I cannot find anywhere in the documentation how to invalidate an FSItem. It seems to be cached indefinitely or am I missing something?

What I'm building An FSUnaryFileSystem that projects a large, read-mostly tree of existing on-disk files into a sandbox namespace — a build sandbox that lays out an action's declared inputs and points outputs at host scratch. This is squarely the "replace a third-party kext (macFUSE-style) with FSKit" use case, and it's a projection/overlay filesystem: nearly every file the volume serves is just a view of a regular file that already exists on a local APFS volume. The problem For file content, the only available path for a file-backed (non-block-device) volume is FSVolumeReadWriteOperations — every read that misses UBC is an XPC round-trip into my extension, where I memcpy from the backing file into the kernel buffer. The kernel already has, or could trivially open, the backing file; instead each page-in becomes: pagein → IPC → extension read → copy → return. FSVolumeKernelOffloadedIOOperations looks like the intended fast path, but it's built around FSBlockDeviceResource — i.e. it assumes the volume is backed by a block device the kernel can do extent I/O against. A projection over regular files has no block device, so there's no way to say "this item is backed by host file X — kernel, please do I/O directly against X and skip my process." What I measured In one representative build action my volume serves ~440 files and the kernel issues ~630 read RPCs (cold). A real build runs thousands of such actions, so this is on the order of millions of round-trips and buffer copies per build, for data that is already sitting in the host page cache. UBC absorbs repeats, but cold reads, cache eviction under memory pressure, and large sequential reads all pay the full RPC+copy cost. It dominates the I/O profile. The ask A passthrough/offload API for file-backed volumes: let the extension associate an FSItem with a backing file descriptor (or vnode) and have the kernel perform reads — and optionally writes — directly against the backing file, bypassing the userspace round-trip. Per-item, opt-in, and read-only-only would already be a huge win for projection/overlay workloads. This is exactly the model that already exists on other platforms: Linux FUSE passthrough (FUSE_PASSTHROUGH, backing-id via FUSE_DEV_IOC_BACKING_OPEN, mainline since 6.9): a FUSE daemon registers a backing fd and the kernel routes I/O straight to it. Windows Projected File System (ProjFS): content is hydrated/served from a provider-supplied source without a per-read user-space hop. FSKit is positioned as the supported replacement for kext-based filesystems, and projection/overlay/caching filesystems are a primary motivation for it — yet those are precisely the volumes that need zero-copy passthrough to be viable at scale. The block-device offload path covers disk-image-style filesystems; the gap is the file-backed case.

Hello there, I am wanting to create a custom FSKit extension, and am trying to get the sample passthrough project running: https://developer.apple.com/documentation/fskit/building-a-passthrough-file-system I have just followed the basic instructions from the tutorial above, so have: Selected my team for the signing certificate (required for FSKit module). Built and run. Enabled "Passthrough file system" under "File System Extensions". Made a test directory to mount to: mkdir ~/test But when I run the mount command, I get the following error: mount -t passthrough ~/Documents ~/test mount: Loading resource: The operation couldn’t be completed. (com.apple.extensionKit.errorDomain error 2.) mount: exec /Library/Filesystems/passthrough.fs/Contents/Resources/mount_passthrough for /Users/xxxxxxxx/test: No such file or directory mount: /Users/xxxxxxxx/test failed with 72 The contents of /Library/Filesystems/ is empty, so I don't know if allowing the extension is meant to add something to this directory or not. Any help would be much appreciated!

