Add support for BPF_LD_MAP_FD pseudo instruction

[Alan-Jowett]

BPF_LD_MAP_FD pseudo instruction is a LD IMM64 with register SRC == BPF_PSEUDO_MAP_FD(1)
Behavior is to resolve the map file descriptor and replace it with the address of the map.

Add new function ubpf_register_map_resolver to allow caller to provide callback to resolve map FD to address.

Resolves #45

[coveralls]

Coverage decreased (-0.7%) to 95.376% when pulling dce24c5 on Alan-Jowett:lddw_maps into 089f627 on iovisor:master.

[jpsamaroo]

This should have an interpreter implementation and some tests, including a test with one program using multiple maps. I also think that the callback should accept the ubpf_vm as first argument, so that in the future we can attach map information directly to the vm structure.

[Alan-Jowett]

Agreed about making the map resolver jitter aware. Modified both the register and the callback to accept void * context so that consumers have the choice of how to implement maps.

[Alan-Jowett]

Added tests for BPF_LD_MAP_FD pseudo instruction for both positive and negative (invalid FD) cases.

[Alan-Jowett]

Note: The goal is to allow consumer of ubpf to add support for maps by implementing the BPF syscall. This change allows the consumer to implement fd -> address mapping.

[jpsamaroo]

Any reason why this was closed?

[Alan-Jowett]

Short version

This overlaps with functionality in another library we are using and is redundant.

Background

My team is building a runtime + execution context (environment) that permits eBPF programs to be hosted in either user mode, kernel mode or an enclave. The runtime invokes the prevail eBPF verifier to parse and verify the eBPF program. As part of verification, callbacks are made to create maps. Net result is that ubpf is agnostic of maps in this model.

Post verification, the runtime then does:

1. Scan the eBPF byte code to extract list of helper functions and map FDs.
2. Resolve the helper function and map addresses (via call to execution context).
3. Replace LD_MAP_FD pseudo instructions with LDDW with the real map address.
4. Provide helper function addresses to jitter.
5. Loads the code into the execution environment.

Note:
Once I have it ready, there will be a smaller patch that allows makes ubpf more flexible about the environment it runs in. Likewise once we exit POC phase for the project we will be pushing it up to public github and will provide a link if you want.