feat(btf): introduce GetBTFFDByID()

This allows to get a FD to a BTF by its ID. This is useful to create a
BPFMapLow passing options set with a BTF ID.

btf.go

@@ -0,0 +1,26 @@
+package libbpfgo
+
+/*
+#cgo LDFLAGS: -lelf -lz
+#include "libbpfgo.h"
+*/
+import "C"
+
+import (
+	"fmt"
+	"syscall"
+)
+
+//
+// BTF (low-level API)
+//
+
+// GetBTFFDByID returns a file descriptor for the BTF with the given ID.
+func GetBTFFDByID(id uint32) (int, error) {
+	fdC := C.bpf_btf_get_fd_by_id(C.uint(id))
+	if fdC < 0 {
+		return int(fdC), fmt.Errorf("could not find BTF id %d: %w", id, syscall.Errno(-fdC))
+	}
+
+	return int(fdC), nil
+}

selftest/getbtffdbyid/Makefile

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

selftest/getbtffdbyid/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/getbtffdbyid/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/getbtffdbyid/main.bpf.c

@@ -0,0 +1,14 @@
+//+build ignore
+
+#include <vmlinux.h>
+
+#include <bpf/bpf_helpers.h>
+
+struct {
+    __uint(type, BPF_MAP_TYPE_ARRAY);
+    __uint(max_entries, 1);
+    __type(key, __u32);
+    __type(value, __u32);
+} inner_array_proto SEC(".maps");
+
+char LICENSE[] SEC("license") = "Dual BSD/GPL";

selftest/getbtffdbyid/main.go

@@ -0,0 +1,60 @@
+package main
+
+import "C"
+
+import (
+	"log"
+	"unsafe"
+
+	bpf "github.com/aquasecurity/libbpfgo"
+)
+
+func main() {
+	bpfModule, err := bpf.NewModuleFromFile("main.bpf.o")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer bpfModule.Close()
+
+	err = bpfModule.BPFLoadObject()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Use the "inner_array_proto" map BTF ID to create a new map with the same
+	// BTF type.
+	innerArrayProto, err := bpfModule.GetMap("inner_array_proto")
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	optsProto, err := bpf.GetMapInfoByFD(innerArrayProto.FileDescriptor())
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// The "inner_array_proto" map is a BTF map, so its ID can be used to create
+	// a new map with the same BTF type.
+	btfFD, err := bpf.GetBTFFDByID(optsProto.BTFID)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	createOpts := &bpf.BPFMapCreateOpts{
+		BTFFD: uint32(btfFD),
+	}
+	innerArray, err := bpf.CreateMap(bpf.MapTypeArray, "inner_array", 4, 4, 1, createOpts)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	// Save an element in the "inner_array" map.
+	key := uint32(0) // index 0
+	keyUnsafe := unsafe.Pointer(&key)
+	value := uint32(191711)
+	valueUnsafe := unsafe.Pointer(&value)
+	err = innerArray.Update(keyUnsafe, valueUnsafe)
+	if err != nil {
+		log.Fatal(err)
+	}
+}

selftest/getbtffdbyid/run.sh

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