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

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 0c857e7..894c9a7 100644 (file)
--- a/tools/objtool/check.c
+++ b/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++;
@@ -524,14 +532,6 @@ static int add_dead_ends(struct objtool_file *file)
        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.
         */
        sec = find_section_by_name(file->elf, ".rela.discard.unreachable");
@@ -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;