eth/downloader: retrieve pivot header from local chain if necessary

eth/downloader/beaconsync.go

@@ -265,7 +265,11 @@ func (d *Downloader) fetchBeaconHeaders(from uint64) error {
  hashes = make([]common.Hash, 0, maxHeadersProcess)
  )
  for i := 0; i < maxHeadersProcess && from <= head.Number.Uint64(); i++ {
- headers = append(headers, d.skeleton.Header(from))
+ header := d.skeleton.Header(from)
+ if header == nil {
+ header = d.lightchain.GetHeaderByNumber(from)
+ }
+ headers = append(headers, header)
  hashes = append(hashes, headers[i].Hash())
  from++
  }

eth/downloader/downloader.go

@@ -78,6 +78,7 @@ var (
  errCanceled = errors.New("syncing canceled (requested)")
  errTooOld = errors.New("peer's protocol version too old")
  errNoAncestorFound = errors.New("no common ancestor found")
+ errNoPivotHeader = errors.New("pivot header is not found")
  ErrMergeTransition = errors.New("legacy sync reached the merge")
 )
 
@@ -158,6 +159,9 @@ type LightChain interface {
  // GetHeaderByHash retrieves a header from the local chain.
  GetHeaderByHash(common.Hash) *types.Header
 
+ // GetHeaderByNumber retrieves a block header from the local chain by number.
+ GetHeaderByNumber(number uint64) *types.Header
+
  // CurrentHeader retrieves the head header from the local chain.
  CurrentHeader() *types.Header
 
@@ -477,7 +481,20 @@ func (d *Downloader) syncWithPeer(p *peerConnection, hash common.Hash, td, ttd *
  return err
  }
  if latest.Number.Uint64() > uint64(fsMinFullBlocks) {
- pivot = d.skeleton.Header(latest.Number.Uint64() - uint64(fsMinFullBlocks))
+ number := latest.Number.Uint64() - uint64(fsMinFullBlocks)
+
+ // Retrieve the pivot header from the skeleton chain segment but
+ // fallback to local chain if it's not found in skeleton space.
+ if pivot = d.skeleton.Header(number); pivot == nil {
+ pivot = d.lightchain.GetHeaderByNumber(number)
+ }
+ // Print an error log and return directly in case the pivot header
+ // is still not found. It means the skeleton chain is not linked
+ // correctly with local chain.
+ if pivot == nil {
+ log.Error("Pivot header is not found", "number", number)
+ return errNoPivotHeader
+ }
  }
  }
  // If no pivot block was returned, the head is below the min full block

eth/downloader/downloader_test.go

@@ -56,6 +56,11 @@ type downloadTester struct {
 
 // newTester creates a new downloader test mocker.
 func newTester() *downloadTester {
+ return newTesterWithNotification(nil)
+}
+
+// newTester creates a new downloader test mocker.
+func newTesterWithNotification(success func()) *downloadTester {
  freezer, err := ioutil.TempDir("", "")
  if err != nil {
  panic(err)
@@ -75,7 +80,7 @@ func newTester() *downloadTester {
  chain: chain,
  peers: make(map[string]*downloadTesterPeer),
  }
- tester.downloader = New(0, db, new(event.TypeMux), tester.chain, nil, tester.dropPeer, nil)
+ tester.downloader = New(0, db, new(event.TypeMux), tester.chain, nil, tester.dropPeer, success)
  return tester
 }
 
@@ -1368,3 +1373,51 @@ func testCheckpointEnforcement(t *testing.T, protocol uint, mode SyncMode) {
  assertOwnChain(t, tester, len(chain.blocks))
  }
 }
+
+// Tests that peers below a pre-configured checkpoint block are prevented from
+// being fast-synced from, avoiding potential cheap eclipse attacks.
+func TestBeaconSync66Full(t *testing.T) { testBeaconSync(t, eth.ETH66, FullSync) }
+func TestBeaconSync66Snap(t *testing.T) { testBeaconSync(t, eth.ETH66, SnapSync) }
+
+func testBeaconSync(t *testing.T, protocol uint, mode SyncMode) {
+ //log.Root().SetHandler(log.LvlFilterHandler(log.LvlInfo, log.StreamHandler(os.Stderr, log.TerminalFormat(true))))
+
+ var cases = []struct {
+ name string // The name of testing scenario
+ local int // The length of local chain(canonical chain assumed), 0 means genesis is the head
+ }{
+ {name: "Beacon sync since genesis", local: 0},
+ {name: "Beacon sync with short local chain", local: 1},
+ {name: "Beacon sync with long local chain", local: blockCacheMaxItems - 15 - fsMinFullBlocks/2},
+ {name: "Beacon sync with full local chain", local: blockCacheMaxItems - 15 - 1},
+ }
+ for _, c := range cases {
+ t.Run(c.name, func(t *testing.T) {
+ success := make(chan struct{})
+ tester := newTesterWithNotification(func() {
+ close(success)
+ })
+ defer tester.terminate()
+
+ chain := testChainBase.shorten(blockCacheMaxItems - 15)
+ tester.newPeer("peer", protocol, chain.blocks[1:])
+
+ // Build the local chain segment if it's required
+ if c.local > 0 {
+ tester.chain.InsertChain(chain.blocks[1 : c.local+1])
+ }
+ if err := tester.downloader.BeaconSync(mode, chain.blocks[len(chain.blocks)-1].Header()); err != nil {
+ t.Fatalf("Failed to beacon sync chain %v %v", c.name, err)
+ }
+ select {
+ case <-success:
+ // Ok, downloader fully cancelled after sync cycle
+ if bs := int(tester.chain.CurrentBlock().NumberU64()) + 1; bs != len(chain.blocks) {
+ t.Fatalf("synchronised blocks mismatch: have %v, want %v", bs, len(chain.blocks))
+ }
+ case <-time.NewTimer(time.Second * 3).C:
+ t.Fatalf("Failed to sync chain in three seconds")
+ }
+ })
+ }
+}