UM API to write into a ring buffer map.

[shankarseal]

Description

Addressess #3572.

Add a new API in eBPF API.dll to write into a ring buffer map from user mode. This API issues an IOCTL which calls bpf_ring_buffer_output function to write user specified data into the ring buffer.

Testing

Updated unit and api tests to write messages into ring buffer interleaved with events that invoke eBPF programs (which in turn write into the same ring buffer).

Documentation

No documentation impact.

Installation

No installation impact.

[saxena-anurag]

nit: should we insert new values in the middle of the list? That will change the operation IDs for the rest of the enums.

[shankarseal]

Since the ebpf API will always ship in conjunction with the ebpf core.sys, why is this a problem?

[dthaler]

Per email thread with Andrii, the correct map type is BPF_MAP_TYPE_USER_RINGBUF and the UM APIs are:

struct ring_buffer_user *
ring_buffer_user__new(int map_fd,
                      const struct ring_buffer_user_opts *opts);
void ring_buffer_user__free(struct ring_buffer_user *rb);
void *ring_buffer_user__reserve(struct ring_buffer_user *rb,
				uint32_t size);
void *ring_buffer_user__poll(struct ring_buffer_user *rb, uint32_t size,
			     int timeout_ms);
void ring_buffer_user__discard(struct ring_buffer_user *rb, void *sample);
void ring_buffer_user__submit(struct ring_buffer_user *rb, void *sample);

per https://lwn.net/Articles/907056/

[Alan-Jowett]

Based on a brief examination of the BPF_MAP_TYPE_USER_RINGBUF map type, I think this will require some heavy lifting from the verifier and/or slightly different implementation in Windows.

1. Support for passing pointers to other functions to a helper function.
2. Dynptr support.