/
btf.go
828 lines (683 loc) · 20 KB
/
btf.go
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package btf
import (
"bufio"
"bytes"
"debug/elf"
"encoding/binary"
"errors"
"fmt"
"io"
"math"
"os"
"reflect"
"sync"
internal "github.com/DataDog/btf-internals"
"github.com/DataDog/btf-internals/sys"
"github.com/DataDog/btf-internals/unix"
)
const btfMagic = 0xeB9F
// Errors returned by BTF functions.
var (
ErrNotSupported = internal.ErrNotSupported
ErrNotFound = errors.New("not found")
ErrNoExtendedInfo = errors.New("no extended info")
)
// ID represents the unique ID of a BTF object.
type ID uint32
// Spec represents decoded BTF.
type Spec struct {
// Data from .BTF.
rawTypes []rawType
strings *stringTable
// All types contained by the spec, the position of a type in the slice
// is its ID.
types types
// Type IDs indexed by type.
typeIDs map[Type]TypeID
// Types indexed by essential name.
// Includes all struct flavors and types with the same name.
namedTypes map[essentialName][]Type
byteOrder binary.ByteOrder
}
type btfHeader struct {
Magic uint16
Version uint8
Flags uint8
HdrLen uint32
TypeOff uint32
TypeLen uint32
StringOff uint32
StringLen uint32
}
// typeStart returns the offset from the beginning of the .BTF section
// to the start of its type entries.
func (h *btfHeader) typeStart() int64 {
return int64(h.HdrLen + h.TypeOff)
}
// stringStart returns the offset from the beginning of the .BTF section
// to the start of its string table.
func (h *btfHeader) stringStart() int64 {
return int64(h.HdrLen + h.StringOff)
}
// LoadSpecFromReader reads from an ELF or a raw BTF blob.
//
// Returns ErrNotFound if reading from an ELF which contains no BTF. ExtInfos
// may be nil.
func LoadSpecFromReader(rd io.ReaderAt) (*Spec, error) {
file, err := internal.NewSafeELFFile(rd)
if err != nil {
if bo := guessRawBTFByteOrder(rd); bo != nil {
// Try to parse a naked BTF blob. This will return an error if
// we encounter a Datasec, since we can't fix it up.
spec, err := loadRawSpec(io.NewSectionReader(rd, 0, math.MaxInt64), bo, nil, nil)
return spec, err
}
return nil, err
}
return loadSpecFromELF(file)
}
// LoadSpecAndExtInfosFromReader reads from an ELF.
//
// ExtInfos may be nil if the ELF doesn't contain section metadta.
// Returns ErrNotFound if the ELF contains no BTF.
func LoadSpecAndExtInfosFromReader(rd io.ReaderAt) (*Spec, *ExtInfos, error) {
file, err := internal.NewSafeELFFile(rd)
if err != nil {
return nil, nil, err
}
spec, err := loadSpecFromELF(file)
if err != nil {
return nil, nil, err
}
extInfos, err := loadExtInfosFromELF(file, spec.types, spec.strings)
if err != nil && !errors.Is(err, ErrNotFound) {
return nil, nil, err
}
return spec, extInfos, nil
}
// variableOffsets extracts all symbols offsets from an ELF and indexes them by
// section and variable name.
//
// References to variables in BTF data sections carry unsigned 32-bit offsets.
// Some ELF symbols (e.g. in vmlinux) may point to virtual memory that is well
// beyond this range. Since these symbols cannot be described by BTF info,
// ignore them here.
func variableOffsets(file *internal.SafeELFFile) (map[variable]uint32, error) {
symbols, err := file.Symbols()
if err != nil {
return nil, fmt.Errorf("can't read symbols: %v", err)
}
variableOffsets := make(map[variable]uint32)
for _, symbol := range symbols {
if idx := symbol.Section; idx >= elf.SHN_LORESERVE && idx <= elf.SHN_HIRESERVE {
// Ignore things like SHN_ABS
continue
}
if symbol.Value > math.MaxUint32 {
// VarSecinfo offset is u32, cannot reference symbols in higher regions.
