Revert "btf: avoid copies of vmlinux BTF for internal callers"

This reverts commit c3a89fc. I've come
to the conclusion that a separate package isn't the right approach.
Parsing vmlinux is still too expensive and will stay this way as long
as we unmarshal the entire type graph when parsing BTF.

I think that the real solution will involve breaking API once more,
probably in the next release. I'm backing out the change to avoid
this churn. The downside is that kfunc, CO-RE and attaching programs
like fentry get slower again. Since they were already slow in the
last release I'm hoping that this doesn't cause too many problems.

Signed-off-by: Lorenz Bauer <lmb@isovalent.com>

btf/btf.go

@@ -10,6 +10,7 @@ import (
 	"math"
 	"os"
 	"reflect"
+	"sync"
 
 	"github.com/cilium/ebpf/internal"
 	"github.com/cilium/ebpf/internal/sys"
@@ -297,6 +298,107 @@ func indexTypes(types []Type, firstTypeID TypeID) (map[Type]TypeID, map[essentia
 	return typeIDs, typesByName
 }
 
+// LoadKernelSpec returns the current kernel's BTF information.
+//
+// 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) {
+	spec, _, err := kernelSpec()
+	if err != nil {
+		return nil, err
+	}
+	return spec.Copy(), nil
+}
+
+var kernelBTF struct {
+	sync.RWMutex
+	spec *Spec
+	// True if the spec was read from an ELF instead of raw BTF in /sys.
+	fallback bool
+}
+
+// FlushKernelSpec removes any cached kernel type information.
+func FlushKernelSpec() {
+	kernelBTF.Lock()
+	defer kernelBTF.Unlock()
+
+	kernelBTF.spec, kernelBTF.fallback = nil, false
+}
+
+func kernelSpec() (*Spec, bool, error) {
+	kernelBTF.RLock()
+	spec, fallback := kernelBTF.spec, kernelBTF.fallback
+	kernelBTF.RUnlock()
+
+	if spec == nil {
+		kernelBTF.Lock()
+		defer kernelBTF.Unlock()
+
+		spec, fallback = kernelBTF.spec, kernelBTF.fallback
+	}
+
+	if spec != nil {
+		return spec, fallback, nil
+	}
+
+	spec, fallback, err := loadKernelSpec()
+	if err != nil {
+		return nil, false, err
+	}
+
+	kernelBTF.spec, kernelBTF.fallback = spec, fallback
+	return spec, fallback, nil
+}
+
+func loadKernelSpec() (_ *Spec, fallback bool, _ error) {
+	fh, err := os.Open("/sys/kernel/btf/vmlinux")
+	if err == nil {
+		defer fh.Close()
+
+		spec, err := loadRawSpec(fh, internal.NativeEndian, nil)
+		return spec, false, err
+	}
+
+	file, err := findVMLinux()
+	if err != nil {
+		return nil, false, err
+	}
+	defer file.Close()
+
+	spec, err := loadSpecFromELF(file)
+	return spec, true, err
+}
+
+// findVMLinux scans multiple well-known paths for vmlinux kernel images.
+func findVMLinux() (*internal.SafeELFFile, error) {
+	release, err := internal.KernelRelease()
+	if err != nil {
+		return nil, err
+	}
+
+	// use same list of locations as libbpf
+	// https://github.com/libbpf/libbpf/blob/9a3a42608dbe3731256a5682a125ac1e23bced8f/src/btf.c#L3114-L3122
+	locations := []string{
+		"/boot/vmlinux-%s",
+		"/lib/modules/%s/vmlinux-%[1]s",
+		"/lib/modules/%s/build/vmlinux",
+		"/usr/lib/modules/%s/kernel/vmlinux",
+		"/usr/lib/debug/boot/vmlinux-%s",
+		"/usr/lib/debug/boot/vmlinux-%s.debug",
+		"/usr/lib/debug/lib/modules/%s/vmlinux",
+	}
+
+	for _, loc := range locations {
+		file, err := internal.OpenSafeELFFile(fmt.Sprintf(loc, release))
+		if errors.Is(err, os.ErrNotExist) {
+			continue
+		}
+		return file, err
+	}
+
+	return nil, fmt.Errorf("no BTF found for kernel version %s: %w", release, internal.ErrNotSupported)
+}
+
 // parseBTFHeader parses the header of the .BTF section.
 func parseBTFHeader(r io.Reader, bo binary.ByteOrder) (*btfHeader, error) {
 	var header btfHeader

btf/btf_test.go

@@ -21,10 +21,13 @@ func vmlinuxSpec(tb testing.TB) *Spec {
 	// through sysfs"), which shipped in Linux 5.4.
 	testutils.SkipOnOldKernel(tb, "5.4", "vmlinux BTF in sysfs")
 
