objtool: Allow stack operations in UNWIND_HINT_UNDEFINED regions

If the code specified UNWIND_HINT_UNDEFINED, skip the "undefined stack
state" warning due to a stack operation.  Just ignore the stack op and
continue to propagate the undefined state.

Link: https://lore.kernel.org/r/820c5b433f17c84e8761fb7465a8d319d706b1cf.1685981486.git.jpoimboe@kernel.org
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

tools/objtool/check.c

@@ -33,6 +33,7 @@ static unsigned long nr_cfi, nr_cfi_reused, nr_cfi_cache;
 static struct cfi_init_state initial_func_cfi;
 static struct cfi_state init_cfi;
 static struct cfi_state func_cfi;
+static struct cfi_state force_undefined_cfi;
 
 struct instruction *find_insn(struct objtool_file *file,
 			      struct section *sec, unsigned long offset)
@@ -2240,6 +2241,11 @@ static int read_unwind_hints(struct objtool_file *file)
 
 		insn->hint = true;
 
+		if (hint->type == UNWIND_HINT_TYPE_UNDEFINED) {
+			insn->cfi = &force_undefined_cfi;
+			continue;
+		}
+
 		if (hint->type == UNWIND_HINT_TYPE_SAVE) {
 			insn->hint = false;
 			insn->save = true;
@@ -2793,6 +2799,10 @@ static int update_cfi_state(struct instruction *insn,
 	struct cfi_reg *cfa = &cfi->cfa;
 	struct cfi_reg *regs = cfi->regs;
 
+	/* ignore UNWIND_HINT_UNDEFINED regions */
+	if (cfi->force_undefined)
+		return 0;
+
 	/* stack operations don't make sense with an undefined CFA */
 	if (cfa->base == CFI_UNDEFINED) {
 		if (insn_func(insn)) {
@@ -4607,6 +4617,8 @@ int check(struct objtool_file *file)
 	init_cfi_state(&init_cfi);
 	init_cfi_state(&func_cfi);
 	set_func_state(&func_cfi);
+	init_cfi_state(&force_undefined_cfi);
+	force_undefined_cfi.force_undefined = true;
 
 	if (!cfi_hash_alloc(1UL << (file->elf->symbol_bits - 3)))
 		goto out;

tools/objtool/include/objtool/cfi.h

@@ -36,6 +36,7 @@ struct cfi_state {
 	bool drap;
 	bool signal;
 	bool end;
+	bool force_undefined;
 };
 
 #endif /* _OBJTOOL_CFI_H */