continue
}
if int(symbol.Section) >= len(file.Sections) {
return nil, fmt.Errorf("symbol %s: invalid section %d", symbol.Name, symbol.Section)
}
secName := file.Sections[symbol.Section].Name
variableOffsets[variable{secName, symbol.Name}] = uint32(symbol.Value)
}
return variableOffsets, nil
}
func loadSpecFromELF(file *internal.SafeELFFile) (*Spec, error) {
var (
btfSection *elf.Section
sectionSizes = make(map[string]uint32)
)
for _, sec := range file.Sections {
switch sec.Name {
case ".BTF":
btfSection = sec
default:
if sec.Type != elf.SHT_PROGBITS && sec.Type != elf.SHT_NOBITS {
break
}
if sec.Size > math.MaxUint32 {
return nil, fmt.Errorf("section %s exceeds maximum size", sec.Name)
}
sectionSizes[sec.Name] = uint32(sec.Size)
}
}
if btfSection == nil {
return nil, fmt.Errorf("btf: %w", ErrNotFound)
}
vars, err := variableOffsets(file)
if err != nil {
return nil, err
}
if btfSection.ReaderAt == nil {
return nil, fmt.Errorf("compressed BTF is not supported")
}
return loadRawSpec(btfSection.ReaderAt, file.ByteOrder, sectionSizes, vars)
}
func loadRawSpec(btf io.ReaderAt, bo binary.ByteOrder, sectionSizes map[string]uint32, variableOffsets map[variable]uint32) (*Spec, error) {
rawTypes, rawStrings, err := parseBTF(btf, bo)
if err != nil {
return nil, err
}
err = fixupDatasec(rawTypes, rawStrings, sectionSizes, variableOffsets)
if err != nil {
return nil, err
}
types, err := inflateRawTypes(rawTypes, rawStrings)
if err != nil {
return nil, err
}
typeIDs, typesByName := indexTypes(types)
return &Spec{
rawTypes: rawTypes,
namedTypes: typesByName,
typeIDs: typeIDs,
types: types,
strings: rawStrings,
byteOrder: bo,
}, nil
}
func indexTypes(types []Type) (map[Type]TypeID, map[essentialName][]Type) {
namedTypes := 0
for _, typ := range types {
if typ.TypeName() != "" {
// Do a pre-pass to figure out how big types by name has to be.
// Most types have unique names, so it's OK to ignore essentialName
// here.
namedTypes++
}
}
typeIDs := make(map[Type]TypeID, len(types))
typesByName := make(map[essentialName][]Type, namedTypes)
for i, typ := range types {
if name := newEssentialName(typ.TypeName()); name != "" {
typesByName[name] = append(typesByName[name], typ)
}
typeIDs[typ] = TypeID(i)
}
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.
func LoadKernelSpec() (*Spec, error) {
kernelBTF.Lock()
defer kernelBTF.Unlock()
if kernelBTF.Spec != nil {
return kernelBTF.Spec, nil
}
var err error
kernelBTF.Spec, err = LoadKernelSpecWithoutCache()
return kernelBTF.Spec, err
}
// LoadKernelSpecWithoutCache 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 LoadKernelSpecWithoutCache() (*Spec, error) {
fh, err := os.Open("/sys/kernel/btf/vmlinux")
if err == nil {
defer fh.Close()
return loadRawSpec(fh, internal.NativeEndian, nil, nil)
}
file, err := findVMLinux()
if err != nil {
return nil, err
}
defer file.Close()
return loadSpecFromELF(file)
}
// 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
if err := binary.Read(r, bo, &header); err != nil {
return nil, fmt.Errorf("can't read header: %v", err)
}
if header.Magic != btfMagic {
return nil, fmt.Errorf("incorrect magic value %v", header.Magic)
}
if header.Version != 1 {
return nil, fmt.Errorf("unexpected version %v", header.Version)
}
if header.Flags != 0 {
return nil, fmt.Errorf("unsupported flags %v", header.Flags)
}
remainder := int64(header.HdrLen) - int64(binary.Size(&header))
if remainder < 0 {
return nil, errors.New("header length shorter than btfHeader size")
}
if _, err := io.CopyN(internal.DiscardZeroes{}, r, remainder); err != nil {
return nil, fmt.Errorf("header padding: %v", err)
}
return &header, nil
}
func guessRawBTFByteOrder(r io.ReaderAt) binary.ByteOrder {
buf := new(bufio.Reader)
for _, bo := range []binary.ByteOrder{
binary.LittleEndian,
binary.BigEndian,
} {
buf.Reset(io.NewSectionReader(r, 0, math.MaxInt64))
if _, err := parseBTFHeader(buf, bo); err == nil {
return bo
}
}
return nil
}
// parseBTF reads a .BTF section into memory and parses it into a list of
// raw types and a string table.