I'm writing a read-only filesystem extension. I see that the documentation for loadResource(resource:options:replyHandler:) claims that the --rdonly option is supported, which suggests that this should be possible. However, I have never seen this option provided to my filesystem extension, even if I return usableButLimited as a probe result (where it doesn't mount at all - FB19241327) or pass the -r or -o rdonly options to the mount(8) command. Instead I see those options on the volume's activate call. But other than saving that "readonly" state (which, in my case, is always the case) and then throwing on all write-related calls I'm not sure how to actually mark the filesystem as "read-only." Without such an indicator, the user is still offered the option to do things like trash items in Finder (although of course those operations do not succeed since I throw an EROFS error in the relevant calls). It also seems like the FSKit extensions that come with the system handle read-only strangely as well. For example, for a FAT32 filesystem, if I mount it like mount -r -F -t msdos /dev/disk15s1 /tmp/mnt Then it acts... weirdly. For example, Finder doesn't know that the volume is read-only, and lets me do some operations like making new folders, although they never actually get written to disk. Writing may or may not lead to errors and/or the change just disappearing immediately (or later), which is pretty much what I'm seeing in my own filesystem extension. If I remove the -F option (thus using the kernel extension version of msdos), this doesn't happen. Are read-only filesystems currently supported by FSKit? The fact that extensions like Apple's own msdos also seem to act weirdly makes me think this is just a current FSKit limitation, although maybe I'm missing something. It's not necessarily a hard blocker given that I can prevent writes from happening in my FSKit module code (or, in my case, just not implement such features at all), but it does make for a strange experience. (I reported this as FB21068845, although I'm mostly asking here because I'm not 100% sure this is not just me missing something.)

