kpatch-build: Add sym->has_func_profiling support for aarch64

The "has_function_profiling" support field in the symbol struct is used to show
that a function symbol is able to be patched. This is necessary to check that
functions which need to be patched are able to be.

On arm64 this means the presence of 2 NOP instructions at function entry
which are patched by ftrace to call the ftrace handling code. These 2 NOPs
are inserted by the compiler and the location of them is recorded in a
section called "__patchable_function_entries". Check whether a symbol has a
corresponding entry in the "__patchable_function_entries" section and if so
mark it as "has_func_profiling".

Additionally updated the error message in kpatch_check_func_profiling_calls()
to accurately reflect the cause of the failure.

Signed-off-by: Suraj Jitindar Singh <surajjs@amazon.com>

Author: Suraj Jitindar Singh

kpatch-build/create-diff-object.c

@@ -1632,8 +1632,13 @@ static void kpatch_check_func_profiling_calls(struct kpatch_elf *kelf)
 		    (sym->parent && sym->parent->status == CHANGED))
 			continue;
 		if (!sym->twin->has_func_profiling) {
+#ifdef __aarch64__
+			log_normal("function %s doesn't have patchable_function_entry,"
+				   " unable to patch\n", sym->name);
+#else
 			log_normal("function %s has no fentry/mcount call, unable to patch\n",
 				   sym->name);
+#endif
 			errs++;
 		}
 	}

kpatch-build/kpatch-elf.c

@@ -322,6 +322,9 @@ static void kpatch_find_func_profiling_calls(struct kpatch_elf *kelf)
 {
 	struct symbol *sym;
 	struct rela *rela;
+#ifdef __aarch64__
+	struct section *patchable_sec = NULL;
+#endif
 	list_for_each_entry(sym, &kelf->symbols, list) {
 		if (sym->type != STT_FUNC || !sym->sec || !sym->sec->rela)
 			continue;
@@ -332,7 +335,27 @@ static void kpatch_find_func_profiling_calls(struct kpatch_elf *kelf)
 				break;
 			}
 		}
-#else
+#elif defined(__aarch64__)
+		if (!patchable_sec) {
+			struct section *sec = find_section_by_name(&kelf->sections,
+					      "__patchable_function_entries");
+			/*
+			 * If we can't find the __patchable_function_entries
+			 * section or there are no relocations in it then not
+			 * patchable.
+			 */
+			if (!sec || !sec->rela)
+				return;
+			patchable_sec = sec->rela;
+		}
+		list_for_each_entry(rela, &patchable_sec->relas,
+				    list) {
+			if (rela->sym->sec && sym->sec == rela->sym->sec) {
+				sym->has_func_profiling = 1;
+				break;
+			}
+		}
+#else /* x86_64 */
 		rela = list_first_entry(&sym->sec->rela->relas, struct rela,
 					list);
 		if ((rela->type != R_X86_64_NONE &&