Correcting a UBSan error in builder

If a table with optional fields is created before the ds-stack is allocated we get an UBSan error.
The ds stack would normally be allocated when a field is added to the table.

/xxxx/flatcc/src/runtime/builder.c:613:16: runtime error: null pointer passed as argument 1, which is declared to never be null
/usr/include/string.h:61:62: note: nonnull attribute specified here
    #0 0x600c6b6e in exit_frame /xxxx/flatcc/src/runtime/builder.c:613:9
    #1 0x600d4520 in flatcc_builder_end_table /xxxx/flatcc/src/runtime/builder.c:1380:5
    #2 0x600a0be8 in optional_scalars_NestedTable_end /xxxx/flatcc/build/Debug/test/optional_scalars_test/generated/optional_scalars_test_builder.h:35:1
    #3 0x600a08a7 in create_scalar_stuff /xxxx/flatcc/test/optional_scalars_test/optional_scalars_test.c:27:20
    #4 0x600ab4f5 in test /xxxx/flatcc/test/optional_scalars_test/optional_scalars_test.c:176:5
    #5 0x600abd1f in main /xxxx/flatcc/test/optional_scalars_test/optional_scalars_test.c:272:9
    #6 0xe9186518  (/lib/i386-linux-gnu/libc.so.6+0x21518) (BuildId: 6f8a2d2f90a25e7865aa6fdfd9a7825d62d53f51)
    #7 0xe91865f2 in __libc_start_main (/lib/i386-linux-gnu/libc.so.6+0x215f2) (BuildId: 6f8a2d2f90a25e7865aa6fdfd9a7825d62d53f51)
    #8 0x6007053a in _start (/xxxx/flatcc/build/Debug/test/optional_scalars_test/optional_scalars_test_d+0x2653a) (BuildId: d47f8a02ee936bc1bc53a0efd6ad360faa6428d7)

SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /xxxx/flatcc/src/runtime/builder.c:613:16

Signed-off-by: Björn Svensson <[email protected]>

Author: bjosv

src/runtime/builder.c

@@ -608,7 +608,10 @@ static int enter_frame(flatcc_builder_t *B, uint16_t align)
 
 static inline void exit_frame(flatcc_builder_t *B)
 {
-    memset(B->ds, 0, B->ds_offset);
+    /* Clear the ds stack (if any struct frames have been allocated). */
+    if (B->ds) {
+        memset(B->ds, 0, B->ds_offset);
+    }
     B->ds_offset = frame(ds_offset);
     B->ds_first = frame(ds_first);
     refresh_ds(B, frame(type_limit));

test/optional_scalars_test/optional_scalars_test.c

@@ -21,6 +21,11 @@ int create_scalar_stuff(flatcc_builder_t *builder)
 {
     ns(ScalarStuff_start_as_root(builder));
 
+    /* Test of creating a table before any fields are added. */
+    ns(NestedTable_ref_t) nested_table;
+    ns(NestedTable_start(builder));
+    nested_table = ns(NestedTable_end(builder));
+
     ns(ScalarStuff_just_i8_add(builder, 10));
     ns(ScalarStuff_maybe_i8_add(builder, 11));
     ns(ScalarStuff_default_i8_add(builder, 12));
@@ -49,6 +54,8 @@ int create_scalar_stuff(flatcc_builder_t *builder)
     ns(ScalarStuff_maybe_xfactor_add)(builder, ns(OptionalFactor_Twice));
     ns(ScalarStuff_default_xfactor_add)(builder, ns(OptionalFactor_Twice));
 
+    ns(ScalarStuff_nested_table_add)(builder,nested_table);
+
     ns(ScalarStuff_end_as_root(builder));
 
     return 0;
@@ -176,7 +183,7 @@ int test(void)
 }
 
 const char *expected_json =
-"{\"just_i8\":10,\"maybe_i8\":11,\"default_i8\":12,\"just_i16\":42,\"maybe_i16\":42,\"maybe_u32\":0,\"default_u32\":0,\"just_f32\":42,\"maybe_f32\":42,\"just_bool\":true,\"maybe_bool\":true,\"just_enum\":\"One\",\"maybe_enum\":\"One\",\"just_xfactor\":\"Twice\",\"maybe_xfactor\":\"Twice\"}";
+"{\"just_i8\":10,\"maybe_i8\":11,\"default_i8\":12,\"just_i16\":42,\"maybe_i16\":42,\"maybe_u32\":0,\"default_u32\":0,\"just_f32\":42,\"maybe_f32\":42,\"just_bool\":true,\"maybe_bool\":true,\"just_enum\":\"One\",\"maybe_enum\":\"One\",\"just_xfactor\":\"Twice\",\"maybe_xfactor\":\"Twice\",\"nested_table\":{}}";
 
 #if 0
 int print_buffer(const void *buf, size_t size)

test/optional_scalars_test/optional_scalars_test.fbs

@@ -68,4 +68,10 @@ table ScalarStuff {
   maybe_yfactor: OptionalFactor = null;
   default_yfactor: OptionalFactor = Twice;
 
+  nested_table: NestedTable;
+}
+
+table NestedTable {
+  u64_0: uint64 = null;
+  u64_1: uint64 = null;
 }