I'm trying to make an FSKit module for NTFS read-write filesystem and at the stage where everything is more or less working fine as long as I mount the volume via mount -F and that volume is a RAM disk. However, since the default NTFS read-only driver is already present in macOS, this introduces an additional challenge. Judging by the DiskArbitration sources, it looks like all FSKit modules are allowed to probe anything only after all kext modules. So, in this situation, any third-party NTFS FSKit module is effectively blocked from using DiskArbitration mechanisms at all because it's always masked during the probing by the system's read-only kext. This leaves mount -F as the only means to mount the NTFS volume via FSKit. However, even that doesn't work for volumes on real (non-RAM) disks due to permission issues. The logs in Console.app hint that the FSKit extension is running; however, it looks like the fskitd itself doesn't have permissions to access real disks if it's initiated from the mount utility? default 16:42:41.939498+0200 fskitd New module list <private> default 16:42:41.939531+0200 fskitd Old modules (null) default 16:42:41.939578+0200 fskitd Added 2 identifiers: <private> default 16:42:41.939651+0200 fskitd [0x7fc58020bf00] activating connection: mach=true listener=true peer=false name=com.apple.filesystems.fskitd debug 16:42:41.939768+0200 fskitd main:RunLoopRun debug 16:42:41.939811+0200 fskitd -[liveFilesMountServiceDelegate listener:shouldAcceptNewConnection:]: start default 16:42:41.939870+0200 fskitd Incomming connection, entitled 0 debug 16:42:41.940021+0200 fskitd -[liveFilesMountServiceDelegate listener:shouldAcceptNewConnection:]: accepting connection default 16:42:41.940048+0200 fskitd [0x7fc580006120] activating connection: mach=false listener=false peer=true name=com.apple.filesystems.fskitd.peer[1816].0x7fc580006120 default 16:42:41.940325+0200 fskitd Hello FSClient! entitlement no default 16:42:41.940977+0200 fskitd About to get current agent for 503 default 16:42:41.941104+0200 fskitd [0x7fc580015480] activating connection: mach=true listener=false peer=false name=com.apple.fskit.fskit_agent info 16:42:41.941227+0200 fskitd About to call to fskit_agent debug 16:42:42.004630+0200 fskitd -[fskitdAgentManager currentExtensionForShortName:auditToken:replyHandler:]_block_invoke: Found extension for fsShortName (<private>) info 16:42:42.005409+0200 fskitd Probe starting on <private> debug 16:42:42.005480+0200 fskitd -[FSResourceManager getResourceState:]:not_found:<private> debug 16:42:42.005528+0200 fskitd -[FSResourceManager addTaskUUID:resource:]:<private>: Adding task (<private>) debug 16:42:42.005583+0200 fskitd applyResource starting with resource <private> kind 1 default 16:42:42.005609+0200 fskitd About to get current agent for 503 info 16:42:42.005629+0200 fskitd About to call to fskit_agent debug 16:42:42.006700+0200 fskitd -[fskitdXPCServer getExtensionModuleFromID:forToken:]_block_invoke: Found extension <private>, attrs <private> default 16:42:42.006829+0200 fskitd About to get current agent for 503 info 16:42:42.006858+0200 fskitd About to call to fskit_agent, bundle ID <private>, instanceUUID <private> default 16:42:42.070923+0200 fskitd About to grab assertion on pid 1820 default 16:42:42.071058+0200 fskitd Initializing connection default 16:42:42.071141+0200 fskitd Removing all cached process handles default 16:42:42.071185+0200 fskitd Sending handshake request attempt #1 to server default 16:42:42.071223+0200 fskitd Creating connection to com.apple.runningboard info 16:42:42.071224+0200 fskitd Acquiring assertion: <RBSAssertionDescriptor| "com.apple.extension.session" ID:(null) target:1820> default 16:42:42.071258+0200 fskitd [0x7fc58001cdc0] activating connection: mach=true listener=false peer=false name=com.apple.runningboard default 16:42:42.075617+0200 fskitd Handshake succeeded default 16:42:42.075660+0200 fskitd Identity resolved as osservice<com.apple.filesystems.fskitd> debug 16:42:42.076337+0200 fskitd Adding assertion 183-1817-1669 to dictionary debug 16:42:42.076385+0200 fskitd +[FSBlockDeviceResource(Project) openWithBSDName:writable:auditToken:replyHandler:]:bsdName:<private> default 16:42:42.076457+0200 fskitd [0x7fc5801092e0] activating connection: mach=true listener=false peer=false name=com.apple.fskit.fskit_helper default 16:42:42.077706+0200 fskitd +[FSBlockDeviceResource(Project) openWithBSDName:writable:auditToken:replyHandler:]_block_invoke: Open device returned error Error Domain=NSPOSIXErrorDomain Code=13 info 16:42:42.077760+0200 fskitd +[FSBlockDeviceResource(Project) openWithBSDName:writable:auditToken:replyHandler:]: failed to open device <private>, Error Domain=NSPOSIXErrorDomain Code=13 default 16:42:42.077805+0200 fskitd [0x7fc5801092e0] invalidated because the current process cancelled the connection by calling xpc_connection_cancel() debug 16:42:42.077830+0200 fskitd +[FSBlockDeviceResource(Project) openWithBSDName:writable:auditToken:replyHandler:]:end info 16:42:42.078459+0200 fskitd openWith returned err Error Domain=NSPOSIXErrorDomain Code=13 dev (null) error 16:42:42.078501+0200 fskitd -[fskitdXPCServer getRealResource:auditToken:reply:]: Unable to convert proxy FSBlockDeviceResource into open resource error 16:42:42.078538+0200 fskitd -[fskitdXPCServer applyResource:targetBundle:instanceID:initiatorAuditToken:authorizingAuditToken:isProbe:usingBlock:]: Can't get the real resource of <private> default 16:42:42.105443+0200 fskitd [0x7fc580006120] invalidated because the client process (pid 1816) either cancelled the connection or exited The mount utility call I use is the same for RAM and real disks with the only difference being the device argument and this permission error is only relevant for real disks case. So, the proper solution (using DiskArbitration) seems to be blocked architecturally in this use case due to FSKit modules being relegated to the fallback role. Is this subject to change in the future? The remaining workaround with using the mount directly doesn't work for unclear reasons. Is that permission error a bug? Or am I missing something?

