GSR

Handling errors with CGO

March 16, 2022

One of the main projects that I work on is libbpfgo. This is a Go wrapper around libbpf, a userspace library for dealing with bpf objects. The wrapper uses CGO to reference individual functions and data types in libbpf, which is written in C.

For example, BPFLoadObject() is a libbpfgo API function which calls bpf_object__load(), a libbpf API function.

func (m *Module) BPFLoadObject() error {
	ret := C.bpf_object__load(m.obj)
	if ret != 0 {
		return fmt.Errorf("failed to load BPF object")
	}

	return nil
}

The goal for libbpfgo is to fully implement the libbpf API (which is pretty big). One complication with this is that libbpf has very inconsistent error handling. Some APIs will return an integer error code directly, some will return a NULL pointer and set the error code in errno, some will return an error code inside of a pointer, and some return an error code which is also in errno. As a result of trying to maintain libbpfgo’s error handling sanity, I’ve picked up some lessons on how you can use CGO to your advantage.

errno is essentially just a global variable. It’s used to convey errors in cases where directly surfacing an error isn’t always easy. Keep in mind that C functions can’t have more than one return value like in Go. errno is also used quite a bit with system calls.

If you’re calling a C function that uses errno, you can add a second return variable to capture the value of errno into an error variable. For example:

func (m *Module) GetMap(mapName string) (*BPFMap, error) {
	cs := C.CString(mapName)
	bpfMap, errno := C.bpf_object__find_map_by_name(m.obj, cs)
	C.free(unsafe.Pointer(cs))
	if bpfMap == nil {
		return nil, fmt.Errorf("failed to find BPF map %s: %w", mapName, errno)
	}

	return &BPFMap{
		bpfMap: bpfMap,
		name:   mapName,
		fd:     C.bpf_map__fd(bpfMap),
		module: m,
	}, nil
}

In the above example, bpf_object__find_map_by_name() is an API function which returns a pointer. That pointer will either be a memory address of a bpf map, or in the case of an error, NULL. The error code is set to errno.

C standards assign a specific cause of error to specific integer values. Go has these defined here. Since these are wrapped in the standard Go error type, you can treat it like any error. You can also cast an integer error code as an syscall.Errno for the same benefit. Like so:

func (b *BPFMap) Resize(maxEntries uint32) error {
	errC := C.bpf_map__set_max_entries(b.bpfMap, C.uint(maxEntries))
	if errC != 0 {
		return fmt.Errorf("failed to resize map %s to %v: %w", b.name, maxEntries, syscall.Errno(-errC))
	}
	return nil
}

In the above example, errC represents an integer error code (libbpf returns them as negative). So syscall.Errno(-errC) gives us a workable error type.

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