Fiddle::Pointer#[]= copies beyond the source pointer size

[kojix2]

require "fiddle"

include Fiddle

dst = Pointer.malloc(32, RUBY_FREE)
src = Pointer.malloc(4, RUBY_FREE)

dst[0, 32] = "\x00" * 32
src[0, 4] = "\xAA\xBB\xCC\xDD"

# src is only 4 bytes, but 16 bytes are copied.
dst[0, 16] = src

puts dst.to_str(32)
        .bytes
        .first(16)
        .map { |b| "%02x" % b }
        .join(" ")

# aa bb cc dd ad 78 00 00 d8 2a a1 98 ad 78 00 00
# (In my environment)

[kojix2]

How much safety guarding (e.g. bounds checking) to include in the design is a project policy decision. Raising the safety threshold reduces the risk of users hitting unexpected undefined behavior or memory corruption. The tradeoff is more code and more complexity, which hurts maintainability. This report is part of my personal practice of bug hunting with AI, and also a contribution to Fiddle, which I use. It is not a proposal to shift the project toward the safer side.

[kou]

Let's add a boundary check for this case.