bpf2go: refactor object file linking into gen and add tests

- replace the direct command execution for linking multiple BPF object files with a new `Link` function in the `gen` package.
- add unit tests for linking

Author: yonch

cmd/bpf2go/gen/link.go

@@ -0,0 +1,34 @@
+//go:build !windows
+
+package gen
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+)
+
+// LinkArgs specifies the arguments for linking multiple BPF object files together.
+type LinkArgs struct {
+	// Destination object file name
+	Dest string
+	// Source object files to link together
+	Sources []string
+}
+
+// Link combines multiple BPF object files into a single object file.
+func Link(args LinkArgs) error {
+	if len(args.Sources) == 0 {
+		return fmt.Errorf("no source files to link")
+	}
+
+	cmd := exec.Command("bpftool", "gen", "object", args.Dest)
+	cmd.Args = append(cmd.Args, args.Sources...)
+	cmd.Stderr = os.Stderr
+
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("bpftool gen object returned error: %w", err)
+	}
+
+	return nil
+}

cmd/bpf2go/gen/link_test.go

@@ -0,0 +1,72 @@
+//go:build !windows
+
+package gen
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/cilium/ebpf/internal/testutils"
+)
+
+const (
+	func1 = `__attribute__((section("socket"), used)) int func1() { return 1; }`
+	func2 = `__attribute__((section("socket"), used)) int func2() { return 2; }`
+)
+
+func TestLink(t *testing.T) {
+	if testing.Short() {
+		t.SkipNow()
+	}
+
+	dir := t.TempDir()
+	mustWriteFile(t, dir, "func1.c", func1)
+	mustWriteFile(t, dir, "func2.c", func2)
+
+	// Compile first object
+	obj1 := filepath.Join(dir, "func1.o")
+	err := Compile(CompileArgs{
+		CC:               testutils.ClangBin(t),
+		DisableStripping: true,
+		Workdir:          dir,
+		Source:           filepath.Join(dir, "func1.c"),
+		Dest:             obj1,
+	})
+	if err != nil {
+		t.Fatal("Can't compile func1:", err)
+	}
+
+	// Compile second object
+	obj2 := filepath.Join(dir, "func2.o")
+	err = Compile(CompileArgs{
+		CC:               testutils.ClangBin(t),
+		DisableStripping: true,
+		Workdir:          dir,
+		Source:           filepath.Join(dir, "func2.c"),
+		Dest:             obj2,
+	})
+	if err != nil {
+		t.Fatal("Can't compile func2:", err)
+	}
+
+	// Link both objects
+	linked := filepath.Join(dir, "linked.o")
+	err = Link(LinkArgs{
+		Dest:    linked,
+		Sources: []string{obj1, obj2},
+	})
+	if err != nil {
+		t.Fatal("Can't link objects:", err)
+	}
+
+	// Verify the linked file exists and has content
+	stat, err := os.Stat(linked)
+	if err != nil {
+		t.Fatal("Can't stat linked file:", err)
+	}
+
+	if stat.Size() == 0 {
+		t.Error("Linked file is empty")
+	}
+}

cmd/bpf2go/main.go

@@ -442,10 +442,11 @@ func (b2g *bpf2go) convert(tgt gen.Target, goarches gen.GoArches) (err error) {
 
 	// If we have multiple object files, link them together
 	if len(tmpObjFileNames) > 1 {
-		cmd := exec.Command("bpftool", "gen", "object", objFileName)
-		cmd.Args = append(cmd.Args, tmpObjFileNames...)
-		cmd.Stderr = os.Stderr
-		if err := cmd.Run(); err != nil {
+		err = gen.Link(gen.LinkArgs{
+			Dest:    objFileName,
+			Sources: tmpObjFileNames,
+		})
+		if err != nil {
 			return fmt.Errorf("link object files: %w", err)
 		}
 	} else {