btf: don't cache kernel BTF in LoadKernelSpec

btf/btf.go

@@ -11,7 +11,6 @@ import (
 	"math"
 	"os"
 	"reflect"
-	"sync"
 
 	"github.com/cilium/ebpf/internal"
 	"github.com/cilium/ebpf/internal/sys"
@@ -255,32 +254,11 @@ func indexTypes(types []Type) (map[Type]TypeID, map[essentialName][]Type) {
 	return typeIDs, typesByName
 }
 
-var kernelBTF struct {
-	sync.Mutex
-	*Spec
-}
-
 // LoadKernelSpec returns the current kernel's BTF information.
 //
-// Requires a >= 5.5 kernel with CONFIG_DEBUG_INFO_BTF enabled. Returns
-// ErrNotSupported if BTF is not enabled.
+// Defaults to /sys/kernel/btf/vmlinux and falls back to scanning the file system
+// for vmlinux ELFs. Returns an error wrapping ErrNotSupported if BTF is not enabled.
 func LoadKernelSpec() (*Spec, error) {
-	kernelBTF.Lock()
-	defer kernelBTF.Unlock()
-
-	if kernelBTF.Spec != nil {
-		return kernelBTF.Spec, nil
-	}
-
-	var err error
-	kernelBTF.Spec, err = loadKernelSpec()
-	return kernelBTF.Spec, err
-}
-
-// loadKernelSpec attempts to load the raw vmlinux BTF blob at
-// /sys/kernel/btf/vmlinux and falls back to scanning the file system
-// for vmlinux ELFs.
-func loadKernelSpec() (*Spec, error) {
 	fh, err := os.Open("/sys/kernel/btf/vmlinux")
 	if err == nil {
 		defer fh.Close()

btf/btf_test.go

@@ -182,7 +182,7 @@ func BenchmarkParseVmlinux(b *testing.B) {
 }
 
 func TestParseCurrentKernelBTF(t *testing.T) {
-	spec, err := loadKernelSpec()
+	spec, err := LoadKernelSpec()
 	testutils.SkipIfNotSupported(t, err)
 	if err != nil {
 		t.Fatal("Can't load BTF:", err)

linker.go

@@ -3,6 +3,7 @@ package ebpf
 import (
 	"errors"
 	"fmt"
+	"sync"
 
 	"github.com/cilium/ebpf/asm"
 	"github.com/cilium/ebpf/btf"
@@ -140,12 +141,9 @@ func applyRelocations(insns asm.Instructions, local, target *btf.Spec) error {
 		return nil
 	}
 
-	if target == nil {
-		var err error
-		target, err = btf.LoadKernelSpec()
-		if err != nil {
-			return err
-		}
+	target, err := maybeLoadKernelBTF(target)
+	if err != nil {
+		return err
 	}
 
 	fixups, err := btf.CORERelocate(local, target, relos)
@@ -200,3 +198,29 @@ func fixupProbeReadKernel(ins *asm.Instruction) {
 		ins.Constant = int64(asm.FnProbeReadStr)
 	}
 }
+
+var kernelBTF struct {
+	sync.Mutex
+	spec *btf.Spec
+}
+
+// maybeLoadKernelBTF loads the current kernel's BTF if spec is nil, otherwise
+// it returns spec unchanged.
+//
+// The kernel BTF is cached for the lifetime of the process.
+func maybeLoadKernelBTF(spec *btf.Spec) (*btf.Spec, error) {
+	if spec != nil {
+		return spec, nil
+	}
+
+	kernelBTF.Lock()
+	defer kernelBTF.Unlock()
+
+	if kernelBTF.spec != nil {
+		return kernelBTF.spec, nil
+	}
+
+	var err error
+	kernelBTF.spec, err = btf.LoadKernelSpec()
+	return kernelBTF.spec, err
+}

prog.go

@@ -801,17 +801,12 @@ func findTargetInKernel(spec *btf.Spec, name string, progType ProgramType, attac
 		return 0, errUnrecognizedAttachType
 	}
 
-	var (
-		target btf.Type
-		err    error
-	)
-	if spec == nil {
-		spec, err = btf.LoadKernelSpec()
-		if err != nil {
-			return 0, fmt.Errorf("load kernel spec: %w", err)
-		}
+	spec, err := maybeLoadKernelBTF(spec)
+	if err != nil {
+		return 0, fmt.Errorf("load kernel spec: %w", err)
 	}
 
+	var target btf.Type
 	if isBTFTypeFunc {
 		var targetFunc *btf.Func
 		err = spec.TypeByName(typeName, &targetFunc)