syscallcompat: implement Getdents()

The Readdir function provided by os is inherently slow because
it calls Lstat on all files.

Getdents gives us all the information we need, but does not have
a proper wrapper in the stdlib.

Implement the "Getdents()" wrapper function that calls
syscall.Getdents() and parses the returned byte blob to a
fuse.DirEntry slice.

1 files changed, 138 insertions, 0 deletions

diff --git a/internal/syscallcompat/getdents_linux.go b/internal/syscallcompat/getdents_linux.go
new file mode 100644
index 0000000..97cd75f
--- /dev/null
+++ b/internal/syscallcompat/getdents_linux.go
@@ -0,0 +1,138 @@
+// +build linux
+
+package syscallcompat
+
+// Other implementations of getdents in Go:
+// https://github.com/ericlagergren/go-gnulib/blob/cb7a6e136427e242099b2c29d661016c19458801/dirent/getdents_unix.go
+// https://github.com/golang/tools/blob/5831d16d18029819d39f99bdc2060b8eff410b6b/imports/fastwalk_unix.go
+
+import (
+	"bytes"
+	"syscall"
+	"unsafe"
+
+	"github.com/hanwen/go-fuse/fuse"
+
+	"github.com/rfjakob/gocryptfs/internal/tlog"
+)
+
+// HaveGetdents is true if we have a working implementation of Getdents
+const HaveGetdents = true
+
+const sizeofDirent = int(unsafe.Sizeof(syscall.Dirent{}))
+
+// Getdents wraps syscall.Getdents and converts the result to []fuse.DirEntry.
+// The function takes a path instead of an fd because we need to be able to
+// call Lstat on files. Fstatat is not yet available in Go as of v1.9:
+// https://github.com/golang/go/issues/14216
+func Getdents(dir string) ([]fuse.DirEntry, error) {
+	fd, err := syscall.Open(dir, syscall.O_RDONLY, 0)
+	if err != nil {
+		return nil, err
+	}
+	defer syscall.Close(fd)
+	// Collect syscall result in smartBuf.
+	// "bytes.Buffer" is smart about expanding the capacity and avoids the
+	// exponential runtime of simple append().
+	var smartBuf bytes.Buffer
+	tmp := make([]byte, 10000)
+	for {
+		n, err := syscall.Getdents(fd, tmp)
+		if err != nil {
+			return nil, err
+		}
+		if n == 0 {
+			break
+		}
+		smartBuf.Write(tmp[:n])
+	}
+	// Make sure we have at least Sizeof(Dirent) of zeros after the last
+	// entry. This prevents a cast to Dirent from reading past the buffer.
+	smartBuf.Grow(sizeofDirent)
+	buf := smartBuf.Bytes()
+	// Count the number of directory entries in the buffer so we can allocate
+	// a fuse.DirEntry slice of the correct size at once.
+	var numEntries, offset int
+	for offset < len(buf) {
+		s := *(*syscall.Dirent)(unsafe.Pointer(&buf[offset]))
+		if s.Reclen == 0 {
+			tlog.Warn.Printf("Getdents: corrupt entry #%d: Reclen=0 at offset=%d. Returning EBADR",
+				numEntries, offset)
+			// EBADR = Invalid request descriptor
+			return nil, syscall.EBADR
+		}
+		if int(s.Reclen) > sizeofDirent {
+			tlog.Warn.Printf("Getdents: corrupt entry #%d: Reclen=%d > %d. Returning EBADR",
+				numEntries, sizeofDirent, s.Reclen)
+			return nil, syscall.EBADR
+		}
+		offset += int(s.Reclen)
+		numEntries++
+	}
+	// Parse the buffer into entries
+	entries := make([]fuse.DirEntry, 0, numEntries)
+	offset = 0
+	for offset < len(buf) {
+		s := *(*syscall.Dirent)(unsafe.Pointer(&buf[offset]))
+		name, err := getdentsName(s)
+		if err != nil {
+			return nil, err
+		}
+		offset += int(s.Reclen)
+		if name == "." || name == ".." {
+			// os.File.Readdir() drops "." and "..". Let's be compatible.
+			continue
+		}
+		mode, err := convertDType(s.Type, dir+"/"+name)
+		if err != nil {
+			// The file may have been deleted in the meantime. Just skip it
+			// and go on.
+			continue
+		}
+		entries = append(entries, fuse.DirEntry{
+			Ino:  s.Ino,
+			Mode: mode,
+			Name: name,
+		})
+	}
+	return entries, nil
+}
+
+// getdentsName extracts the filename from a Dirent struct and returns it as
+// a Go string.
+func getdentsName(s syscall.Dirent) (string, error) {
+	// After the loop, l contains the index of the first '\0'.
+	l := 0
+	for l = range s.Name {
+		if s.Name[l] == 0 {
+			break
+		}
+	}
+	if l < 1 {
+		tlog.Warn.Printf("Getdents: invalid name length l=%d. Returning EBADR", l)
+		// EBADR = Invalid request descriptor
+		return "", syscall.EBADR
+	}
+	// Copy to byte slice.
+	name := make([]byte, l)
+	for i := range name {
+		name[i] = byte(s.Name[i])
+	}
+	return string(name), nil
+}
+
+// convertDType converts a Dirent.Type to at Stat_t.Mode value.
+func convertDType(dtype uint8, file string) (uint32, error) {
+	if dtype != syscall.DT_UNKNOWN {
+		// Shift up by four octal digits = 12 bits
+		return uint32(dtype) << 12, nil
+	}
+	// DT_UNKNOWN: we have to call Lstat()
+	var st syscall.Stat_t
+	err := syscall.Lstat(file, &st)
+	if err != nil {
+		return 0, err
+	}
+	// The S_IFMT bit mask extracts the file type from the mode.
+	return st.Mode & syscall.S_IFMT, nil
+}