feat(map): introduce SetInnerMapFD()

This allows to set the inner map prototype of a map of maps which
doesn't have a pre-allocated inner map.

Initial effort: #213

Co-authored-by: Kemal Akkoyun <kakkoyun@gmail.com>

map.go

@@ -283,9 +283,25 @@ func (m *BPFMap) Unpin(pinPath string) error {
 // func (m *BPFMap) InnerMap() *BPFMap {
 // }
 
-// TODO: implement `bpf_map__set_inner_map_fd` wrapper
-// func (m *BPFMap) SetInnerMapFD(innerMapFD int) error {
-// }
+// SetInnerMap configures the inner map prototype for a BPFMap that represents
+// a map of maps.
+//
+// This function accepts the file descriptor of another map, which will serve as
+// a prototype.
+//
+// NOTE: It must be called before the module is loaded.
+func (m *BPFMap) SetInnerMap(templateMapFD int) error {
+	if templateMapFD < 0 {
+		return fmt.Errorf("invalid inner map fd %d", templateMapFD)
+	}
+
+	retC := C.bpf_map__set_inner_map_fd(m.bpfMap, C.int(templateMapFD))
+	if retC < 0 {
+		return fmt.Errorf("failed to set inner map for %s: %w", m.Name(), syscall.Errno(-retC))
+	}
+
+	return nil
+}
 
 //
 // BPFMap Operations

selftest/map-setinner/Makefile

@@ -0,0 +1 @@
+../common/Makefile

selftest/map-setinner/go.mod

@@ -0,0 +1,7 @@
+module github.com/aquasecurity/libbpfgo/selftest/map-update
+
+go 1.18
+
+require github.com/aquasecurity/libbpfgo v0.4.7-libbpf-1.2.0-b2e29a1
+
+replace github.com/aquasecurity/libbpfgo => ../../

selftest/map-setinner/go.sum

@@ -0,0 +1,4 @@
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/stretchr/testify v1.8.2 h1:+h33VjcLVPDHtOdpUCuF+7gSuG3yGIftsP1YvFihtJ8=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=

selftest/map-setinner/main.bpf.c

@@ -0,0 +1,15 @@
+//+build ignore
+
+#include <vmlinux.h>
+
+#include <bpf/bpf_helpers.h>
+
+// Hash map of maps with no inner maps preallocated.
+struct {
+    __uint(type, BPF_MAP_TYPE_HASH_OF_MAPS);
+    __uint(max_entries, 1);
+    __type(key, __u32);
+    __type(value, __u32);
+} outer_hash SEC(".maps");
+
+char LICENSE[] SEC("license") = "Dual BSD/GPL";

selftest/map-setinner/main.go

@@ -0,0 +1,59 @@
+package main
+
+import "C"
+
+import (
+	"log"
+	"syscall"
+
+	bpf "github.com/aquasecurity/libbpfgo"
+)
+
+func main() {
+	bpfModule, err := bpf.NewModuleFromFile("main.bpf.o")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer bpfModule.Close()
+
+	outerHash, err := bpfModule.GetMap("outer_hash")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	templateInnerMap, err := bpf.CreateMap(bpf.MapTypeHash, "template_inner_map", 4, 4, 420, nil)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// As the "outer_hash" map does not have an inner map prototype pre-allocated,
+	// an active map (from any origin) must be used as a template, by calling
+	// SetInnerMap() before the object is loaded, otherwise the BPF program will
+	// fail to load. The template map can be removed after the object is loaded.
+	err = outerHash.SetInnerMap(templateInnerMap.FileDescriptor())
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	err = bpfModule.BPFLoadObject()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	//
+	// "outer_hash" map of maps is now fully loaded and can be used.
+	//
+
+	// Attempting to set inner map after the object is loaded will fail.
+	err = outerHash.SetInnerMap(templateInnerMap.FileDescriptor())
+	if err == nil {
+		log.Fatal("should fail after object is loaded")
+	}
+
+	// If not needed anymore, remove the "template_inner_map",
+	// freeing up resources.
+	err = syscall.Close(templateInnerMap.FileDescriptor())
+	if err != nil {
+		log.Fatal(err)
+	}
+}

selftest/map-setinner/run.sh

@@ -0,0 +1 @@
+../common/run.sh