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Absolute native Blob payload pointers can select memory outside the current isolate group if native state is corrupted. Store sandboxed byte views as a 32-bit offset from the current JS cage plus 4 GiB, with a 32-bit length, and decode using the supplied isolate's base. Use the public sandbox address-space API with guard regions disabled. Require exactly two 4 GiB cages at compile time. Unrepresentable sandbox views log a fatal error and abort. Decoding needs no range check; bounded overruns rely on protected neighbors. Non-sandbox builds retain ordinary pointers and recoverable size errors. Thread the isolate lock through Blob readers, File views, FormData, and buffered streams. Cover nested slices, empty payloads, File data, and R2 Blob bodies, including their instrumentation expectations. No JavaScript API or valid Blob behavior changes, so no compatibility flag is required. Ten focused downstream test targets passed five runs each. Clang-tidy and formatting checks passed, and the non-sandbox helper compiles.
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Blob payload views use a 32-bit offset relative to the current isolate's JS cage plus 4 GiB, so a corrupted stored offset cannot select another isolate group's memory.
SandboxedBytesalso stores a 32-bit length and aborts if a sandboxed view cannot be represented at construction; reads reconstruct the pointer through the public sandbox-address-space API without a range check. Non-sandbox builds retain ordinary pointers.Sandbox-enabled backing-store allocations must fit in that second 4 GiB region, with sandbox guard regions disabled. The downstream 8 GiB sandbox layout already provides it, so this needs no V8 changes. The length bounds overrun distance. The helper does not validate allocation ownership or object type.
Validation: ten focused downstream test targets passed with
--runs_per_test=5, including corruption and cross-isolate cage tests, Blob C++/JS tests, FormData, streams, and R2 variants. Clang-tidy and formatting checks passed. The helper also passed a syntax check with sandbox support disabled.