fuse: wire up passthrough support

The passthrough feature lets a filesystem register an open file
descriptor with the kernel to avoid roundtrips for read/write
operations.

Change-Id: Ia8bde502a3450028f4d87ba61fa9c76ea3ea6c63

fuse/opcode.go

@@ -103,7 +103,7 @@ func doInit(server *Server, req *request) {
 	server.reqMu.Lock()
 	server.kernelSettings = *input
 	kernelFlags &= (CAP_ASYNC_READ | CAP_BIG_WRITES | CAP_FILE_OPS |
-		CAP_READDIRPLUS | CAP_NO_OPEN_SUPPORT | CAP_PARALLEL_DIROPS | CAP_MAX_PAGES | CAP_RENAME_SWAP)
+		CAP_READDIRPLUS | CAP_NO_OPEN_SUPPORT | CAP_PARALLEL_DIROPS | CAP_MAX_PAGES | CAP_RENAME_SWAP | CAP_PASSTHROUGH)
 
 	if server.opts.EnableLocks {
 		kernelFlags |= CAP_FLOCK_LOCKS | CAP_POSIX_LOCKS
@@ -154,6 +154,7 @@ func doInit(server *Server, req *request) {
 		CongestionThreshold: uint16(server.opts.MaxBackground * 3 / 4),
 		MaxBackground:       uint16(server.opts.MaxBackground),
 		MaxPages:            uint16(maxPages),
+		MaxStackDepth:       1,
 	}
 	out.setFlags(kernelFlags)
 	if server.opts.MaxReadAhead != 0 && uint32(server.opts.MaxReadAhead) < out.MaxReadAhead {

fuse/passthrough_linux.go

@@ -0,0 +1,50 @@
+// Copyright 2024 the Go-FUSE 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 fuse
+
+import (
+	"syscall"
+	"unsafe"
+)
+
+const (
+	_DEV_IOC_BACKING_OPEN  = 0x4010e501
+	_DEV_IOC_BACKING_CLOSE = 0x4004e502
+)
+
+type backingMap struct {
+	Fd      int32
+	Flags   uint32
+	padding uint64
+}
+
+// RegisterBackingFd registers the given file descriptor in the
+// kernel, so the kernel can bypass FUSE and access the backing file
+// directly for read and write calls. On success a backing ID is
+// returned. The backing ID should unregistered using
+// UnregisterBackingFd() once the file is released. For now, the flags
+// argument is unused, and should be 0.
+func (ms *Server) RegisterBackingFd(fd int, flags uint32) (int32, syscall.Errno) {
+	m := backingMap{
+		Fd:    int32(fd),
+		Flags: flags,
+	}
+
+	ms.writeMu.Lock()
+	id, _, errno := syscall.Syscall(syscall.SYS_IOCTL, uintptr(ms.mountFd), uintptr(_DEV_IOC_BACKING_OPEN), uintptr(unsafe.Pointer(&m)))
+	ms.writeMu.Unlock()
+
+	return int32(id), errno
+}
+
+// UnregisterBackingFd unregisters the given ID in the kernel. The ID
+// should have been acquired before using RegisterBackingFd.
+func (ms *Server) UnregisterBackingFd(id int32) syscall.Errno {
+	ms.writeMu.Lock()
+	_, _, ep := syscall.Syscall(syscall.SYS_IOCTL, uintptr(ms.mountFd), uintptr(_DEV_IOC_BACKING_CLOSE), uintptr(unsafe.Pointer(&id)))
+	ms.writeMu.Unlock()
+
+	return ep
+}