }
+
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+}
+
+static int pre_revert_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        BUG_ON(func->applied != LIVEPATCH_FUNC_APPLIED);
+        printk(KERN_DEBUG "%s: pre reverted: %s\n", __func__, func->name);
+    }
+
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+
+    return 0;
+}
+
+static void post_revert_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        BUG_ON(func->applied == LIVEPATCH_FUNC_APPLIED);
+        printk(KERN_DEBUG "%s: post reverted: %s\n", __func__, func->name);
+    }
+
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+}
+
+LIVEPATCH_PREAPPLY_HOOK(pre_apply_hook);
+LIVEPATCH_POSTAPPLY_HOOK(post_apply_hook);
+LIVEPATCH_PREREVERT_HOOK(pre_revert_hook);
+LIVEPATCH_POSTREVERT_HOOK(post_revert_hook);
+
+struct livepatch_func __section(".livepatch.funcs") livepatch_xen_hello_world = {
+    .version = LIVEPATCH_PAYLOAD_VERSION,
+    .name = hello_world_patch_this_fnc,
+    .new_addr = xen_hello_world,
+    .old_addr = xen_extra_version,
+    .new_size = NEW_CODE_SZ,
+    .old_size = OLD_CODE_SZ,
+};
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * tab-width: 4
+ * indent-tabs-mode: nil
+ * End:
+ */

xen/test/livepatch/xen_action_hooks_noapply.c

@@ -0,0 +1,136 @@
+/*
+ * Copyright (c) 2019 Amazon.com, Inc. or its affiliates. All rights reserved.
+ *
+ */
+
+#include "config.h"
+#include <xen/lib.h>
+#include <xen/types.h>
+#include <xen/version.h>
+#include <xen/livepatch.h>
+#include <xen/livepatch_payload.h>
+
+#include <public/sysctl.h>
+
+static const char hello_world_patch_this_fnc[] = "xen_extra_version";
+extern const char *xen_hello_world(void);
+
+static unsigned int apply_cnt;
+
+static int pre_apply_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        BUG_ON(func->applied == LIVEPATCH_FUNC_APPLIED);
+        printk(KERN_DEBUG "%s: pre applied: %s\n", __func__, func->name);
+    }
+
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+
+    return 0;
+}
+
+static int apply_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        apply_cnt++;
+        printk(KERN_DEBUG "%s: applying: %s\n", __func__, func->name);
+    }
+
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+
+    return -EINVAL; /* Mark action as inconsistent */
+}
+
+static void post_apply_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        BUG_ON(apply_cnt != 1);
+        BUG_ON(func->applied == LIVEPATCH_FUNC_APPLIED);
+        printk(KERN_DEBUG "%s: post applied: %s\n", __func__, func->name);
+    }
+
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+}
+
+static int pre_revert_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        BUG_ON(func->applied == LIVEPATCH_FUNC_APPLIED);
+        printk(KERN_DEBUG "%s: pre reverted: %s\n", __func__, func->name);
+    }
+
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+
+    return 0;
+}
+
+static void post_revert_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        BUG_ON(func->applied == LIVEPATCH_FUNC_APPLIED);
+        printk(KERN_DEBUG "%s: post reverted: %s\n", __func__, func->name);
+    }
+
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+}
+
+LIVEPATCH_APPLY_HOOK(apply_hook);
+
+LIVEPATCH_PREAPPLY_HOOK(pre_apply_hook);
+LIVEPATCH_POSTAPPLY_HOOK(post_apply_hook);
+LIVEPATCH_PREREVERT_HOOK(pre_revert_hook);
+LIVEPATCH_POSTREVERT_HOOK(post_revert_hook);
+
+struct livepatch_func __section(".livepatch.funcs") livepatch_xen_hello_world = {
+    .version = LIVEPATCH_PAYLOAD_VERSION,
+    .name = hello_world_patch_this_fnc,
+    .new_addr = xen_hello_world,
+    .old_addr = xen_extra_version,
+    .new_size = NEW_CODE_SZ,
+    .old_size = OLD_CODE_SZ,
+};
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * tab-width: 4
+ * indent-tabs-mode: nil
+ * End:
+ */

