lima-vm/lima · error

read APFS superblock at offset %d: %w

Error message

read APFS superblock at offset %d: %w

What it means

Thrown by openContainer after findAPFSPartitionGPT succeeds: the ReadAt of the NX superblock at the discovered APFS partition byte offset failed. The underlying error (short read near end of file, I/O error) is wrapped with the offset. This indicates the located APFS partition region is unreadable or the image is truncated partway into the partition.

Source

Thrown at pkg/apfs/chown.go:91

	if _, err := f.ReadAt(hdr, 0); err != nil {
		f.Close()
		return nil, fmt.Errorf("read block 0: %w", err)
	}

	if le.Uint32(hdr[nxMagicOff:]) == nxMagic {
		// Raw APFS container (no partition table).
		c.blockSize = le.Uint32(hdr[nxBlockSizeOff:])
	} else {
		// Look for a GPT partition table and find the APFS partition.
		offset, err := findAPFSPartitionGPT(f)
		if err != nil {
			f.Close()
			return nil, fmt.Errorf("finding APFS partition: %w", err)
		}
		c.baseOffset = offset
		if _, err := f.ReadAt(hdr, offset); err != nil {
			f.Close()
			return nil, fmt.Errorf("read APFS superblock at offset %d: %w", offset, err)
		}
		if le.Uint32(hdr[nxMagicOff:]) != nxMagic {
			f.Close()
			return nil, fmt.Errorf("APFS partition at offset %d has bad magic", offset)
		}
		c.blockSize = le.Uint32(hdr[nxBlockSizeOff:])
	}

	if c.blockSize < 4096 {
		f.Close()
		return nil, fmt.Errorf("invalid block size %d", c.blockSize)
	}
	return c, nil
}

// GPT constants.
const (
	gptHeaderSignature = "EFI PART"

View on GitHub (pinned to dd909d0973)

Solutions

1. Verify the image file size covers the partition's first LBA (firstLBA*512 + 4096 bytes); re-copy the full image if truncated.
2. Regenerate or re-export the disk image from a healthy source.
3. Check the underlying storage/disk for I/O errors (smartctl, dmesg).
4. If the image was shrunk, restore its original size so partition offsets are in range.

Example fix

// before
truncate -s 1G macos.img   // smaller than APFS partition start -> read fails
// after
cp --sparse=always full-macos.img macos.img   # copy the complete image

fi, err := os.Stat(diskPath)
// ensure the file is large enough for a plausible partition start + superblock
if err == nil && fi.Size() < 1024*1024 {
    return fmt.Errorf("image %q suspiciously small; likely truncated", diskPath)
}

if err := apfs.Chown(diskPath, role, uid, gid, paths...); err != nil {
    if strings.Contains(err.Error(), "read APFS superblock at offset") {
        // partition region unreadable/truncated: restore full image
    }
    return err
}

When it happens

Trigger: apfs.Chown on a GPT image where the APFS partition entry's first LBA points beyond the end of the file (truncated image) or to an unreadable region of a device; storage I/O errors at that offset.

Common situations: A partially copied/cloned disk image where the partition table survived but the partition data did not; a sparse image with an unmaterialized region; resizing an image smaller than the partition layout expects; faulty disk or flaky USB-attached target.

Related errors

• open disk: %w
• read block 0: %w
• finding APFS partition: %w
• resolving filesystem root tree OID %d: %w
• resolving path %#q: %w

AI-assisted analysis of lima-vm/lima@dd909d0973 (2026-09-01). Data as JSON: /api/errors/f5506de34661deba. Report an issue: GitHub.