Lesson 10 · Guarding malformed sizes - Build a Video Container Parser

Guarding malformed sizes

A real parser has to survive a corrupt or truncated file without crashing or spinning forever. Today you guard the box walk against sizes that are too small or too large, returning a clear error instead of looping.

The goal

Make the box walk return an error on a size smaller than the header or larger than the remaining bytes.

Start here - the target

TO DO

Scenario: Bad box sizes produce an error, never a hang

Givenan 8-byte buffer whose box declares size field 3 (smaller than the 8-byte header)

Whenthe buffer is walked

Thenthe walk returns an error and does not loop forever

Anda box declaring size 1024 inside an 8-byte buffer also returns an error (size exceeds the remaining bytes)

Background

The box walk advances by each box’s declared size, so a malformed size is dangerous. A size smaller than the header (say 3, when the header alone is 8) would move the cursor backward or not at all - an infinite loop. A size larger than the bytes that remain would read past the buffer into nothing. Both are signs of a truncated or corrupt file, and both must become a clean error rather than a crash or a hang.

The two guards are: reject size < headerSize (the box cannot even contain its own header), and reject offset + size > len(buffer) (the box claims more bytes than exist). With those in place the parser makes forward progress on every well-formed box and refuses every ill-formed one. This is the robustness the finalized inspector relies on to reject a non-MP4 file gracefully.

Make it work

func parseBoxes(b []byte) ([]Box, error) {
  pos := 0
  for pos < len(b) {
    box := parseHeaderAt(b, pos, len(b))
    // reject a size that cannot advance past the header,
    // or that runs past the end of the buffer
    if int(box.Size) < box.HeaderSize || pos+int(box.Size) > len(b) {
      // return an error here
    }
    pos += int(box.Size)
  }
  return boxes, nil
}

CheckpointDONE

Your walk rejects malformed sizes. Commit and stop here.