If I'm in an enumerateDirectory call, I can very quickly fill in the fileID, parentID, and (maybe) the type attributes based on the directory entry I have loaded. That is, I can quickly fill in anything that is contained in the dirent structure in dirent.h, plus the parentID. However, if any other attributes are requested (say, flags), or if the file system doesn't store the filetype in the directory entry, then I need to do additional I/O and load an inode. If I have to load an inode, I might keep a reference to it and assume that I can clean it up later whenever there is a matching call to reclaimItem. But in the enumerateDirectory call, I never provide an FSItem to the system! By observation, I see that normally, a call to enumerateDirectory of this nature is followed up by a lookupItem call for every single fetched item, and then assumedly the system can later reclaim it if need be. At least, I tried various ways of listing directories, and each way I tried showed this behavior. If that's the case, then I can rely on a later reclaimItem call telling me when to clean up this cached data from memory. Is this guaranteed, however? I don't see a mention of this in the documentation, so I'm not sure if I can rely on this. Or, do I need to handle a case where, if I do additional I/O after enumerateDirectory, I might need to figure out when cached data should be cleaned up to avoid a "leak?" (Using the term "leak" loosely here, since in theory looking up the file later would make it reclaimable, but perhaps that might not happen.)

Shouldn't it be supported? Also is there a way to disable spotlight indexing on a mounted folder? mds_stores is going wild on fskit volumes. mount -o nobrowse -t passthrough ~/Downloads ~/mnt alexf@MacBook-Pro-3 build % mount file:///Users/alexf/Downloads/ on /Users/alexf/mnt (passthrough, local, nodev, nosuid, noowners, noatime, fskit, mounted by alexf)```

After building the sample and enabling the file system extension the mount command is freezing. Any tips how to diagnose that? The logs show the following: log stream --style compact --info --debug --predicate 'subsystem == "com.apple.FSKit" OR process CONTAINS[c] "samplecode"' Filtering the log data using "subsystem == "com.apple.FSKit" OR process CONTAINS[c] "samplecode"" Timestamp Ty Process[PID:TID] 2026-03-17 15:15:51.832 I mount[16111:d88caa] [com.apple.FSKit:default] FSClient setting up connection to fskitd 2026-03-17 15:15:51.833 Db fskitd[589:d88a5f] [com.apple.FSKit:default] -[liveFilesMountServiceDelegate listener:shouldAcceptNewConnection:]: start 2026-03-17 15:15:51.833 Df fskitd[589:d88a5f] [com.apple.FSKit:default] Incomming connection, entitled 0 2026-03-17 15:15:51.833 Db fskitd[589:d88a5f] [com.apple.FSKit:default] -[liveFilesMountServiceDelegate listener:shouldAcceptNewConnection:]: accepting connection 2026-03-17 15:15:51.833 Df fskitd[589:d88a5f] [com.apple.FSKit:default] Hello FSClient! entitlement no 2026-03-17 15:15:51.834 Df mount[16111:d88caa] [com.apple.FSKit:default] Setting remote protocol to all XPC 2026-03-17 15:15:51.834 Df fskitd[589:d88a5f] [com.apple.FSKit:default] About to get current agent for 501 2026-03-17 15:15:51.834 I fskitd[589:d88a5f] [com.apple.FSKit:default] About to call to fskit_agent 2026-03-17 15:15:51.835 I fskit_agent[10123:d877d9] [com.apple.FSKit:default] Getting extensions 2026-03-17 15:15:51.836 Db fskitd[589:d88a5f] [com.apple.FSKit:default] -[fskitdAgentManager currentExtensionForShortName:auditToken:replyHandler:]_block_invoke: Found extension for fsShortName () 2026-03-17 15:15:51.837 I fskitd[589:d88a5f] [com.apple.FSKit:default] Probe starting on 2026-03-17 15:15:51.837 Db fskitd[589:d87c31] [com.apple.FSKit:default] -[FSResourceManager getResourceState:]:found: 2026-03-17 15:15:51.837 Db fskitd[589:d87c31] [com.apple.FSKit:default] -[FSResourceManager addTaskUUID:resource:]:: Adding task () 2026-03-17 15:15:51.837 Df fskitd[589:d87c31] [com.apple.FSKit:default] About to get current agent for 501 2026-03-17 15:15:51.837 I fskitd[589:d87c31] [com.apple.FSKit:default] About to call to fskit_agent 2026-03-17 15:15:51.837 I fskit_agent[10123:d877d9] [com.apple.FSKit:default] Getting extensions 2026-03-17 15:15:51.838 Db fskitd[589:d87c31] [com.apple.FSKit:default] -[fskitdXPCServer getExtensionModuleFromID:forToken:]_block_invoke: Found extension , attrs 2026-03-17 15:15:51.838 Db fskitd[589:d87c31] [com.apple.FSKit:default] applyResource starting with resource kind 4

