1 files changed, 84 insertions, 0 deletions
diff --git a/src/bytes/boundary_test.go b/src/bytes/boundary_test.go
new file mode 100644
index 0000000..ea84f1e
--- /dev/null
+++ b/src/bytes/boundary_test.go
@@ -0,0 +1,84 @@
+// Copyright 2017 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.
+//
+// +build linux
+
+package bytes_test
+
+import (
+	. "bytes"
+	"syscall"
+	"testing"
+)
+
+// This file tests the situation where byte operations are checking
+// data very near to a page boundary. We want to make sure those
+// operations do not read across the boundary and cause a page
+// fault where they shouldn't.
+
+// These tests run only on linux. The code being tested is
+// not OS-specific, so it does not need to be tested on all
+// operating systems.
+
+// dangerousSlice returns a slice which is immediately
+// preceded and followed by a faulting page.
+func dangerousSlice(t *testing.T) []byte {
+	pagesize := syscall.Getpagesize()
+	b, err := syscall.Mmap(0, 0, 3*pagesize, syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_ANONYMOUS|syscall.MAP_PRIVATE)
+	if err != nil {
+		t.Fatalf("mmap failed %s", err)
+	}
+	err = syscall.Mprotect(b[:pagesize], syscall.PROT_NONE)
+	if err != nil {
+		t.Fatalf("mprotect low failed %s\n", err)
+	}
+	err = syscall.Mprotect(b[2*pagesize:], syscall.PROT_NONE)
+	if err != nil {
+		t.Fatalf("mprotect high failed %s\n", err)
+	}
+	return b[pagesize : 2*pagesize]
+}
+
+func TestEqualNearPageBoundary(t *testing.T) {
+	t.Parallel()
+	b := dangerousSlice(t)
+	for i := range b {
+		b[i] = 'A'
+	}
+	for i := 0; i <= len(b); i++ {
+		Equal(b[:i], b[len(b)-i:])
+		Equal(b[len(b)-i:], b[:i])
+	}
+}
+
+func TestIndexByteNearPageBoundary(t *testing.T) {
+	t.Parallel()
+	b := dangerousSlice(t)
+	for i := range b {
+		idx := IndexByte(b[i:], 1)
+		if idx != -1 {
+			t.Fatalf("IndexByte(b[%d:])=%d, want -1\n", i, idx)
+		}
+	}
+}
+
+func TestIndexNearPageBoundary(t *testing.T) {
+	t.Parallel()
+	var q [64]byte
+	b := dangerousSlice(t)
+	if len(b) > 256 {
+		// Only worry about when we're near the end of a page.
+		b = b[len(b)-256:]
+	}
+	for j := 1; j < len(q); j++ {
+		q[j-1] = 1 // difference is only found on the last byte
+		for i := range b {
+			idx := Index(b[i:], q[:j])
+			if idx != -1 {
+				t.Fatalf("Index(b[%d:], q[:%d])=%d, want -1\n", i, j, idx)
+			}
+		}
+		q[j-1] = 0
+	}
+}