[Swift] Verify size-prefixed roots from reader position

[carrerasdarren-cell]

Summary

• verify Swift roots from the active ByteBuffer.reader position
• verify optional file identifiers from that same position
• add regression coverage for malformed decoy roots, truncated identifiers, and a valid prefixed identifier

Problem

Both size-prefixed checked-root APIs call skipPrefix() before delegating to getCheckedRoot. The returned table is constructed from the post-prefix reader position, but verification previously followed a root offset from byte zero and checked a file identifier at byte four.

A malformed size-prefixed buffer could therefore place a valid decoy root where the verifier looked while returning a different malformed table from the post-prefix root. The same malformed table is rejected when checked directly, but the prefixed wrappers accepted it. Generated accessors or mutators could then operate outside the buffer bounds despite the checked API succeeding.

Fix

Capture the active reader position once and use it consistently for file-identifier verification, root verification, and table construction. The file-identifier bounds check now accounts for that position without wrapping on short buffers.

Validation

• added a Swift regression covering both size-prefixed checked-root APIs
• added short file-identifier bounds coverage and a valid prefixed control
• parsed all modified Swift source and test files with swiftc
• compiled the production Swift sources with a deterministic harness; direct malformed verification rejected, both patched prefixed paths rejected, a truncated identifier rejected, and a valid prefixed identifier was accepted
• git diff --check passes

[mustiikhalil]

@carrerasdarren-cell Thanks for opening the PR. I was looking at the cpp approach. And it would be better if the swift implementation also follows a similar convention. Where we only skip after we have verified the prefixed value too. It's fine if we duplicate the logic for the verification here getPrefixedSizeCheckedRoot.

flatbuffers/include/flatbuffers/verifier.h

Line 256 in 81edeb1

bool VerifySizePrefixedBuffer(const char* const identifier) {

flatbuffers/tests/monster_test.cpp

Line 609 in 81edeb1

TEST_EQ(VerifySizePrefixedMonsterBuffer(verifier), true);

[carrerasdarren-cell]

Thanks, updated in 83b8c03. Both checked size-prefixed entry points now read and validate the UOffset prefix before calling skipPrefix(). getPrefixedSizeCheckedRoot applies the C++-style prefix <= available bytes check, while getCheckedPrefixedSizeRoot preserves its exact-size requirement. I also added an oversized-prefix regression that verifies rejection occurs without advancing ByteBuffer.reader. The production Swift modules compile with Swift 6.3.3, the modified sources/tests parse, git diff --check passes, and the deterministic malformed/valid harness still passes.

[mustiikhalil]

LGTM! amazing work! one of the comments can be disregarded if not needed

[mustiikhalil]

@carrerasdarren-cell just pinging you so we can get this PR to the finish line

[carrerasdarren-cell]

Thanks for the ping. I addressed the remaining wrapping-arithmetic nits in b76d5d5, replied to the optional placement suggestion, and resolved all three review threads. The production Swift module compiles cleanly locally with Swift 6.3.3; the refreshed CI run is now starting.

[mustiikhalil]

@carrerasdarren-cell thanks for your contribution