func parseBTF(btf io.ReaderAt, bo binary.ByteOrder) ([]rawType, *stringTable, error) {
buf := internal.NewBufferedSectionReader(btf, 0, math.MaxInt64)
header, err := parseBTFHeader(buf, bo)
if err != nil {
return nil, nil, fmt.Errorf("parsing .BTF header: %v", err)
}
rawStrings, err := readStringTable(io.NewSectionReader(btf, header.stringStart(), int64(header.StringLen)))
if err != nil {
return nil, nil, fmt.Errorf("can't read type names: %w", err)
}
buf.Reset(io.NewSectionReader(btf, header.typeStart(), int64(header.TypeLen)))
rawTypes, err := readTypes(buf, bo, header.TypeLen)
if err != nil {
return nil, nil, fmt.Errorf("can't read types: %w", err)
}
return rawTypes, rawStrings, nil
}
type variable struct {
section string
name string
}
func fixupDatasec(rawTypes []rawType, rawStrings *stringTable, sectionSizes map[string]uint32, variableOffsets map[variable]uint32) error {
for i, rawType := range rawTypes {
if rawType.Kind() != kindDatasec {
continue
}
name, err := rawStrings.Lookup(rawType.NameOff)
if err != nil {
return err
}
if name == ".kconfig" || name == ".ksyms" {
return fmt.Errorf("reference to %s: %w", name, ErrNotSupported)
}
if rawTypes[i].SizeType != 0 {
continue
}
size, ok := sectionSizes[name]
if !ok {
return fmt.Errorf("data section %s: missing size", name)
}
rawTypes[i].SizeType = size
secinfos := rawType.data.([]btfVarSecinfo)
for j, secInfo := range secinfos {
id := int(secInfo.Type - 1)
if id >= len(rawTypes) {
return fmt.Errorf("data section %s: invalid type id %d for variable %d", name, id, j)
}
varName, err := rawStrings.Lookup(rawTypes[id].NameOff)
if err != nil {
return fmt.Errorf("data section %s: can't get name for type %d: %w", name, id, err)
}
offset, ok := variableOffsets[variable{name, varName}]
if !ok {
return fmt.Errorf("data section %s: missing offset for variable %s", name, varName)
}
secinfos[j].Offset = offset
}
}
return nil
}
// Copy creates a copy of Spec.
func (s *Spec) Copy() *Spec {
types := copyTypes(s.types, nil)
typeIDs, typesByName := indexTypes(types)
// NB: Other parts of spec are not copied since they are immutable.
return &Spec{
s.rawTypes,
s.strings,
types,
typeIDs,
typesByName,
s.byteOrder,
}
}
type marshalOpts struct {
ByteOrder binary.ByteOrder
StripFuncLinkage bool
}
func (s *Spec) marshal(opts marshalOpts) ([]byte, error) {
var (
buf bytes.Buffer
header = new(btfHeader)
headerLen = binary.Size(header)
)
// Reserve space for the header. We have to write it last since
// we don't know the size of the type section yet.
_, _ = buf.Write(make([]byte, headerLen))
// Write type section, just after the header.
for _, raw := range s.rawTypes {
switch {
case opts.StripFuncLinkage && raw.Kind() == kindFunc:
raw.SetLinkage(StaticFunc)
}
if err := raw.Marshal(&buf, opts.ByteOrder); err != nil {
return nil, fmt.Errorf("can't marshal BTF: %w", err)
}
}
typeLen := uint32(buf.Len() - headerLen)
// Write string section after type section.
stringsLen := s.strings.Length()
buf.Grow(stringsLen)
if err := s.strings.Marshal(&buf); err != nil {
return nil, err
}
// Fill out the header, and write it out.
header = &btfHeader{
Magic: btfMagic,
Version: 1,
Flags: 0,
HdrLen: uint32(headerLen),
TypeOff: 0,
TypeLen: typeLen,
StringOff: typeLen,
StringLen: uint32(stringsLen),
}
raw := buf.Bytes()
err := binary.Write(sliceWriter(raw[:headerLen]), opts.ByteOrder, header)
if err != nil {
return nil, fmt.Errorf("can't write header: %v", err)
}
return raw, nil
}
type sliceWriter []byte
func (sw sliceWriter) Write(p []byte) (int, error) {
if len(p) != len(sw) {
return 0, errors.New("size doesn't match")
}
return copy(sw, p), nil
}
// TypeByID returns the BTF Type with the given type ID.