I developed a cloud drive using fskit, but after mounting it, it did not appear in the Finder sidebar and the disk tool could not list it. How should I adapt? The mounting looks successful, and you can also open and see the fixed files I wrote in the code. I have also turned on the Finder sidebar settings function

Is there an FSKit sample available? I have tried sample code from https://github.com/KhaosT/FSKitSample on macOS Tahoe 26. I am able to compile the code, with signed/unsigned code, the mount always hangs on Apple Silicon macOS Tahoe 26. Here is the mount script: mkfile -n 100m dummy // create a dummy file hdiutil attach -imagekey diskimage-class=CRawDiskImage -nomount dummy // mount the newly created file as a raw block device mkdir /tmp/TestVol mount -F -t MyFS diskX /tmp/TestVol Any idea why the mount hangs.

I'm trying to test the update process for an app containing an FSKit module that I'm distributing on the Mac App Store. (I'm also distributing the same app directly with Developer ID, but here I'll focus on App Store because that's the behavior I've been looking at first.) To do that I'm using an internal tester group on TestFlight and then testing an update with TestFlight. Below is the behavior I'm seeing on macOS 15.7.2 (24G325). I've noticed that if an app update is triggered while a disk is mounted using the FSKit extension, the disk is automatically unmounted without warning (FB21287341). That's already undesirable itself in my opinion, but on top of the unmount, there are two other problems: That unmount doesn't seem to be a "clean" unmount and doesn't call functions like synchronize (FB21287688). Now, in my case, my app only provides read-only access, so that doesn't actually matter much in my case. However, I'd imagine if I were to add write access at some point in the future, this would go from "doesn't matter" to "very bad." I've seen a few cases where quitting or crashing the FSModule process while a volume is mounted without actually doing a clean unmount causes a lot of "disk-related actions" (for lack of a better term) to freeze (FB21305906). For example, a use of the mount(8) command or trying to mount a disk at all freezes, and opening Disk Utility stalls on a "Loading disks" spinning indicator. This happens until the Mac is rebooted. I did notice this issue once while testing updates via TestFlight a few times. The same applies if I simply delete the app with Finder instead of updating it. Is there a way to prevent the extension's process from terminating in this case and/or another workaround I could use without waiting for a macOS update to hopefully change this behavior? And does observing this kind of behavior with TestFlight's update behavior suggest the same thing could happen on the App Store with its automatic updates? I'm concerned that pushing an update via the App Store will unexpectedly unmount disks or cause the system-wide issues described in FB21305906 at a random time, which is a pretty big disruption for users.

I've developed a custom FSKit module which all seems to work OK when tested with a dummy test FSResource. However, the end goal is to mount a user's entire home directory via my FSKit module. I obviously need the mount to occur before that user logs in for the first time, since that is when their home directory will be first populated. I also have to perform the mount as the user in question since their home directory must be owned by them. I have written a LaunchDaemon module to call /sbin/mount, running as that user, however I notice that whether FSKit modules are enabled or disabled seems to be a per-user setting. If I enable my FSKit module as User A in 'System Settings > Login Items and Extensions' and then log in as User B and go into Login Items and Extensions, the module shows as disabled. Is there a way to enable my FSKit module globally so that it is enabled for all users without each user having to go into System Settings and manually enable it? Or a way of enabling it via command line that I can run with "sudo -u" ? And would this enable me to mount a filesystem before any user has logged in?

