objtool: Rework ASM_REACHABLE

Currently ASM_REACHABLE only works for UD2 instructions; reorder
things to also allow over-riding dead_end_function().

To that end:

 - Mark INSN_BUG instructions in decode_instructions(), this saves
   having to iterate all instructions yet again.

 - Have add_call_destinations() set insn->dead_end for
   dead_end_function() calls.

 - Move add_dead_ends() *after* add_call_destinations() such that
   ASM_REACHABLE can clear the ->dead_end mark.

 - have validate_branch() only check ->dead_end.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/20220308154319.410010807@infradead.org

tools/objtool/check.c

@@ -395,6 +395,14 @@ static int decode_instructions(struct objtool_file *file)
 			if (ret)
 				goto err;
 
+			/*
+			 * By default, "ud2" is a dead end unless otherwise
+			 * annotated, because GCC 7 inserts it for certain
+			 * divide-by-zero cases.
+			 */
+			if (insn->type == INSN_BUG)
+				insn->dead_end = true;
+
 			hash_add(file->insn_hash, &insn->hash, sec_offset_hash(sec, insn->offset));
 			list_add_tail(&insn->list, &file->insn_list);
 			nr_insns++;
@@ -523,14 +531,6 @@ static int add_dead_ends(struct objtool_file *file)
 	struct reloc *reloc;
 	struct instruction *insn;
 
-	/*
-	 * By default, "ud2" is a dead end unless otherwise annotated, because
-	 * GCC 7 inserts it for certain divide-by-zero cases.
-	 */
-	for_each_insn(file, insn)
-		if (insn->type == INSN_BUG)
-			insn->dead_end = true;
-
 	/*
 	 * Check for manually annotated dead ends.
 	 */
@@ -1114,6 +1114,9 @@ static void annotate_call_site(struct objtool_file *file,
 		list_add_tail(&insn->call_node, &file->mcount_loc_list);
 		return;
 	}
+
+	if (!sibling && dead_end_function(file, sym))
+		insn->dead_end = true;
 }
 
 static void add_call_dest(struct objtool_file *file, struct instruction *insn,
@@ -2089,10 +2092,6 @@ static int decode_sections(struct objtool_file *file)
 	if (ret)
 		return ret;
 
-	ret = add_dead_ends(file);
-	if (ret)
-		return ret;
-
 	add_ignores(file);
 	add_uaccess_safe(file);
 
@@ -2131,6 +2130,14 @@ static int decode_sections(struct objtool_file *file)
 	if (ret)
 		return ret;
 
+	/*
+	 * Must be after add_call_destinations() such that it can override
+	 * dead_end_function() marks.
+	 */
+	ret = add_dead_ends(file);
+	if (ret)
+		return ret;
+
 	ret = add_jump_table_alts(file);
 	if (ret)
 		return ret;
@@ -3138,7 +3145,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 				return 1;
 			}
 
-			if (dead_end_function(file, insn->call_dest))
+			if (insn->dead_end)
 				return 0;
 
 			break;