fuse: fix mounting with MaxWrite < 8kiB

The readPool size was calculated from MaxWrite not taking
into account that the kernel fails all reads below 8 kiB.

Fix that by always using at least 8kiB.

Change-Id: I3bd9013be44b00bdc370bac00d00b4a106ec2d07

fuse/mount_linux_test.go

@@ -64,3 +64,37 @@ func TestMountDevFd(t *testing.T) {
 		t.Error(err)
 	}
 }
+
+// TestMountMaxWrite makes sure that mounting works with all MaxWrite settings.
+// We used to fail with EINVAL below 8k because readPool got too small.
+func TestMountMaxWrite(t *testing.T) {
+	opts := []MountOptions{
+		{MaxWrite: 0}, // go-fuse default
+		{MaxWrite: 1},
+		{MaxWrite: 123},
+		{MaxWrite: 1 * 1024},
+		{MaxWrite: 4 * 1024},
+		{MaxWrite: 8 * 1024},
+		{MaxWrite: 64 * 1024},  // go-fuse default
+		{MaxWrite: 128 * 1024}, // limit in Linux v4.19 and older
+		{MaxWrite: 999 * 1024},
+		{MaxWrite: 1024 * 1024}, // limit in Linux v4.20+
+	}
+	for _, o := range opts {
+		name := fmt.Sprintf("MaxWrite%d", o.MaxWrite)
+		t.Run(name, func(t *testing.T) {
+			mnt, err := ioutil.TempDir("", name)
+			if err != nil {
+				t.Fatal(err)
+			}
+			fs := NewDefaultRawFileSystem()
+			srv, err := NewServer(fs, mnt, &o)
+			if err != nil {
+				t.Error(err)
+			} else {
+				go srv.Serve()
+				srv.Unmount()
+			}
+		})
+	}
+}

fuse/server.go

@@ -24,6 +24,10 @@ const (
 	// The kernel caps writes at 128k.
 	MAX_KERNEL_WRITE = 128 * 1024
 
+	// Linux kernel constant from include/uapi/linux/fuse.h
+	// Reads from /dev/fuse that are smaller fail with EINVAL.
+	_FUSE_MIN_READ_BUFFER = 8192
+
 	minMaxReaders = 2
 	maxMaxReaders = 16
 )
@@ -207,8 +211,12 @@ func NewServer(fs RawFileSystem, mountPoint string, opts *MountOptions) (*Server
 		}
 	}
 	ms.readPool.New = func() interface{} {
-		buf := make([]byte, o.MaxWrite+int(maxInputSize)+logicalBlockSize)
-		buf = alignSlice(buf, unsafe.Sizeof(WriteIn{}), logicalBlockSize, uintptr(o.MaxWrite)+maxInputSize)
+		targetSize := o.MaxWrite + int(maxInputSize)
+		if targetSize < _FUSE_MIN_READ_BUFFER {
+			targetSize = _FUSE_MIN_READ_BUFFER
+		}
+		buf := make([]byte, targetSize+logicalBlockSize)
+		buf = alignSlice(buf, unsafe.Sizeof(WriteIn{}), logicalBlockSize, uintptr(targetSize))
 		return buf
 	}
 	mountPoint = filepath.Clean(mountPoint)