[dev.link] cmd/link: fix hash collision check

For content-addressable symbols, we build its content hash based
on the symbol data and relocations. When the compiler builds the
symbol data, it may not always include the trailing zeros, e.g.
the data of [10]int64{1,2,3} is only the first 24 bytes.
Therefore, we may end up with symbols with the same contents
(thus same hash) but different sizes. This is not actually a hash
collision. In this case, we can deduplicate them and keep the one
with the larger size.

Change-Id: If6834542d7914cc00f917d7db151955e5aee6f30
Reviewed-on: https://go-review.googlesource.com/c/go/+/243718
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

src/cmd/link/internal/loader/loader.go

@@ -434,24 +434,46 @@ func (l *Loader) addSym(name string, ver int, r *oReader, li uint32, kind int, o
 		l.symsByName[ver][name] = i
 		addToGlobal()
 		return i, true
-	case hashed64Def:
+	case hashed64Def, hashedDef:
 		// Hashed (content-addressable) symbol. Check the hash
 		// but don't add to name lookup table, as they are not
 		// referenced by name. Also no need to do overwriting
 		// check, as same hash indicates same content.
-		hash := r.Hash64(li - uint32(r.ndef))
+		var checkHash func() (symSizeAlign, bool)
+		var addToHashMap func(symSizeAlign)
+		var h64 uint64         // only used for hashed64Def
+		var h *goobj2.HashType // only used for hashedDef
+		if kind == hashed64Def {
+			checkHash = func() (symSizeAlign, bool) {
+				h64 = r.Hash64(li - uint32(r.ndef))
+				s, existed := l.hashed64Syms[h64]
+				return s, existed
+			}
+			addToHashMap = func(ss symSizeAlign) { l.hashed64Syms[h64] = ss }
+		} else {
+			checkHash = func() (symSizeAlign, bool) {
+				h = r.Hash(li - uint32(r.ndef+r.nhashed64def))
+				s, existed := l.hashedSyms[*h]
+				return s, existed
+			}
+			addToHashMap = func(ss symSizeAlign) { l.hashedSyms[*h] = ss }
+		}
 		siz := osym.Siz()
 		align := osym.Align()
-		if s, existed := l.hashed64Syms[hash]; existed {
-			// For short symbols, the content hash is the identity function of the
-			// 8 bytes, and trailing zeros doesn't change the hash value, e.g.
+		if s, existed := checkHash(); existed {
+			// The content hash is built from symbol data and relocations. In the
+			// object file, the symbol data may not always contain trailing zeros,
+			// e.g. for [5]int{1,2,3} and [100]int{1,2,3}, the data is same
+			// (although the size is different).
+			// Also, for short symbols, the content hash is the identity function of
+			// the 8 bytes, and trailing zeros doesn't change the hash value, e.g.
 			// hash("A") == hash("A\0\0\0").
 			// So when two symbols have the same hash, we need to use the one with
-			// larget size.
+			// larger size.
 			if siz <= s.size {
 				if align > s.align { // we need to use the biggest alignment
 					l.SetSymAlign(s.sym, int32(align))
-					l.hashed64Syms[hash] = symSizeAlign{s.sym, s.size, align}
+					addToHashMap(symSizeAlign{s.sym, s.size, align})
 				}
 			} else {
 				// New symbol has larger size, use the new one. Rewrite the index mapping.
@@ -460,34 +482,11 @@ func (l *Loader) addSym(name string, ver int, r *oReader, li uint32, kind int, o
 					align = s.align // keep the biggest alignment
 					l.SetSymAlign(s.sym, int32(align))
 				}
-				l.hashed64Syms[hash] = symSizeAlign{s.sym, siz, align}
+				addToHashMap(symSizeAlign{s.sym, siz, align})
 			}
 			return s.sym, false
 		}
-		l.hashed64Syms[hash] = symSizeAlign{i, siz, align}
-		addToGlobal()
-		return i, true
-	case hashedDef:
-		// Hashed (content-addressable) symbol. Check the hash
-		// but don't add to name lookup table, as they are not
-		// referenced by name. Also no need to do overwriting
-		// check, as same hash indicates same content.
-		hash := r.Hash(li - uint32(r.ndef+r.nhashed64def))
-		if s, existed := l.hashedSyms[*hash]; existed {
-			if s.size != osym.Siz() {
-				fmt.Printf("hash collision: %v (size %d) and %v (size %d), hash %x\n", l.SymName(s.sym), s.size, osym.Name(r.Reader), osym.Siz(), *hash)
-				panic("hash collision")
-			}
-			if l.flags&FlagStrictDups != 0 {
-				l.checkdup(name, r, li, s.sym)
-			}
-			if a := osym.Align(); a > s.align { // we need to use the biggest alignment
-				l.SetSymAlign(s.sym, int32(a))
-				l.hashedSyms[*hash] = symSizeAlign{s.sym, s.size, a}
-			}
-			return s.sym, false
-		}
-		l.hashedSyms[*hash] = symSizeAlign{i, osym.Siz(), osym.Align()}
+		addToHashMap(symSizeAlign{i, siz, align})
 		addToGlobal()
 		return i, true
 	}

src/cmd/link/link_test.go

@@ -778,3 +778,23 @@ func TestPErsrc(t *testing.T) {
 		t.Fatalf("binary does not contain expected content")
 	}
 }
+
+func TestContentAddressableSymbols(t *testing.T) {
+	// Test that the linker handles content-addressable symbols correctly.
+	testenv.MustHaveGoBuild(t)
+
+	t.Parallel()
+
+	tmpdir, err := ioutil.TempDir("", "TestContentAddressableSymbols")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tmpdir)
+
+	src := filepath.Join("testdata", "testHashedSyms", "p.go")
+	cmd := exec.Command(testenv.GoToolPath(t), "run", src)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Errorf("command %s failed: %v\n%s", cmd, err, out)
+	}
+}

src/cmd/link/testdata/testHashedSyms/p.go

@@ -0,0 +1,33 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This test case contains two static temps (the array literals)
+// with same contents but different sizes. The linker should not
+// report a hash collision. The linker can (and actually does)
+// dedup the two symbols, by keeping the larger symbol. The dedup
+// is not a requirement for correctness and not checked in this test.
+// We do check the emitted symbol contents are correct, though.
+
+package main
+
+func main() {
+	F([10]int{1, 2, 3, 4, 5, 6}, [20]int{1, 2, 3, 4, 5, 6})
+}
+
+//go:noinline
+func F(x, y interface{}) {
+	x1 := x.([10]int)
+	y1 := y.([20]int)
+	for i := range y1 {
+		if i < 6 {
+			if x1[i] != i+1 || y1[i] != i+1 {
+				panic("FAIL")
+			}
+		} else {
+			if (i < len(x1) && x1[i] != 0) || y1[i] != 0 {
+				panic("FAIL")
+			}
+		}
+	}
+}