xen/test/livepatch/xen_action_hooks_norevert.c

@@ -0,0 +1,143 @@
+/*
+ * Copyright (c) 2019 Amazon.com, Inc. or its affiliates. All rights reserved.
+ *
+ */
+
+#include "config.h"
+#include <xen/lib.h>
+#include <xen/types.h>
+#include <xen/version.h>
+#include <xen/livepatch.h>
+#include <xen/livepatch_payload.h>
+
+#include <public/sysctl.h>
+
+static const char hello_world_patch_this_fnc[] = "xen_extra_version";
+extern const char *xen_hello_world(void);
+
+static unsigned int revert_cnt;
+
+static int pre_apply_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        BUG_ON(func->applied == LIVEPATCH_FUNC_APPLIED);
+        printk(KERN_DEBUG "%s: pre applied: %s\n", __func__, func->name);
+    }
+
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+
+    return 0;
+}
+
+static void post_apply_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        BUG_ON(func->applied != LIVEPATCH_FUNC_APPLIED);
+        printk(KERN_DEBUG "%s: post applied: %s\n", __func__, func->name);
+    }
+
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+}
+
+static int pre_revert_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        BUG_ON(func->applied != LIVEPATCH_FUNC_APPLIED);
+        printk(KERN_DEBUG "%s: pre reverted: %s\n", __func__, func->name);
+    }
+
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+
+    return 0;
+}
+
+static int revert_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        revert_cnt++;
+        printk(KERN_DEBUG "%s: reverting: %s\n", __func__, func->name);
+    }
+
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+
+    return -EINVAL; /* Mark action as inconsistent */
+}
+
+static void post_revert_hook(livepatch_payload_t *payload)
+{
+    int i;
+
+    printk(KERN_DEBUG "%s: Hook starting.\n", __func__);
+
+    for (i = 0; i < payload->nfuncs; i++)
+    {
+        struct livepatch_func *func = &payload->funcs[i];
+
+        BUG_ON(revert_cnt != 1);
+        BUG_ON(func->applied != LIVEPATCH_FUNC_APPLIED);
+
+        /* Outside of quiesce zone: MAY TRIGGER HOST CRASH/UNDEFINED BEHAVIOR */
+        arch_livepatch_quiesce();
+        common_livepatch_revert(payload);
+        arch_livepatch_revive();
+        BUG_ON(func->applied == LIVEPATCH_FUNC_APPLIED);
+
+        printk(KERN_DEBUG "%s: post reverted: %s\n", __func__, func->name);
+    }
+
+    printk(KERN_DEBUG "%s: Hook done.\n", __func__);
+}
+
+LIVEPATCH_APPLY_HOOK(revert_hook);
+
+LIVEPATCH_PREAPPLY_HOOK(pre_apply_hook);
+LIVEPATCH_POSTAPPLY_HOOK(post_apply_hook);
+LIVEPATCH_PREREVERT_HOOK(pre_revert_hook);
+LIVEPATCH_POSTREVERT_HOOK(post_revert_hook);
+
+struct livepatch_func __section(".livepatch.funcs") livepatch_xen_hello_world = {
+    .version = LIVEPATCH_PAYLOAD_VERSION,
+    .name = hello_world_patch_this_fnc,
+    .new_addr = xen_hello_world,
+    .old_addr = xen_extra_version,
+    .new_size = NEW_CODE_SZ,
+    .old_size = OLD_CODE_SZ,
+};
+
+/*
+ * Local variables:
+ * mode: C
+ * c-file-style: "BSD"
+ * c-basic-offset: 4
+ * tab-width: 4
+ * indent-tabs-mode: nil
+ * End:
+ */