//
// Returns an error wrapping ErrNotFound if a Type with the given ID
// does not exist in the Spec.
func (s *Spec) TypeByID(id TypeID) (Type, error) {
return s.types.ByID(id)
}
// TypeID returns the ID for a given Type.
//
// Returns an error wrapping ErrNoFound if the type isn't part of the Spec.
func (s *Spec) TypeID(typ Type) (TypeID, error) {
if _, ok := typ.(*Void); ok {
// Equality is weird for void, since it is a zero sized type.
return 0, nil
}
id, ok := s.typeIDs[typ]
if !ok {
return 0, fmt.Errorf("no ID for type %s: %w", typ, ErrNotFound)
}
return id, nil
}
// AnyTypesByName returns a list of BTF Types with the given name.
//
// If the BTF blob describes multiple compilation units like vmlinux, multiple
// Types with the same name and kind can exist, but might not describe the same
// data structure.
//
// Returns an error wrapping ErrNotFound if no matching Type exists in the Spec.
func (s *Spec) AnyTypesByName(name string) ([]Type, error) {
types := s.namedTypes[newEssentialName(name)]
if len(types) == 0 {
return nil, fmt.Errorf("type name %s: %w", name, ErrNotFound)
}
// Return a copy to prevent changes to namedTypes.
result := make([]Type, 0, len(types))
for _, t := range types {
// Match against the full name, not just the essential one
// in case the type being looked up is a struct flavor.
if t.TypeName() == name {
result = append(result, t)
}
}
return result, nil
}
// AnyTypeByName returns a Type with the given name.
//
// Returns an error if multiple types of that name exist.
func (s *Spec) AnyTypeByName(name string) (Type, error) {
types, err := s.AnyTypesByName(name)
if err != nil {
return nil, err
}
if len(types) > 1 {
return nil, fmt.Errorf("found multiple types: %v", types)
}
return types[0], nil
}
// TypeByName searches for a Type with a specific name. Since multiple
// Types with the same name can exist, the parameter typ is taken to
// narrow down the search in case of a clash.
//
// typ must be a non-nil pointer to an implementation of a Type.
// On success, the address of the found Type will be copied to typ.
//
// Returns an error wrapping ErrNotFound if no matching
// Type exists in the Spec. If multiple candidates are found,
// an error is returned.
func (s *Spec) TypeByName(name string, typ interface{}) error {
typValue := reflect.ValueOf(typ)
if typValue.Kind() != reflect.Ptr {
return fmt.Errorf("%T is not a pointer", typ)
}
typPtr := typValue.Elem()
if !typPtr.CanSet() {
return fmt.Errorf("%T cannot be set", typ)
}
wanted := typPtr.Type()
if !wanted.AssignableTo(reflect.TypeOf((*Type)(nil)).Elem()) {
return fmt.Errorf("%T does not satisfy Type interface", typ)
}
types, err := s.AnyTypesByName(name)
if err != nil {
return err
}
var candidate Type
for _, typ := range types {
if reflect.TypeOf(typ) != wanted {
continue
}
if candidate != nil {
return fmt.Errorf("type %s: multiple candidates for %T", name, typ)
}
candidate = typ
}
if candidate == nil {
return fmt.Errorf("type %s: %w", name, ErrNotFound)
}
typPtr.Set(reflect.ValueOf(candidate))
return nil
}
// Handle is a reference to BTF loaded into the kernel.
type Handle struct {
spec *Spec
fd *sys.FD
}
// NewHandle loads BTF into the kernel.