I'm developing a custom file system using FSKit. After testing some example projects on github and have encountered an issue where the removeItem(_:named:fromDirectory:) method in my FSVolume.Operations implementation is not being invoked when attempting to delete files or directories through Finder or the command line. Environment: macOS: macOS 26.1 Xcode: 16.2 / 17A400 Sample Project: https://github.com/KhaosT/FSKitSample https://github.com/debox-network/FSKitBridge My volume implements the required FSVolume.Operations protocol with the following removeItem implementation: func removeItem( _ item: FSItem, named name: FSFileName, fromDirectory directory: FSItem ) async throws { logger.info("remove: \(name)") if let item = item as? MyFSItem, let directory = directory as? MyFSItem { directory.removeItem(item) } else { throw fs_errorForPOSIXError(POSIXError.EIO.rawValue) } } Steps to Reproduce: Mount the custom FSKit-based file system Create files using Finder or terminal (works correctly - createItem is called) Attempt to delete a file using: Terminal command: rm -rf /path/to/mounted/file option+cmd+delete force to delete file Expected Behavior: The removeItem(_:named:fromDirectory:) method should be called, logging "remove: [filename]" and removing the item from the directory's children collection. Actual Behavior: The removeItem method is never invoked. No logs appear from this method. The deletion operation either fails silently or returns an error, but the callback never occurs. Are there specific volume capabilities or entitlements required for removeItem to be invoked or are there any known issues with deletion operations in the current FSKit implementation? Any guidance would be greatly appreciated.

Hi, I am currently in the process of writing a fskit extension. My goal is it to implement something similar like unionfs/mergerfs with fskit. For this to work my extension requires access to a set of user provided file paths. I use FSGenericURLResource with query parameters for this. But the sandbox restrictions make this impossible. This is why I tried to implement a privileged helper, but this makes it even more complicated and slower. Is there a way to disable the sandbox restrictions for the extension? I don't plan any app store publishing which makes this even more frustrating. When I remove the sandbox entitlement, I can't load the plugin with pluginkit -a anymore. Or is there any other recommend way, except a privileged helper? Another question I have on my mind: How to write proper tests for an fskit extension? You can load the extension via pluginkit -a and also remove it, but you can't enable it in the system panel. I have no idea how to build automatic tests with this restriction. Lovely greetings, Nils

I'm trying to use FSKit to create a File System Extension that can read MFS-formatted disk images, following the old MFSLives sample project for reference. I have a well-formed MFS formatted img file that I'm trying to mount, but I'm having trouble getting the system to actually use my FSModule. DiskImageMounter fails to mount the img file, but I'm able to use it to attach the image as a device by clicking "Ignore" when it prompts me that it isn't able to read the disk. This is effectively the same as using the hdiutil command in Terminal. hdiutil attach -imagekey diskimage-class=CRawDiskImage -nomount Sample.img I've read that FSKit isn't fully integrated with Disk Arbitration yet, so I decided to see if I could force the system to use my extension by using the mount command. mkdir /tmp/Sample mount -F -t MFS disk54 /tmp/Sample Watching the logs in Console, I can see that fskit_agent sees my extension in its "New Modules List", and I see an MFS process gets launched and logs messages from com.apple.running and com.apple.xpc. However, the logs from the MFS process end there, and don't include any of my debug logs, which should be posted when my FSFileSystem subclass is created or when probeResource is called. Ultimately the mount command fails with exit code 69 and prints the following error message: mount: Probing resource: The operation couldn’t be completed. Permission denied mount: Unable to invoke task I've checked everything I could think of: The extension is enabled in System Settings. The extension has the FSKit Module capability added in Xcode. The Info.plist sets the FSSupportsBlockResources key to YES. The Info.plist sets both the FSName and FSShortName keys to MFS. The extension has its Team set to my developer account, with Xcode setting the Provisioning Profile and Signing Certificate automatically. The hosting app has its Team set to my developer account with the "Development" signing certificate. I wanted to see if it was something with my project configuration or implementation, so I downloaded the KhaosT/FSKitSample project from GitHub. Once I got that building, I tried mounting a disk image using the MyFS extesnion, but my system wouldn't run that either. Is there something about the system configuration I should be aware of to enable File System Extensions? I have my MFS extension showing up and enabled, but I'm not sure if there's something I'm missing that I still have to do. Is there a capability or signing requirement I didn't list that's required for the extension to run? The documentation doesn't specify anything about the entitlements, signing capabilities, or Info.plist keys, so I'm not sure what I should be looking for. I'm running macOS Sequoia 15.6.1 on an M2 Max MacBook Pro, and I'm building my project with Xcode 26 beta 6.

