Function read_block 

pub fn read_block<'a>(
    partition: &'a [u8],
    sb: &Superblock,
    addr: u64,
    len: usize,
) -> Result<&'a [u8], UfsError>

Read len bytes of a file’s block addr from partition (the filesystem partition bytes, filesystem byte 0). addr is a fragment address as stored in an inode’s di_db[]/di_ib[]; the byte offset is addr * fs_fsize.

A directory’s data lives in whole blocks; the tail block may be a partial fragment run, so len bounds how much is read (typically fs_bsize, or the remaining di_size for the last block). Reading past the partition end is not an error here — the returned slice is clamped to what is present, so a truncated image yields a short (possibly empty) block rather than a failure, and the caller’s d_reclen/di_size bounds still hold.

Errors

UfsError::ImpossibleGeometry if fs_fsize <= 0 (the multiplier for the byte offset), so the address cannot be computed — never a panic.