x86/livepatch: Validate __fentry__ location

Currently livepatch assumes __fentry__ lives at func+0, which is most
likely untrue with IBT on. Instead make it use ftrace_location() by
default which both validates and finds the actual ip if there is any
in the same symbol.

Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220308154318.285971256@infradead.org

arch/powerpc/include/asm/livepatch.h

@@ -19,16 +19,6 @@ static inline void klp_arch_set_pc(struct ftrace_regs *fregs, unsigned long ip)
 	regs_set_return_ip(regs, ip);
 }
 
-#define klp_get_ftrace_location klp_get_ftrace_location
-static inline unsigned long klp_get_ftrace_location(unsigned long faddr)
-{
-	/*
-	 * Live patch works only with -mprofile-kernel on PPC. In this case,
-	 * the ftrace location is always within the first 16 bytes.
-	 */
-	return ftrace_location_range(faddr, faddr + 16);
-}
-
 static inline void klp_init_thread_info(struct task_struct *p)
 {
 	/* + 1 to account for STACK_END_MAGIC */

kernel/livepatch/patch.c

@@ -124,19 +124,6 @@ static void notrace klp_ftrace_handler(unsigned long ip,
 	ftrace_test_recursion_unlock(bit);
 }
 
-/*
- * Convert a function address into the appropriate ftrace location.
- *
- * Usually this is just the address of the function, but on some architectures
- * it's more complicated so allow them to provide a custom behaviour.
- */
-#ifndef klp_get_ftrace_location
-static unsigned long klp_get_ftrace_location(unsigned long faddr)
-{
-	return faddr;
-}
-#endif
-
 static void klp_unpatch_func(struct klp_func *func)
 {
 	struct klp_ops *ops;
@@ -153,8 +140,7 @@ static void klp_unpatch_func(struct klp_func *func)
 	if (list_is_singular(&ops->func_stack)) {
 		unsigned long ftrace_loc;
 
-		ftrace_loc =
-			klp_get_ftrace_location((unsigned long)func->old_func);
+		ftrace_loc = ftrace_location((unsigned long)func->old_func);
 		if (WARN_ON(!ftrace_loc))
 			return;
 
@@ -186,8 +172,7 @@ static int klp_patch_func(struct klp_func *func)
 	if (!ops) {
 		unsigned long ftrace_loc;
 
-		ftrace_loc =
-			klp_get_ftrace_location((unsigned long)func->old_func);
+		ftrace_loc = ftrace_location((unsigned long)func->old_func);
 		if (!ftrace_loc) {
 			pr_err("failed to find location for function '%s'\n",
 				func->old_name);