Flatbuffer fails to verify after adding a table offset - flatbuffers

Question

I’ve been staring at this too long and I’m sure it’s something I’m doing wrong. My flatbuffer fails to verify after trying to add a table member. It verifies fine if I only add the integer at the top of the struct.
Root Schema:
table TestRootForBasicTypeTables
{
test_int_value:int;
test_ubyte:ubyte_table;
…
‘C’ structure definition for schema above
struct TestRootForBasicTypeTables
{
int test_int_value;
////
//// Structures for unary types
////
ubyte_table test_ubyte;
byte_table test_byte;
…
Schema for ubyte_table:
table ubyte_table
{
ubyte_value:ubyte;
}
Structure definition of ubyte_table
struct ubyte_table
{
UCHAR ubyte_value;
};
Byte buffer when only adding the test_int_value:
48 0 0 0
44 0
8 0 <= size of data
4 0 <= offset from root to integer value
0 0 <= all other offsets are zero
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
44 0 0 0 <= root table
41 0 0 0 <= integer value
Byte buffer when adding ubyte_table
48 0 0 0
44 0
14 0 <= size of data
4 0 <= offset from root to integer value
8 0 <= offset from root to test_ubyte
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
44 0 0 0
41 0 0 0 <= integer value
12 0 <= length of test_ubyte data
0 0
0 0
6 0
8 0
7 0
6 0
0 0 0 0 0 55
Here is the code:
flatbuffers::Offset< FBS_NS::TestRootForSonusBasicTypeTables> writeFlatbuffers(flatbuffers::FlatBufferBuilder &fbb)
{
return FBS_NS::CreateTestRootForBasicTypeTables(fbb,
41,
SONUS_FBS_NS::Createubyte_table(fbb, 55));
}
void BasicTypeTablesUnitTest::testHelper_(void)
{
flatbuffers::FlatBufferBuilder fbb;
// Set test value and serialize data
FBS_NS::FinishTestRootForBasicTypeTablesBuffer(fbb, ::writeFlatbuffers(fbb, input));
#if (DBG_PRT==1)
// print byte data for debugging:
auto p = fbb.GetBufferPointer();
for (flatbuffers::uoffset_t i = 0; i < fbb.GetSize(); i++)
printf("%d ", p[i]);
printf("\n");
#endif /* DBG_PRT */
auto *buf = fbb.GetBufferPointer();
auto size = fbb.GetSize();
fbb.ReleaseBufferPointer();
flatbuffers::Verifier verifier(buf, size);
CPPUNIT_ASSERT(FBS_NS::VerifyTestRootForBasicTypeTablesBuffer(verifier));
// deserialize data into the output structure
const FBS_NS::TestRootForBasicTypeTables *root = FBS_NS::GetTestRootForBasicTypeTables((const void*)buf);
::readFlatbuffers(root, output);
}
Stack trace for validation failure
(gdb) where
#0 0x00007ffff6ed7125 in raise () from /sonus/p4/ws/dmccracken/dsbc_cmnthirdparty/kernel/3.2/debug/lib/x86_64-linux-gnu/libc.so.6
#1 0x00007ffff6eda3a0 in abort () from /sonus/p4/ws/dmccracken/dsbc_cmnthirdparty/kernel/3.2/debug/lib/x86_64-linux-gnu/libc.so.6
#2 0x00007ffff6ed0311 in __assert_fail () from /sonus/p4/ws/dmccracken/dsbc_cmnthirdparty/kernel/3.2/debug/lib/x86_64-linux-gnu/libc.so.6
#3 0x000000000043428e in flatbuffers::Verifier::Check (this=0x7fffffffca50, ok=false) at /sonus/p4/ws/dmccracken/dsbc_cmnthirdparty/flatbuffers/include/flatbuffers/flatbuffers.h:942
#4 0x0000000000434308 in flatbuffers::Verifier::Verify (this=0x7fffffffca50, elem=0x6967f3, elem_len=4) at /sonus/p4/ws/dmccracken/dsbc_cmnthirdparty/flatbuffers/include/flatbuffers/flatbuffers.h:949
#5 0x0000000000437f8c in flatbuffers::Verifier::Verify<int> (this=0x7fffffffca50, elem=0x6967f3) at /sonus/p4/ws/dmccracken/dsbc_cmnthirdparty/flatbuffers/include/flatbuffers/flatbuffers.h:954
#6 0x0000000000434451 in flatbuffers::Table::VerifyTableStart (this=0x6967f3, verifier=...) at /sonus/p4/ws/dmccracken/dsbc_cmnthirdparty/flatbuffers/include/flatbuffers/flatbuffers.h:1146
#7 0x0000000000434e9b in FBS_NS::TestRootForBasicTypeTables::Verify (this=0x6967f3, verifier=...) at TestRootForBasicTypeTables_generated.h:71
#8 0x0000000000439410 in flatbuffers::Verifier::VerifyBuffer< FBS_NS::TestRootForBasicTypeTables> (this=0x7fffffffca50) at /sonus/p4/ws/dmccracken/dsbc_cmnthirdparty/flatbuffers/include/flatbuffers/flatbuffers.h:1020
#9 0x0000000000435acb in FBS_NS::VerifyTestRootForBasicTypeTablesBuffer (verifier=...) at TestRootForSonusBasicTypeTables_generated.h:202
#10 0x000000000042d97b in BasicTypeTablesUnitTest::testHelper_ (this=0x66ec80) at BasicTypeTablesUnitTest.cpp:324
#11 0x000000000042db25 in BasicTypeTablesUnitTest::test_ubyte (this=0x66ec80) at BasicTypeTablesUnitTest.cpp:349

Answer 1

One thing that is problematic is that GetBufferPointer gives you a pointer to the internal buffer that fbb owns, and then ReleaseBufferPointer creates a smart pointer that owns the buffer, and removes it from fbb.
Since you don't even store the smart pointer, I'm guessing you probably don't want to be calling ReleaseBufferPointer at all.
In most cases the original pointer buf would still be valid, however, so I don't know if that is the cause of the verifier failure. It fails to read the vtable offset of the root table, which would indicate the entire buffer is bogus.