[PATCH 8/9] Avoid undefined behavior in expression dumping

[Tom Tromey <tom@tromey.com>]

-fsanitize=undefined pointed out undefined behavior in
dump_raw_expression like:

    runtime error: load of value 2887952, which is not a valid value for type 'exp_opcode'

dump_raw_expression will try to print the opcode for each element of
the expression, even when it is not valid.  To allow this, but have it
avoid undefined behavior, this patch sets the underlying type of enum
exp_opcode, and arranges for op_name to handle invalid opcodes more
nicely.

ChangeLog
2018-08-27  Tom Tromey  <tom@tromey.com>

	* expression.h (enum exp_opcode): Use uint8_t as base type.
	* expprint.c (op_name): Handle invalid opcodes.
---
 gdb/ChangeLog    | 5 +++++
 gdb/expprint.c   | 6 ++++++
 gdb/expression.h | 2 +-
 3 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/gdb/expprint.c b/gdb/expprint.c
index d6ed41253ed..7c91cc73749 100644
--- a/gdb/expprint.c
+++ b/gdb/expprint.c
@@ -687,6 +687,12 @@ static int dump_subexp_body (struct expression *exp, struct ui_file *, int);
 const char *
 op_name (struct expression *exp, enum exp_opcode opcode)
 {
+  if (opcode >= OP_UNUSED_LAST)
+    {
+      char *cell = get_print_cell ();
+      xsnprintf (cell, PRINT_CELL_SIZE, "unknown opcode: %d", int (opcode));
+      return cell;
+    }
   return exp->language_defn->la_exp_desc->op_name (opcode);
 }
 
diff --git a/gdb/expression.h b/gdb/expression.h
index 9f26bb8d60b..db572efe2a3 100644
--- a/gdb/expression.h
+++ b/gdb/expression.h
@@ -39,7 +39,7 @@
    and skip that many.  Strings, like numbers, are indicated
    by the preceding opcode.  */
 
-enum exp_opcode
+enum exp_opcode : uint8_t
   {
 #define OP(name) name ,
 
-- 
2.13.6