-	spec, err := LoadSpec("/sys/kernel/btf/vmlinux")
+	spec, fallback, err := kernelSpec()
 	if err != nil {
 		tb.Fatal(err)
 	}
+	if fallback {
+		tb.Fatal("/sys/kernel/btf/vmlinux is not available")
+	}
 	return spec.Copy()
 }
 
@@ -240,6 +243,24 @@ func TestParseCurrentKernelBTF(t *testing.T) {
 	t.Logf("Average string size: %.0f", float64(totalBytes)/float64(len(spec.strings.strings)))
 }
 
+func TestFindVMLinux(t *testing.T) {
+	file, err := findVMLinux()
+	testutils.SkipIfNotSupported(t, err)
+	if err != nil {
+		t.Fatal("Can't find vmlinux:", err)
+	}
+	defer file.Close()
+
+	spec, err := loadSpecFromELF(file)
+	if err != nil {
+		t.Fatal("Can't load BTF:", err)
+	}
+
+	if len(spec.namedTypes) == 0 {
+		t.Fatal("Empty kernel BTF")
+	}
+}
+
 func TestLoadSpecFromElf(t *testing.T) {
 	testutils.Files(t, testutils.Glob(t, "../testdata/loader-e*.elf"), func(t *testing.T, file string) {
 		spec := parseELFBTF(t, file)
@@ -295,6 +316,17 @@ func TestVerifierError(t *testing.T) {
 	}
 }
 
+func TestLoadKernelSpec(t *testing.T) {
+	if _, err := os.Stat("/sys/kernel/btf/vmlinux"); os.IsNotExist(err) {
+		t.Skip("/sys/kernel/btf/vmlinux not present")
+	}
+
+	_, err := LoadKernelSpec()
+	if err != nil {
+		t.Fatal("Can't load kernel spec:", err)
+	}
+}
+
 func TestGuessBTFByteOrder(t *testing.T) {
 	bo := guessRawBTFByteOrder(vmlinuxTestdataReader(t))
 	if bo != binary.LittleEndian {

btf/core.go

@@ -166,8 +166,11 @@ func (k coreKind) String() string {
 // for relos[i].
 func CORERelocate(relos []*CORERelocation, target *Spec, bo binary.ByteOrder) ([]COREFixup, error) {
 	if target == nil {
-		// Explicitly check for nil here since the argument used to be optional.
-		return nil, fmt.Errorf("target must be provided")
+		var err error
+		target, _, err = kernelSpec()
+		if err != nil {
+			return nil, fmt.Errorf("load kernel spec: %w", err)
+		}
 	}
 
 	if bo != target.byteOrder {

linker.go

@@ -10,7 +10,6 @@ import (
 	"github.com/cilium/ebpf/asm"
 	"github.com/cilium/ebpf/btf"
 	"github.com/cilium/ebpf/internal"
-	"github.com/cilium/ebpf/internal/linux"
 )
 
 // handles stores handle objects to avoid gc cleanup
@@ -124,14 +123,6 @@ func applyRelocations(insns asm.Instructions, target *btf.Spec, bo binary.ByteOr
 		bo = internal.NativeEndian
 	}
 
-	if target == nil {
-		var err error
-		target, err = linux.TypesNoCopy()
-		if err != nil {
-			return err
-		}
-	}
-
 	fixups, err := btf.CORERelocate(relos, target, bo)
 	if err != nil {
 		return err
@@ -253,7 +244,7 @@ func fixupKfuncs(insns asm.Instructions) (handles, error) {
 
 fixups:
 	// only load the kernel spec if we found at least one kfunc call
-	kernelSpec, err := linux.TypesNoCopy()
+	kernelSpec, err := btf.LoadKernelSpec()
 	if err != nil {
 		return nil, err
 	}