bpf() shim don't follow upstream error conventions

[lmb]

Describe the bug

The efW libbpf shim uses the convention of returning -1 and setting errno. Upstream libbpf returns errno directly from most functions, except when returning a pointer. See https://libbpf.readthedocs.io/en/latest/api.html

This problem extends to the bpf() syscall wrapper, which doesn't follow Linux ABI for this reason. See https://man7.org/linux/man-pages/man2/bpf.2.html

OS information

No response

Steps taken to reproduce bug

Call a libbpf function and inspect the return code.

Expected behavior

errno should not be modified, and errors returned according to upstream behaviour.

Actual outcome

errno is modified and -1 returned.

Additional details

No response

[lmb]

I think I misunderstood how things work, at least partially:

ebpf-for-windows/libs/api/libbpf_internal.h

Lines 8 to 27 in 0b4142d

	static inline int
	libbpf_err(int ret)
	{
	if (ret < 0)
	errno = -ret;
	return ret;
	}
	static inline int
	libbpf_result_err(ebpf_result_t result)
	{
	return libbpf_err(-ebpf_result_to_errno(result));
	}
	static inline void*
	libbpf_err_ptr(int err)
	{
	errno = -err;
	return NULL;
	}

The libbpf wrappers use these functions to return integer errors in the correct way. What confused me is that they also set errno, and some tests inspect errno.

The bpf() wrapper does indeed contain some error returns which aren't according to spec though:

• ebpf-for-windows/libs/api/bpf_syscall.cpp

  Lines 10 to 11 in 0b4142d

  	errno = EINVAL; \
  	return -1; \

• ebpf-for-windows/libs/api/bpf_syscall.cpp

  Lines 56 to 57 in 0b4142d

  	errno = EINVAL;
  	return -1;

• etc.

It might make sense to reword this issue to only be about the bpf() wrapper.