`offset` parameter of `lseek` is defined as unsigned

[kastixx]

The seek offset may be negative if whence is SEEK_END or SEEK_CUR

[kastixx]

Looks like FUSE forwards lseek to userspace processes only for special whence values like SEEK_DATA and SEEK_HOLE that accept non-negative offsets only, while all normal seek operations are handled internally.
Maybe this issue is not that important after all.