syscall: correctly pad with NUL in FreeBSD convertFromDirents11

We weren't writing a terminating NUL after dstDirent.Namlen bytes of dstDirent.Name.
And we weren't filling the possible additional bytes until dstDirent.Reclen.

Fixes #28131

Change-Id: Id691c25225795c0dbb0d7004bfca7bb7fc706de9
Reviewed-on: https://go-review.googlesource.com/c/141297
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/dirent_bsd_test.go

+// Copyright 2018 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 darwin dragonfly freebsd netbsd openbsd
+
+package syscall_test
+
+import (
+	"bytes"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"sort"
+	"strconv"
+	"strings"
+	"syscall"
+	"testing"
+)
+
+func TestDirent(t *testing.T) {
+	const (
+		direntBufSize   = 2048
+		filenameMinSize = 11
+	)
+
+	d, err := ioutil.TempDir("", "dirent-test")
+	if err != nil {
+		t.Fatalf("tempdir: %v", err)
+	}
+	defer os.RemoveAll(d)
+	t.Logf("tmpdir: %s", d)
+
+	for i, c := range []byte("0123456789") {
+		name := string(bytes.Repeat([]byte{c}, filenameMinSize+i))
+		err = ioutil.WriteFile(filepath.Join(d, name), nil, 0644)
+		if err != nil {
+			t.Fatalf("writefile: %v", err)
+		}
+	}
+
+	buf := bytes.Repeat([]byte("DEADBEAF"), direntBufSize/8)
+	fd, err := syscall.Open(d, syscall.O_RDONLY, 0)
+	defer syscall.Close(fd)
+	if err != nil {
+		t.Fatalf("syscall.open: %v", err)
+	}
+	n, err := syscall.ReadDirent(fd, buf)
+	if err != nil {
+		t.Fatalf("syscall.readdir: %v", err)
+	}
+	buf = buf[:n]
+
+	names := make([]string, 0, 10)
+	for len(buf) > 0 {
+		var bc int
+		bc, _, names = syscall.ParseDirent(buf, -1, names)
+		buf = buf[bc:]
+	}
+
+	sort.Strings(names)
+	t.Logf("names: %q", names)
+
+	if len(names) != 10 {
+		t.Errorf("got %d names; expected 10", len(names))
+	}
+	for i, name := range names {
+		ord, err := strconv.Atoi(name[:1])
+		if err != nil {
+			t.Fatalf("names[%d] is non-integer %q: %v", i, names[i], err)
+		}
+		if expected := string(strings.Repeat(name[:1], filenameMinSize+ord)); name != expected {
+			t.Errorf("names[%d] is %q (len %d); expected %q (len %d)", i, name, len(name), expected, len(expected))
+		}
+	}
+}

src/syscall/syscall_freebsd.go

@@ -344,18 +344,17 @@ func (s *Statfs_t) convertFrom(old *statfs_freebsd11_t) {
 	copy(s.Mntonname[:], old.Mntonname[:n])
 }
 
-func convertFromDirents11(oldBuf []byte, buf []byte) int {
-	src := unsafe.Pointer(&oldBuf[0])
-	esrc := unsafe.Pointer(uintptr(src) + uintptr(len(oldBuf)))
-	dst := unsafe.Pointer(&buf[0])
-	edst := unsafe.Pointer(uintptr(dst) + uintptr(len(buf)))
-
-	for uintptr(src) < uintptr(esrc) && uintptr(dst) < uintptr(edst) {
-		srcDirent := (*dirent_freebsd11)(src)
-		dstDirent := (*Dirent)(dst)
-
-		reclen := roundup(int(unsafe.Offsetof(dstDirent.Name)+uintptr(srcDirent.Namlen)+1), 8)
-		if uintptr(dst)+uintptr(reclen) >= uintptr(edst) {
+func convertFromDirents11(old []byte, buf []byte) int {
+	oldFixedSize := int(unsafe.Offsetof((*dirent_freebsd11)(nil).Name))
+	fixedSize := int(unsafe.Offsetof((*Dirent)(nil).Name))
+	srcPos := 0
+	dstPos := 0
+	for dstPos+fixedSize < len(buf) && srcPos+oldFixedSize < len(old) {
+		srcDirent := (*dirent_freebsd11)(unsafe.Pointer(&old[srcPos]))
+		dstDirent := (*Dirent)(unsafe.Pointer(&buf[dstPos]))
+
+		reclen := roundup(fixedSize+int(srcDirent.Namlen)+1, 8)
+		if dstPos+reclen >= len(buf) {
 			break
 		}
 
@@ -367,18 +366,17 @@ func convertFromDirents11(oldBuf []byte, buf []byte) int {
 		dstDirent.Namlen = uint16(srcDirent.Namlen)
 		dstDirent.Pad1 = 0
 
-		sl := srcDirent.Name[:]
-		n := clen(*(*[]byte)(unsafe.Pointer(&sl)))
-		copy(dstDirent.Name[:], srcDirent.Name[:n])
-		for i := n; i < int(dstDirent.Namlen); i++ {
-			dstDirent.Name[i] = 0
+		copy(dstDirent.Name[:], srcDirent.Name[:srcDirent.Namlen])
+		padding := buf[dstPos+fixedSize+int(dstDirent.Namlen) : dstPos+reclen]
+		for i := range padding {
+			padding[i] = 0
 		}
 
-		src = unsafe.Pointer(uintptr(src) + uintptr(srcDirent.Reclen))
-		dst = unsafe.Pointer(uintptr(dst) + uintptr(reclen))
+		dstPos += int(dstDirent.Reclen)
+		srcPos += int(srcDirent.Reclen)
 	}
 
-	return int(uintptr(dst) - uintptr(unsafe.Pointer((&buf[0]))))
+	return dstPos
 }
 
 /*