Hello. I'm working on a sample project of a network file system using FSKit. The problem I'm currently facing is FSItems not getting reclaimed after they disappear from the file system. If an item deletion goes through the file system, I get the removeItem() and reclaimItem() callbacks. When deletion goes though a different client to the network file system I force an enumeration of the parent directory and FSKit gets the directory contents. The problem is the FSItem instances that are now missing from the contents don't get reclaimed or removed in any way and remain in memory. This gets reproduced both on macOS 15.6.1 + Xcode 16.4 and the latest beta software versions. Is there any way to work around this? Thanks

Hi, I'm trying to mount my FSKit volume with a client app (SwiftUI). I already successfully did it with the "mount" command and I can instantiate my file-system with FSPathURLResource. Also, I managed to mount the file-system with DiskArbitration in a SwiftUI app, but I only managed to get it working with FSBlockDeviceResource. Is there a way to programmatically do it in a client app? Or is "mount" command currently the only option?

I've gotten to the point where I can use the mount(8) command line tool and the -t option to mount a file system using my FSKit file system extension, in which case I can see a process for my extension launch, probe, and perform the other necessary actions. However, when plugging in my USB flash drive or trying to mount with diskutil mount, the file system does not mount: $ diskutil mount disk20s3 Volume on disk20s3 failed to mount If you think the volume is supported but damaged, try the "readOnly" option $ diskutil mount readOnly disk20s3 Volume on disk20s3 failed to mount If you think the volume is supported but damaged, try the "readOnly" option Initially I thought it would be enough to just implement probeExtension(resource:replyHandler:) and the system would handle the rest, but this doesn't seem to be the case. Even a trivial implementation that always returns .usable doesn't cause the system to use my FSModule, even though I've enabled my extension in System Settings > General > Login Items & Extensions > File System Extensions. From looking at some of the open source msdos and Disk Arb code, it seems like my app extension needs to list FSMediaTypes to probe. I eventually tried putting this in my Info.plist of the app extension: <key>FSMediaTypes</key> <dict> <key>EBD0A0A2-B9E5-4433-87C0-68B6B72699C7</key> <dict> <key>FSMediaProperties</key> <dict> <key>Content Hint</key> <string>EBD0A0A2-B9E5-4433-87C0-68B6B72699C7</string> <key>Leaf</key> <true/> </dict> </dict> <key>0FC63DAF-8483-4772-8E79-3D69D8477DE4</key> <dict> <key>FSMediaProperties</key> <dict> <key>Content Hint</key> <string>0FC63DAF-8483-4772-8E79-3D69D8477DE4</string> <key>Leaf</key> <true/> </dict> </dict> <key>Whole</key> <dict> <key>FSMediaProperties</key> <dict> <key>Leaf</key> <true/> <key>Whole</key> <true/> </dict> </dict> <key>ext4</key> <dict> <key>FSMediaProperties</key> <dict> <key>Content Hint</key> <string>ext4</string> <key>Leaf</key> <true/> </dict> </dict> </dict> </plist> (For reference, the partition represented by disk20s3 has a Content Hint of 0FC63DAF-8483-4772-8E79-3D69D8477DE4 and Leaf is True which I verified using IORegistryExplorer.app from the Xcode additional tools.) Looking in Console it does appear now that the system is trying to use my module (ExtendFS_fskit) to probe when I plug in my USB drive, but I never see a process for my extension actually launch when trying to attach to it from Xcode by name (unlike when I use mount(8), where I can do this). However I do see a Can't find the extension for <private> error which I'm not sure is related but does sound like the system can't find the extension for some reason. The below messages are when filtering for "FSKit": default 19:14:53.455826-0400 diskarbitrationd probed disk, id = /dev/disk20s3, with ExtendFS_fskit, ongoing. default 19:14:53.456038-0400 fskitd Incomming connection, entitled 1 default 19:14:53.456064-0400 fskitd [0x7d4172e40] activating connection: mach=false listener=false peer=true name=com.apple.filesystems.fskitd.peer[350].0x7d4172e40 default 19:14:53.456123-0400 fskitd Hello FSClient! entitlement yes default 19:14:53.455902-0400 diskarbitrationd [0x7461d8dc0] activating connection: mach=true listener=false peer=false name=com.apple.filesystems.fskitd default 19:14:53.456151-0400 diskarbitrationd Setting remote protocol to all XPC default 19:14:53.456398-0400 fskitd About to get current agent for 501 default 19:14:53.457185-0400 diskarbitrationd probed disk, id = /dev/disk20s3, with ExtendFS_fskit, failure. error 19:14:53.456963-0400 fskitd -[fskitdXPCServer applyResource:targetBundle:instanceID:initiatorAuditToken:authorizingAuditToken:isProbe:usingBlock:]: Can't find the extension for <private> (I only see these messages after plugging my USB drive in. When running diskutil mount, I see no messages in the console when filtering by FSKit, diskarbitrationd, or ExtendFS afterward. It just fails.) Is there a step I'm missing to get this to work, or would this be an FSKit bug/current limitation?

