This check is redundant. The header slice was just created with make([]byte, 4) on line 250, so it will always have length 4 and never be less than 1. This check can never be true.

The logic here is incorrect. On BSD systems, os.Dev is an int32, and when it's negative, using max(os.Dev, int32(0)) will result in 0, not the actual device number. This differs from the Linux implementation which uses bitwise masking to preserve the full value. If os.Dev is negative (which is a valid signed value), this code will incorrectly map it to 0, potentially causing device number collisions. Consider using the same approach as Linux: convert to uint32 directly or use bitwise masking to preserve all bits.

The file permission was changed from 0o644 (read/write for owner, read-only for group and others) to 0o600 (read/write for owner only). While this is more restrictive and better from a security perspective, this changes the behavior of test files. The comment on line 237-239 states these are fixture files that need write permissions to avoid errors when deleting or modifying. Consider if 0o600 is intentional here or if the original 0o644 was sufficient for the test's purpose.

The check hashBits > 62 on line 791 may be too permissive. According to the comment and the guard in the grow() function (line 798), hashBits is capped at 30. However, if hashBits is between 31 and 62, the shift operation 1 << uint(hashBits) on line 794 could overflow for values above 30. Consider either using the same limit as grow() (hashBits > 30) or documenting why 62 is the correct upper bound.