sysenc: special case unmarshaling into []byte

[lmb]

map: use t.Cleanup in createArray

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

sysenc: special case unmarshaling into []byte

The library has somewhat complicated semantics when unmarshaling map keys
and values. It turns out that the following was supported
(but not documented) until the zero-allocation marshaling work broke it:

    var s []byte
   m.Lookup(key, &s)

The important thing is that the slice here is empty. The current code treats
this as "the key should be zero length" while originally we'd assign the
temporary buffer used by the syscall to s as a way to reduce allocations. In
hindsight this wasn't a great idea, but it is what it is.

Reintroduce the byte slice special case.

Fixes https://github.com/cilium/ebpf/issues/1175

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

[ti-mo]

Interesting, but couldn't we have left it the way it was and explicitly returned an error? The old behaviour is a little surprising; you give it a pointer to a slice (allocated by you!) and the lib updates the pointer to point to a new slice? That's not ideal if you're expecting to read a value from the original slice's backing memory.

[lmb]

I think we can still decide to do that, but it should be a deliberate choice.