objtool: Move the IRET hack into the arch decoder

Quoting Julien:

  "And the other suggestion is my other email was that you don't even
  need to add INSN_EXCEPTION_RETURN. You can keep IRET as
  INSN_CONTEXT_SWITCH by default and x86 decoder lookups the symbol
  conaining an iret. If it's a function symbol, it can just set the type
  to INSN_OTHER so that it caries on to the next instruction after
  having handled the stack_op."
Suggested-by: Julien Thierry <jthierry@redhat.com>
Signed-off-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/20200428191659.913283807@infradead.org

tools/objtool/arch.h

@@ -19,7 +19,6 @@ enum insn_type {
 	INSN_CALL,
 	INSN_CALL_DYNAMIC,
 	INSN_RETURN,
-	INSN_EXCEPTION_RETURN,
 	INSN_CONTEXT_SWITCH,
 	INSN_BUG,
 	INSN_NOP,

tools/objtool/arch/x86/decode.c

@@ -94,6 +94,7 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
 		      rex_x = 0, modrm = 0, modrm_mod = 0, modrm_rm = 0,
 		      modrm_reg = 0, sib = 0;
 	struct stack_op *op = NULL;
+	struct symbol *sym;
 
 	x86_64 = is_x86_64(elf);
 	if (x86_64 == -1)
@@ -469,17 +470,24 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
 		break;
 
 	case 0xcf: /* iret */
-		*type = INSN_EXCEPTION_RETURN;
-
-		ADD_OP(op) {
-			/* add $40, %rsp */
-			op->src.type = OP_SRC_ADD;
-			op->src.reg = CFI_SP;
-			op->src.offset = 5*8;
-			op->dest.type = OP_DEST_REG;
-			op->dest.reg = CFI_SP;
+		/*
+		 * Handle sync_core(), which has an IRET to self.
+		 * All other IRET are in STT_NONE entry code.
+		 */
+		sym = find_symbol_containing(sec, offset);
+		if (sym && sym->type == STT_FUNC) {
+			ADD_OP(op) {
+				/* add $40, %rsp */
+				op->src.type = OP_SRC_ADD;
+				op->src.reg = CFI_SP;
+				op->src.offset = 5*8;
+				op->dest.type = OP_DEST_REG;
+				op->dest.reg = CFI_SP;
+			}
+			break;
 		}
-		break;
+
+		/* fallthrough */
 
 	case 0xca: /* retf */
 	case 0xcb: /* retf */

tools/objtool/check.c

@@ -2320,17 +2320,6 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 
 			break;
 
-		case INSN_EXCEPTION_RETURN:
-			/*
-			 * This handles x86's sync_core() case, where we use an
-			 * IRET to self. All 'normal' IRET instructions are in
-			 * STT_NOTYPE entry symbols.
-			 */
-			if (func)
-				break;
-
-			return 0;
-
 		case INSN_CONTEXT_SWITCH:
 			if (func && (!next_insn || !next_insn->hint)) {
 				WARN_FUNC("unsupported instruction in callable function",

tools/objtool/elf.c

@@ -61,7 +61,7 @@ static void rb_add(struct rb_root *tree, struct rb_node *node,
 	rb_insert_color(node, tree);
 }
 
-static struct rb_node *rb_find_first(struct rb_root *tree, const void *key,
+static struct rb_node *rb_find_first(const struct rb_root *tree, const void *key,
 			       int (*cmp)(const void *key, const struct rb_node *))
 {
 	struct rb_node *node = tree->rb_node;
@@ -189,7 +189,7 @@ struct symbol *find_func_by_offset(struct section *sec, unsigned long offset)
 	return NULL;
 }
 
-struct symbol *find_symbol_containing(struct section *sec, unsigned long offset)
+struct symbol *find_symbol_containing(const struct section *sec, unsigned long offset)
 {
 	struct rb_node *node;
 

tools/objtool/elf.h

@@ -124,7 +124,7 @@ struct section *find_section_by_name(const struct elf *elf, const char *name);
 struct symbol *find_func_by_offset(struct section *sec, unsigned long offset);
 struct symbol *find_symbol_by_offset(struct section *sec, unsigned long offset);
 struct symbol *find_symbol_by_name(const struct elf *elf, const char *name);
-struct symbol *find_symbol_containing(struct section *sec, unsigned long offset);
+struct symbol *find_symbol_containing(const struct section *sec, unsigned long offset);
 struct rela *find_rela_by_dest(const struct elf *elf, struct section *sec, unsigned long offset);
 struct rela *find_rela_by_dest_range(const struct elf *elf, struct section *sec,
 				     unsigned long offset, unsigned int len);