Narrow field values with truncatingIfNeeded instead of the trapping initializer - #15
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…nitializer rzfit_swift_string_for_type narrows a FIT_UINT32 into the field's own type (FIT_ENUM, FIT_UINT8, FIT_UINT16, ...) with the exact initializer, which traps when the value does not fit. A structurally valid file can carry such a value - a vendor's private enum, or one corrupt byte - and then FitFile.init takes the whole process down before the caller sees a line of it. This changes the generator template and the generated map to truncatingIfNeeded:, which is what the C SDK does when it reads the same bytes into the narrower type, and adds a test with a value that does not fit an enum and one that does not fit a uint16. Found by a mutation fuzz over a corpus of Garmin recordings in a wing-foiling app that imports a rider's whole year from intervals.icu.
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What
rzfit_swift_string_for_typenarrows aFIT_UINT32into the field's own type (FIT_ENUM,FIT_UINT8,FIT_UINT16, …) with the exact initializer:FIT_ENUM(val)traps whenvaldoes not fit a byte. A structurally valid FIT can carry such a value — a vendor's private enum, or one corrupt byte in an otherwise good file — and thenFitFile.inittakes the whole process down before the caller sees a line of it. There is nothing to catch: it is a Swift runtime trap, not an error.This PR changes the one line in the generator (
python/fitsdkparser.py,swift_stmt_case_type_function_call) and the generatedrzfit_swift_map.swiftit produces totruncatingIfNeeded:, which is what the C SDK does when it reads the same bytes into the narrower type, and adds a test with a value that does not fit an enum and one that does not fit auint16. The generated diff is 179 lines, all of the shapeFIT_X(val)→FIT_X(truncatingIfNeeded: val).How it was found
A mutation fuzz over a corpus of Garmin recordings in a wing-foiling app that imports a rider's whole year from intervals.icu: one tester's first sync crashed the app on their first activity, the crash was inside the decoder, and no amount of validation on our side (header, CRC, message framing) could see it, because the file is valid. We are shipping a vendored copy with this change until it is upstream; thank you for the library.