Fix Stack Overflow DoS via Unbounded Recursion in idl_gen_text.cpp

[YLChen-007]

Fix Stack Overflow DoS via Unbounded Recursion in idl_gen_text.cpp

Description

A Denial of Service (DoS) vulnerability exists in the flatc compiler when converting binary FlatBuffers to text/JSON format. A missing recursion depth guard in idl_gen_text.cpp allows a maliciously crafted, deeply nested binary FlatBuffer (e.g., via a self-referencing table) to cause unbounded recursion between the GenStruct and PrintOffset functions. Processing such a file exhausts the process stack, resulting in an immediate segmentation fault (SIGSEGV).

This PR adds a depth_limit check to JsonPrinter (matching FLATBUFFERS_MAX_PARSING_DEPTH), guarded via a DepthGuard scope manager, to safely catch arbitrarily nested inputs and return a text generation error instead of exhausting the C++ call stack.

Testing

Compiled flatc and successfully tested against an evil.bin (depth > 20,000) generated targeting an explicitly recursive schema (table Node { child: Node; }).

Without the patch:
./flatc --raw-binary -t nest.fbs -- evil.bin -> Segmentation fault

With the patch:
./flatc --raw-binary -t nest.fbs -- evil.bin -> error: Unable to generate text for evil (error: text generation depth limit exceeded)

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.