Hello, I'm playing around with the FSKit API and I'm looking for programmatic ways to mount a file system backed by an FSKit extension. Currently, I could only find the new -F option in the mount command but I would prefer if I could avoid executing comman line tools. I checked the DiskArbitration framework but couldn't find anything specific for FSKit there. Thanks

I reported this as a bug (FB18614667), but also wanted to ask here in case this is actually just me doing something wrong, or maybe I'm misunderstanding the entire use case of metadataRead. (My understanding is that metadataRead is basically read but it checks a cache that the kernel manages before trying to read the physical resource, and in the case of a cache miss it would just go to the physical resource and then add the bytes to the cache. Is that right?) I’m encountering an issue in an FSKit file system extension where (for example) read(into: buf, startingAt: 0, length: Int(physicalBlockSize)) works, but metadataRead(into: buf, startingAt: 0, length: Int(physicalBlockSize)) throws an EIO error (Input/output error) no matter what I do. (Note: physicalBlockSize is 512 in this example.) The documentation (https://developer.apple.com/documentation/fskit/fsblockdeviceresource/metadataread(into:startingat:length:)) indicates that the restrictions on metadataRead are that the operations must be sector-addressed (which is the case here, especially as regular read has the same restriction and succeeds) and that partial reading of metadata is not supported. (I don’t think that applies here?) In a sample project I was able to replicate this behavior where the module only ever reads the block device in its enumerateDirectory implementation, and so trying to list the contents of a directory leads to an "Input/output error" when e.g. running ls on the volume. The enumerateDirectory sample implementation is like so: func enumerateDirectory(_ directory: FSItem, startingAt cookie: FSDirectoryCookie, verifier: FSDirectoryVerifier, attributes: FSItem.GetAttributesRequest?, packer: FSDirectoryEntryPacker) async throws -> FSDirectoryVerifier { let buf = UnsafeMutableRawBufferPointer.allocate(byteCount: Int(blockDevice.physicalBlockSize), alignment: 1) defer { buf.deallocate() } // metadataRead will throw... try blockDevice.metadataRead(into: buf, startingAt: 0, length: Int(blockDevice.physicalBlockSize)) // but read will work. // try await blockDevice.read(into: buf, startingAt: 0, length: Int(blockDevice.physicalBlockSize)) // ... return dummy file here (won't reach this point because metadataRead throws) } I'm observing this behavior on both macOS 15.5 (24F74) and macOS 15.6 beta 3 (24G5074c). Has anyone been able to get metadataRead to work? I see it used in Apple's msdos FSKit implementation so it seems like it has to work at some level.