//
// Returns ErrNotSupported if BTF is not supported.
func NewHandle(spec *Spec) (*Handle, error) {
if err := haveBTF(); err != nil {
return nil, err
}
if spec.byteOrder != internal.NativeEndian {
return nil, fmt.Errorf("can't load %s BTF on %s", spec.byteOrder, internal.NativeEndian)
}
btf, err := spec.marshal(marshalOpts{
ByteOrder: internal.NativeEndian,
StripFuncLinkage: haveFuncLinkage() != nil,
})
if err != nil {
return nil, fmt.Errorf("can't marshal BTF: %w", err)
}
if uint64(len(btf)) > math.MaxUint32 {
return nil, errors.New("BTF exceeds the maximum size")
}
attr := &sys.BtfLoadAttr{
Btf: sys.NewSlicePointer(btf),
BtfSize: uint32(len(btf)),
}
fd, err := sys.BtfLoad(attr)
if err != nil {
logBuf := make([]byte, 64*1024)
attr.BtfLogBuf = sys.NewSlicePointer(logBuf)
attr.BtfLogSize = uint32(len(logBuf))
attr.BtfLogLevel = 1
_, logErr := sys.BtfLoad(attr)
// NB: The syscall will never return ENOSPC as of 5.18-rc4.
return nil, internal.ErrorWithLog(err, logBuf, logErr)
}
return &Handle{spec.Copy(), fd}, nil
}
// NewHandleFromID returns the BTF handle for a given id.
//
// Returns ErrNotExist, if there is no BTF with the given id.
//
// Requires CAP_SYS_ADMIN.
func NewHandleFromID(id ID) (*Handle, error) {
fd, err := sys.BtfGetFdById(&sys.BtfGetFdByIdAttr{
Id: uint32(id),
})
if err != nil {
return nil, fmt.Errorf("get BTF by id: %w", err)
}
info, err := newInfoFromFd(fd)
if err != nil {
_ = fd.Close()
return nil, fmt.Errorf("get BTF spec for handle: %w", err)
}
return &Handle{info.BTF, fd}, nil
}
// Spec returns the Spec that defined the BTF loaded into the kernel.
func (h *Handle) Spec() *Spec {
return h.spec
}
// Close destroys the handle.
//
// Subsequent calls to FD will return an invalid value.
func (h *Handle) Close() error {
return h.fd.Close()
}
// FD returns the file descriptor for the handle.
func (h *Handle) FD() int {
return h.fd.Int()
}
func marshalBTF(types interface{}, strings []byte, bo binary.ByteOrder) []byte {
const minHeaderLength = 24
typesLen := uint32(binary.Size(types))
header := btfHeader{
Magic: btfMagic,
Version: 1,
HdrLen: minHeaderLength,
TypeOff: 0,
TypeLen: typesLen,
StringOff: typesLen,
StringLen: uint32(len(strings)),
}
buf := new(bytes.Buffer)
_ = binary.Write(buf, bo, &header)
_ = binary.Write(buf, bo, types)
buf.Write(strings)
return buf.Bytes()
}
var haveBTF = internal.FeatureTest("BTF", "5.1", func() error {
var (
types struct {
Integer btfType
Var btfType
btfVar struct{ Linkage uint32 }
}
strings = []byte{0, 'a', 0}
)
// We use a BTF_KIND_VAR here, to make sure that
// the kernel understands BTF at least as well as we
// do. BTF_KIND_VAR was introduced ~5.1.
types.Integer.SetKind(kindPointer)
types.Var.NameOff = 1
types.Var.SetKind(kindVar)
types.Var.SizeType = 1
btf := marshalBTF(&types, strings, internal.NativeEndian)
fd, err := sys.BtfLoad(&sys.BtfLoadAttr{
Btf: sys.NewSlicePointer(btf),
BtfSize: uint32(len(btf)),
})
if errors.Is(err, unix.EINVAL) || errors.Is(err, unix.EPERM) {
// Treat both EINVAL and EPERM as not supported: loading the program
// might still succeed without BTF.
return internal.ErrNotSupported
}
if err != nil {
return err
}
fd.Close()
return nil
})
var haveFuncLinkage = internal.FeatureTest("BTF func linkage", "5.6", func() error {
if err := haveBTF(); err != nil {
return err
}
var (
types struct {
FuncProto btfType
Func btfType
}
strings = []byte{0, 'a', 0}
)
types.FuncProto.SetKind(kindFuncProto)
types.Func.SetKind(kindFunc)
types.Func.SizeType = 1 // aka FuncProto
types.Func.NameOff = 1
types.Func.SetLinkage(GlobalFunc)
btf := marshalBTF(&types, strings, internal.NativeEndian)
fd, err := sys.BtfLoad(&sys.BtfLoadAttr{
Btf: sys.NewSlicePointer(btf),
BtfSize: uint32(len(btf)),
})
if errors.Is(err, unix.EINVAL) {
return internal.ErrNotSupported
}
if err != nil {
return err
}
fd.Close()
return nil
})