Fix seek undefined behavior on signed integer overflow#1027
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Original tests provided by m-kostrzewa, these identify signed overflow (undefined behavior) when compiled with -fsanitize=undefined.
In the previous implementation of lfs_file_seek, we calculated the new offset using signed arithmetic before checking for possible overflow/underflow conditions. This results in undefined behavior in C. Fortunately for us, littlefs is now limited to 31-bit file sizes for API reasons, so we don't have to be too clever here. Doing the arithmetic with unsigned integers and just checking if we're in a valid range afterwards should work. Found by m-kostrzewa and lucic71
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Tests passed ✓, Code: 17060 B, Stack: 1440 B, Structs: 812 B
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In the previous implementation of lfs_file_seek, we calculated the new offset using signed arithmetic before checking for possible overflow/underflow conditions. This results in undefined behavior in C.
Fortunately for us, littlefs is now limited to 31-bit file sizes for API reasons, so we don't have to be too clever here. Doing the arithmetic with unsigned integers and just checking if we're in a valid range afterwards should work.
Found by @m-kostrzewa and @lucic71
Original tests showing the issue provided by @m-kostrzewa here: #1002
Alternative fix proposed by @lucic71 here: #1017