libdrgn: dwarf_info: ignore DW_OP_{,GNU_}entry_value

These opcodes appear in practice, and we choke on them with an exception
like "unknown DWARF expression opcode 0xf3" or "unknown DWARF expression
opcode 0xa3". In some cases, it'd be possible to recover the entry value
by looking at call site information, but that's pretty involved. For
now, just treat these operations as optimized out so we stop failing
hard.

Closes #233.

Signed-off-by: Omar Sandoval <[email protected]>

libdrgn/dwarf_info.c

@@ -4450,6 +4450,16 @@ drgn_eval_dwarf_expression(struct drgn_dwarf_expression_context *ctx,
 		/* Special operations. */
 		case DW_OP_nop:
 			break;
+		case DW_OP_entry_value:
+		case DW_OP_GNU_entry_value:
+			// TODO: DW_OP_(GNU_)entry_value followed by
+			// DW_OP_reg<n> means the value of the register when the
+			// current subprogram was entered. We could recover this
+			// by finding the DW_TAG_(GNU_)call_site for the return
+			// address and using the DW_AT_(GNU_)call_value of a
+			// DW_TAG_(GNU_)call_parameter with a DW_AT_location
+			// matching that register.
+			return &drgn_not_found;
 		/* Location description operations. */
 		case DW_OP_reg0 ... DW_OP_reg31:
 		case DW_OP_regx:
@@ -4465,7 +4475,6 @@ drgn_eval_dwarf_expression(struct drgn_dwarf_expression_context *ctx,
 		 *
 		 * - DW_OP_push_object_address
 		 * - DW_OP_form_tls_address
-		 * - DW_OP_entry_value
 		 *   DW_OP_implicit_pointer
 		 * - Procedure calls: DW_OP_call2, DW_OP_call4, DW_OP_call_ref.
 		 * - Typed operations: DW_OP_const_type, DW_OP_regval_type,