patch-hook: fix possible cast errors

Livepatch callbacks are packed into specific sections
by gcc by the macros defined in kpatch-macros.h. Callbacks
take the form of:

    struct example {
        type *(fn)(struct klp_object *obj);
        char* objname;
    }

However, create-diff-objects.c can't include the kernel livepatch
header because it is compiled separately from the kernel module.
So the solution before was to define the structs in the header file as:

    struct example {
                type *(fn)(void *obj);
                char* objname;
        }

and then cast fn to (type *(fn)(struct klp_object *obj)) if necessary.
This way, the same header file and definitions can be used in
both places.

But! If compiled with the right GCC flags, such as -Wbad-function-cast,
this casting may throw an error during livepatch building, which is what
I am seeing (not sure why, since it was always fine before...).

Solution: Define the callback function parameter as the correct type, which
allows us to get rid of the troublesome casting.

Since we need to know if callbacks are present in kpatch-patch.h, we can move
the definition of HAVE_CALLBACKS to from livepatch-patch-hook.c to kpatch-patch.h.

IMPORTANT: For correctness, this change requires proper ordering of include
statements! kpatch-patch.h must always be included AFTER linux/livepatch.h
(if using linux/livepatch.h).

Signed-off-by: Brennan Lamoreaux <[email protected]>

kmod/patch/kpatch-patch-hook.c

@@ -24,6 +24,7 @@
 #include <linux/slab.h>
 #include <linux/kallsyms.h>
 #include "kpatch.h"
+#include <linux/livepatch.h>
 #include "kpatch-patch.h"
 
 static bool replace;

kmod/patch/kpatch-patch.h

@@ -22,6 +22,34 @@
 #ifndef _KPATCH_PATCH_H_
 #define _KPATCH_PATCH_H_
 
+#include <linux/version.h>
+
+/* Support for livepatch callbacks? */
+#ifdef RHEL_RELEASE_CODE
+# if RHEL_RELEASE_CODE >= RHEL_RELEASE_VERSION(7, 5)
+#  define HAVE_CALLBACKS
+# endif
+#elif LINUX_VERSION_CODE >= KERNEL_VERSION(4, 15, 0)
+# define HAVE_CALLBACKS
+#endif
+
+#ifdef HAVE_CALLBACKS
+/* IMPORTANT: include ordering is critical! */
+# ifdef _LINUX_LIVEPATCH_H_
+typedef struct klp_object klp_obj;
+# else
+/*
+ * Basically just a placeholder for when we can't include linux/livepatch.h.
+ * The correct type, which is what gets packed in the section, is
+ * struct klp_object.
+ */
+typedef void klp_obj;
+# endif /* _LINUX_LIVEPATCH_H_ */
+#else
+#include "kpatch.h"
+typedef struct kpatch_object klp_obj;
+#endif /* HAVE_CALLBACKS */
+
 struct kpatch_patch_func {
 	unsigned long new_addr;
 	unsigned long new_size;
@@ -43,20 +71,21 @@ struct kpatch_patch_dynrela {
 	long addend;
 };
 
+
 struct kpatch_pre_patch_callback {
-	int (*callback)(void *obj);
+	int (*callback)(klp_obj *obj);
 	char *objname;
 };
 struct kpatch_post_patch_callback {
-	void (*callback)(void *obj);
+	void (*callback)(klp_obj *obj);
 	char *objname;
 };
 struct kpatch_pre_unpatch_callback {
-	void (*callback)(void *obj);
+	void (*callback)(klp_obj *obj);
 	char *objname;
 };
 struct kpatch_post_unpatch_callback {
-	void (*callback)(void *obj);
+	void (*callback)(klp_obj *obj);
 	char *objname;
 };
 

kmod/patch/livepatch-patch-hook.c

@@ -1,6 +1,6 @@
 /*
  * Copyright (C) 2013-2014 Josh Poimboeuf <[email protected]>
- * Copyright (C) 2014 Seth Jennings <[email protected]> 
+ * Copyright (C) 2014 Seth Jennings <[email protected]>
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License
@@ -56,14 +56,6 @@
 # define HAVE_IMMEDIATE
 #endif
 
-#ifdef RHEL_RELEASE_CODE
-# if RHEL_RELEASE_CODE >= RHEL_RELEASE_VERSION(7, 5)
-#  define HAVE_CALLBACKS
-# endif
-#elif LINUX_VERSION_CODE >= KERNEL_VERSION(4, 15, 0)
-# define HAVE_CALLBACKS
-#endif
-
 #ifdef RHEL_RELEASE_CODE
 # if (RHEL_RELEASE_CODE >= RHEL_RELEASE_VERSION(7, 8) && 		\
 	 RHEL_RELEASE_CODE < RHEL_RELEASE_VERSION(8, 0)) || 		\
@@ -356,8 +348,7 @@ static int add_callbacks_to_patch_objects(void)
 				object->name ? object->name : "vmlinux");
 			return -EINVAL;
 		}
-		object->callbacks.pre_patch = (int (*)(struct klp_object *))
-					       p_pre_patch_callback->callback;
+		object->callbacks.pre_patch = p_pre_patch_callback->callback;
 	}
 
 	for (p_post_patch_callback = __kpatch_callbacks_post_patch;
@@ -371,8 +362,7 @@ static int add_callbacks_to_patch_objects(void)
 				object->name ? object->name : "vmlinux");
 			return -EINVAL;
 		}
-		object->callbacks.post_patch = (void (*)(struct klp_object *))
-						p_post_patch_callback->callback;
+		object->callbacks.post_patch = p_post_patch_callback->callback;
 	}
 
 	for (p_pre_unpatch_callback = __kpatch_callbacks_pre_unpatch;
@@ -386,8 +376,7 @@ static int add_callbacks_to_patch_objects(void)
 				object->name ? object->name : "vmlinux");
 			return -EINVAL;
 		}
-		object->callbacks.pre_unpatch = (void (*)(struct klp_object *))
-						p_pre_unpatch_callback->callback;
+		object->callbacks.pre_unpatch = p_pre_unpatch_callback->callback;
 	}
 
 	for (p_post_unpatch_callback = __kpatch_callbacks_post_unpatch;
@@ -401,8 +390,7 @@ static int add_callbacks_to_patch_objects(void)
 				object->name ? object->name : "vmlinux");
 			return -EINVAL;
 		}
-		object->callbacks.post_unpatch = (void (*)(struct klp_object *))
-						p_post_unpatch_callback->callback;
+		object->callbacks.post_unpatch = p_post_unpatch_callback->callback;
 	}
 
 	return 0;