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Copy pathlinker.go
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244 lines (203 loc) · 6.6 KB
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package ebpf
import (
"encoding/binary"
"errors"
"fmt"
"sync"
"github.com/cilium/ebpf/asm"
"github.com/cilium/ebpf/btf"
"github.com/cilium/ebpf/internal"
)
// splitSymbols splits insns into subsections delimited by Symbol Instructions.
// insns cannot be empty and must start with a Symbol Instruction.
//
// The resulting map is indexed by Symbol name.
func splitSymbols(insns asm.Instructions) (map[string]asm.Instructions, error) {
if len(insns) == 0 {
return nil, errors.New("insns is empty")
}
if insns[0].Symbol() == "" {
return nil, errors.New("insns must start with a Symbol")
}
var name string
progs := make(map[string]asm.Instructions)
for _, ins := range insns {
if sym := ins.Symbol(); sym != "" {
if progs[sym] != nil {
return nil, fmt.Errorf("insns contains duplicate Symbol %s", sym)
}
name = sym
}
progs[name] = append(progs[name], ins)
}
return progs, nil
}
// The linker is responsible for resolving bpf-to-bpf calls between programs
// within an ELF. Each BPF program must be a self-contained binary blob,
// so when an instruction in one ELF program section wants to jump to
// a function in another, the linker needs to pull in the bytecode
// (and BTF info) of the target function and concatenate the instruction
// streams.
//
// Later on in the pipeline, all call sites are fixed up with relative jumps
// within this newly-created instruction stream to then finally hand off to
// the kernel with BPF_PROG_LOAD.
//
// Each function is denoted by an ELF symbol and the compiler takes care of
// register setup before each jump instruction.
// hasFunctionReferences returns true if insns contains one or more bpf2bpf
// function references.
func hasFunctionReferences(insns asm.Instructions) bool {
for _, i := range insns {
if i.IsFunctionReference() {
return true
}
}
return false
}
// applyRelocations collects and applies any CO-RE relocations in insns.
//
// Passing a nil target will relocate against the running kernel. insns are
// modified in place.
func applyRelocations(insns asm.Instructions, target *btf.Spec, bo binary.ByteOrder) error {
var relos []*btf.CORERelocation
var reloInsns []*asm.Instruction
iter := insns.Iterate()
for iter.Next() {
if relo := btf.CORERelocationMetadata(iter.Ins); relo != nil {
relos = append(relos, relo)
reloInsns = append(reloInsns, iter.Ins)
}
}
if len(relos) == 0 {
return nil
}
if bo == nil {
bo = internal.NativeEndian
}
target, err := maybeLoadKernelBTF(target)
if err != nil {
return err
}
fixups, err := btf.CORERelocate(relos, target, bo)
if err != nil {
return err
}
for i, fixup := range fixups {
if err := fixup.Apply(reloInsns[i]); err != nil {
return fmt.Errorf("apply fixup %s: %w", &fixup, err)
}
}
return nil
}
// flattenPrograms resolves bpf-to-bpf calls for a set of programs.
//
// Links all programs in names by modifying their ProgramSpec in progs.
func flattenPrograms(progs map[string]*ProgramSpec, names []string) {
// Pre-calculate all function references.
refs := make(map[*ProgramSpec][]string)
for _, prog := range progs {
refs[prog] = prog.Instructions.FunctionReferences()
}
// Create a flattened instruction stream, but don't modify progs yet to
// avoid linking multiple times.
flattened := make([]asm.Instructions, 0, len(names))
for _, name := range names {
flattened = append(flattened, flattenInstructions(name, progs, refs))
}
// Finally, assign the flattened instructions.
for i, name := range names {
progs[name].Instructions = flattened[i]
}
}
// flattenInstructions resolves bpf-to-bpf calls for a single program.
//
// Flattens the instructions of prog by concatenating the instructions of all
// direct and indirect dependencies.
//
// progs contains all referenceable programs, while refs contain the direct
// dependencies of each program.
func flattenInstructions(name string, progs map[string]*ProgramSpec, refs map[*ProgramSpec][]string) asm.Instructions {
prog := progs[name]
insns := make(asm.Instructions, len(prog.Instructions))
copy(insns, prog.Instructions)
// Add all direct references of prog to the list of to be linked programs.
pending := make([]string, len(refs[prog]))
copy(pending, refs[prog])
// All references for which we've appended instructions.
linked := make(map[string]bool)
// Iterate all pending references. We can't use a range since pending is
// modified in the body below.
for len(pending) > 0 {
var ref string
ref, pending = pending[0], pending[1:]
if linked[ref] {
// We've already linked this ref, don't append instructions again.
continue
}
progRef := progs[ref]
if progRef == nil {
// We don't have instructions that go with this reference. This
// happens when calling extern functions.
continue
}
insns = append(insns, progRef.Instructions...)
linked[ref] = true
// Make sure we link indirect references.
pending = append(pending, refs[progRef]...)
}
return insns
}
// fixupAndValidate is called by the ELF reader right before marshaling the
// instruction stream. It performs last-minute adjustments to the program and
// runs some sanity checks before sending it off to the kernel.
func fixupAndValidate(insns asm.Instructions) error {
iter := insns.Iterate()
for iter.Next() {
ins := iter.Ins
// Map load was tagged with a Reference, but does not contain a Map pointer.
if ins.IsLoadFromMap() && ins.Reference() != "" && ins.Map() == nil {
return fmt.Errorf("instruction %d: map %s: %w", iter.Index, ins.Reference(), asm.ErrUnsatisfiedMapReference)
}
fixupProbeReadKernel(ins)
}
return nil
}
// fixupProbeReadKernel replaces calls to bpf_probe_read_{kernel,user}(_str)
// with bpf_probe_read(_str) on kernels that don't support it yet.
func fixupProbeReadKernel(ins *asm.Instruction) {
if !ins.IsBuiltinCall() {
return
}
// Kernel supports bpf_probe_read_kernel, nothing to do.
if haveProbeReadKernel() == nil {
return
}
switch asm.BuiltinFunc(ins.Constant) {
case asm.FnProbeReadKernel, asm.FnProbeReadUser:
ins.Constant = int64(asm.FnProbeRead)
case asm.FnProbeReadKernelStr, asm.FnProbeReadUserStr:
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
}