syscall: swap src, dest arguments in convertFromDirents11 for FreeBSD

make fixedSize, oldFixedSize constants.
use st instead of stat for function arg so that we do not shadow the stat() function.

dstPos+reclen == len(buf) is a valid write location, update the break condition.

Change-Id: I55f9210f54d24a3f9cda1ebab52437436254f8f4
Reviewed-on: https://go-review.googlesource.com/c/143637
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/export_freebsd_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.
+
+package syscall
+
+type Dirent_freebsd11 = dirent_freebsd11
+
+var (
+	Roundup              = roundup
+	ConvertFromDirents11 = convertFromDirents11
+)

src/syscall/syscall_freebsd.go

@@ -223,31 +223,31 @@ func Fstat(fd int, st *Stat_t) (err error) {
 	return nil
 }
 
-func Statfs(path string, stat *Statfs_t) (err error) {
+func Statfs(path string, st *Statfs_t) (err error) {
 	var oldStatfs statfs_freebsd11_t
 	if supportsABI(_ino64First) {
-		return statfs_freebsd12(path, stat)
+		return statfs_freebsd12(path, st)
 	}
 	err = statfs(path, &oldStatfs)
 	if err != nil {
 		return err
 	}
 
-	stat.convertFrom(&oldStatfs)
+	st.convertFrom(&oldStatfs)
 	return nil
 }
 
-func Fstatfs(fd int, stat *Statfs_t) (err error) {
+func Fstatfs(fd int, st *Statfs_t) (err error) {
 	var oldStatfs statfs_freebsd11_t
 	if supportsABI(_ino64First) {
-		return fstatfs_freebsd12(fd, stat)
+		return fstatfs_freebsd12(fd, st)
 	}
 	err = fstatfs(fd, &oldStatfs)
 	if err != nil {
 		return err
 	}
 
-	stat.convertFrom(&oldStatfs)
+	st.convertFrom(&oldStatfs)
 	return nil
 }
 
@@ -262,7 +262,7 @@ func Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error) {
 	oldBuf := make([]byte, oldBufLen)
 	n, err = getdirentries(fd, oldBuf, basep)
 	if err == nil && n > 0 {
-		n = convertFromDirents11(oldBuf[:n], buf)
+		n = convertFromDirents11(buf, oldBuf[:n])
 	}
 	return
 }
@@ -344,17 +344,20 @@ func (s *Statfs_t) convertFrom(old *statfs_freebsd11_t) {
 	copy(s.Mntonname[:], old.Mntonname[:n])
 }
 
-func convertFromDirents11(old []byte, buf []byte) int {
-	oldFixedSize := int(unsafe.Offsetof((*dirent_freebsd11)(nil).Name))
-	fixedSize := int(unsafe.Offsetof((*Dirent)(nil).Name))
-	srcPos := 0
+func convertFromDirents11(buf []byte, old []byte) int {
+	const (
+		fixedSize    = int(unsafe.Offsetof(Dirent{}.Name))
+		oldFixedSize = int(unsafe.Offsetof(dirent_freebsd11{}.Name))
+	)
+
 	dstPos := 0
+	srcPos := 0
 	for dstPos+fixedSize < len(buf) && srcPos+oldFixedSize < len(old) {
-		srcDirent := (*dirent_freebsd11)(unsafe.Pointer(&old[srcPos]))
 		dstDirent := (*Dirent)(unsafe.Pointer(&buf[dstPos]))
+		srcDirent := (*dirent_freebsd11)(unsafe.Pointer(&old[srcPos]))
 
 		reclen := roundup(fixedSize+int(srcDirent.Namlen)+1, 8)
-		if dstPos+reclen >= len(buf) {
+		if dstPos+reclen > len(buf) {
 			break
 		}
 

src/syscall/syscall_freebsd_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 freebsd
+
+package syscall_test
+
+import (
+	"fmt"
+	"syscall"
+	"testing"
+	"unsafe"
+)
+
+func TestConvertFromDirent11(t *testing.T) {
+	const (
+		filenameFmt  = "%04d"
+		numFiles     = 64
+		fixedHdrSize = int(unsafe.Offsetof(syscall.Dirent_freebsd11{}.Name))
+	)
+
+	namlen := len(fmt.Sprintf(filenameFmt, 0))
+	reclen := syscall.Roundup(fixedHdrSize+namlen+1, 4)
+	old := make([]byte, numFiles*reclen)
+	for i := 0; i < numFiles; i++ {
+		dent := syscall.Dirent_freebsd11{
+			Fileno: uint32(i + 1),
+			Reclen: uint16(reclen),
+			Type:   syscall.DT_REG,
+			Namlen: uint8(namlen),
+		}
+		rec := make([]byte, reclen)
+		copy(rec, (*[fixedHdrSize]byte)(unsafe.Pointer(&dent))[:])
+		copy(rec[fixedHdrSize:], fmt.Sprintf(filenameFmt, i+1))
+		copy(old[i*reclen:], rec)
+	}
+
+	buf := make([]byte, 2*len(old))
+	n := syscall.ConvertFromDirents11(buf, old)
+
+	names := make([]string, 0, numFiles)
+	_, _, names = syscall.ParseDirent(buf[:n], -1, names)
+
+	if len(names) != numFiles {
+		t.Errorf("expected %d files, have %d; names: %q", numFiles, len(names), names)
+	}
+
+	for i, name := range names {
+		if expected := fmt.Sprintf(filenameFmt, i+1); name != expected {
+			t.Errorf("expected names[%d] to be %q; got %q", i, expected, name)
+		}
+	}
+}