Why does this set ctxOut?

ctxOut is unmarshaled from later.

We need it to be the same as the syscall input (ctxIn == ctxOut). We can't ctxIn = ctxOut since it has been possibly marshaled to, therefore ctxOut = ctxIn.

I think that this should just fail if no input is provided, since that is what upstream does as well. We don't support in == nil && out != nil for other types, no?

We don't support in == nil && out != nil for other types, no?

It looks like it is supposed to work — see for instance bpf_ctx_init — but it doesn't, the below program fails with EINVAL (arguments to bpf syscall look reasonable).

I still feel that we shouldn't repurpose Context for output. I will DID simplify by dropping support for in == nil && out != nil.

package main

import (
	"log"

	"github.com/cilium/ebpf"
	"github.com/cilium/ebpf/asm"
)

func main() {
	prog, err := ebpf.NewProgram(&ebpf.ProgramSpec{
		Type: ebpf.XDP,
		Instructions: []asm.Instruction{
			asm.Mov.Imm(asm.R2, -8),
			asm.FnXdpAdjustMeta.Call(),
			asm.Return(),
		},
	})
	if err != nil {
		log.Fatal(err)
	}

	data := make([]byte, 128)
	dataOut := make([]byte, 1024)
	ctx := struct{ Data, DataEnd, DataMeta, I1, I2, I3 uint32 }{DataEnd: 128}

	rc, err := prog.Run(&ebpf.RunOptions{Data: data, DataOut: dataOut, ContextOut: &ctx})
	log.Print(rc, err, ctx)
}