slayers: possible panic in the SerializeTo with receiver of type Raw

[jcp19]

Hi :)
In the following excerpt, it seems to me that there is the implicit assumption that len(s.Raw) is at least MetaLen. Is this always true? if not, then it is possible that a panic occurs in the slicing operation (s.Raw[:MetaLen]).

scion/go/lib/slayers/path/scion/raw.go

Lines 52 to 56 in e4b728b

	// XXX(roosd): This modifies the underlying buffer. Consider writing to data
	// directly.
	if err := s.PathMeta.SerializeTo(s.Raw[:MetaLen]); err != nil {
	return err
	}

If this is indeed a problem, then the simplest solution seems to be to remove the slicing operation because s.PathMeta.SerializeTo already performs a bounds check before writing to the buffer passed as argument.

[oncilla]

Hi @jcp19

There is an implicit assumption here. The only place that raw is set, here:

scion/go/lib/slayers/path/scion/raw.go

Line 39 in e4b728b

s.Raw = data[:pathLen]

This means that s.Raw is non-nil, it is guaranteed to be of at least MetaLen length.
However, to protect against future changes, I think we should just drop the slicing as you suggested.

Care to create a PR?

[jcp19]

Yes, I will take care of this issue and the other occurrences of s.Raw[:MetaLen].

[jiceatscion]

Since we haven't heard from jcp19, attempting to refactor as suggested.