Apple’s Darwin OS and XNU Kernel Deep Dive

https://tansanrao.com/blog/2025/04/xnu-kernel-and-darwin-evolution-and-architecture/

by tansanrao • 1 year ago

485 180 1 year ago

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whalesalad• 1 year ago

I’ve been wanting to understand Darwin at this depth for a long time. Great read!

tansanrao4• 1 year ago

It’s my first time condensing my research notes into a blog post like this, glad you liked it!

ForOldHack5• 1 year ago

It was easily comprehensive enough, that I printed it out and was showing people today what the differences were in the last couple of os releases, while ruminating about snow leopard. No one mentions that the blue box became Rosetta, and Rosetta II did the same thing in the switch from Intel to ARM.

There are some very tiny points, but this is easily very best to date. ( I started with Rhapsody, and Linux in swedish, and NT 3.1 )( Ran MKLinux on a 7100, but never got accelerated video to work. )

jshier• 1 year ago

Mac OS X Internals by Singh is one of my favorite books, such a great in depth examination of Mac OS X circa 10.4. I really wish there was an updated version.

Edit: I see it's even cited at the end of this article. Truly a source for the (macOS) ages.

wpm3• 1 year ago

Jonathan Levin’s three part series “*OS Internals” is that update, but they stopped working on and writing about Darwin around Catalina.

kccqzy• 1 year ago

I've also wanted to understand Windows NT at this depth for a while. Skip the Win32 stuff, and discuss what's underneath it. As I understand Win32 is just one personality; there was also Windows Services for UNIX in the Windows XP days and Subsystem for UNIX-based Applications in Windows Vista. The underlying NT kernel is flexible enough to allow POSIX compliance. That would be an interesting read.

p_ing5• 1 year ago

Windows Internals is the book you want.

Or Inside Windows NT, if you want "version 1" of the Internals series. Or read the Windows NT OS/2 Design Workbook - https://computernewb.com/~lily/files/Documents/NTDesignWorkb....

Yes, Win32 is just one personality, but a required one. OpenNT, Interix, SFU, SUA will ride alongside Win32. And of course there was the official OS/2 personality.

skissane8• 1 year ago

As I understand Win32 is just one personality

Not really... although NT was designed to run multiple "personalities" (or "environment subsystems" to use the official term), relatively early in its development they decided to make Win32 the "primary" environment subsystem, with the result that the other two subsystems (OS/2 and POSIX) ended up relying on Win32 for essential system services.

I think this multiple personalities thing was the original vision but it never really took off in the way its original architects intended – although there used to be OS/2 and POSIX subsystems, Microsoft never put a great deal of effort into them, and now them are both dead, so Win32 is the only environment subsystem left.

Yes, there is WSL, but: WSL1 is not an environment subsystem in the classic NT sense – it has a radically different implementation from the old OS/2 and POSIX subsystems, a "picoprocess provider". And WSL2 is just a Linux virtual machine.

bch3• 1 year ago

Mach’s virtual memory (VM) system was influential beyond the project – it was adopted by 4.4BSD and later FreeBSD as their memory management subsystem.

…and NetBSD[0], OpenBSD[1], but apparently not DragonFly BSD[2].

[0] https://netbsd.org/docs/kernel/uvm.html

[1] https://man.openbsd.org/OpenBSD-3.0/uvm.9

[2] https://www.dragonflybsd.org/mailarchive/kernel/2011-04/msg0...

tansanrao4• 1 year ago

Ohhh interesting! I’ll update the post to include this soon, thanks!

inkyoto9• 1 year ago

Sadly, that is not entirely correct.

Whilst all three BSDs (386BSD, FreeBSD, and NetBSD; there was no OpenBSD in the beginning) did inherit the legacy Mach 2.5-style design, it did not live on in FreeBSD, whose core team started pretty quickly replacing all remaining vestiges of the Mach VM[0] with a complete, modern, and highly performant rewrite of the entire VM. FreeBSD 4 had none of the original Mach code left in the kernel codebase, and that happened in the late 1990s. Therefore, FreeBSD can't be referenced in a relationship to Mach apart from the initial separation/very early foundation stage.

NetBSD (and OpenBSD) went on for a while but also quickly hit the wall with the Mach design (performance, SMP/scalability, networking) and also set out on a complete rewrite with UVM (unified virtual memory) designed and led by Chuck Cranor, who wrote his dissertation on the UVM. OpenBSD later borrowed and adopted the UVM implementation, which remains in use today.

So out of all living BSD's[1], only XNU/Darwin continues to use Mach, and not Mach 2.5 but Mach 3. There have been Mach 2.5, 3 and 4 (GNU/Hurd uses Mach 4) in existence, and the compatibility between them is rather low, and remains mostly at the overall architectural level. They are better to be treated as distinct design with shared influence.

[0] Of which there were not that many to start off with.

[1] I am not sure whether DragonBSD is dead or alive today at all.

o11c• 1 year ago

I am not sure whether DragonBSD is dead or alive today at all.

It seems to have about the same level of activity as NetBSD. Take that how you will.

bch3• 1 year ago

I am not sure whether DragonBSD is dead or alive today at all.

Oof, yeah.[0][1]. I hope they're doing alright - technically fascinating, and charming as they march to the beat of their own accordion.[2][3][4][5]

[0] https://www.dragonflybsd.org/release64/

[1] https://gitweb.dragonflybsd.org/dragonfly.git

[2] https://www.dragonflybsd.org/mailarchive/kernel/2012-03/msg0...

[3] http://www.bsdnewsletter.com/2007/02/Features176.html

[4] https://en.wikipedia.org/wiki/Vkernel

[5] https://en.wikipedia.org/wiki/HAMMER_(file_system)

swatson7412• 1 year ago

Whenever I see the Darwin kernel brought into the discussion I can't help but wonder how different things could have been if Apple had just forked Linux and ran their OS services on top of that.

Especially when I think about how committed they are to Darwin it really paints a poor image in my mind. The loss that open source suffers from that, and the time and money Apple has to dedicate to this with a disproportionate return.

skissane8• 1 year ago

Whenever I see the Darwin kernel brought into the discussion I can't help but wonder how different things could have been if Apple had just forked Linux

XNU is only partially open sourced – the core is open sourced, but significant chunks are missing, e.g. APFS filesystem.

Forking Linux might have legally compelled them to make all kernel modules open source–which while that would likely be a positive for humanity, isn't what Apple wants to do

mattl• 1 year ago

At one point NeXT considered distributing GCC under the GPL with some proprietary parts linked at first boot into the binary.

Stallman after speaking with lawyers rejected this.

https://sourceforge.net/p/clisp/clisp/ci/default/tree/doc/Wh...

Look for "NeXT" on this page.

threeseed2• 1 year ago

1) We are talking about the late 90s, well before Ubuntu, where Desktop Linux was pretty poor in terms of features and polish.

2) Apple had no money or time to invest in rewriting NeXTStep for a completely new kernel they had no experience in. Especially when so many of the dev team was involved in sorting out Apple's engineering and tech strategy as well as all the features needed to make it more Mac like.

3) Apple was still using PowerPC at the time which NeXTStep supported but Linux did not. It took IBM a couple of years to get Linux running.

CharlesW2• 1 year ago

Apple had no money or time to invest in rewriting NeXTStep for a completely new kernel they had no experience in.

And even if they had had the money and time, Avie Tevanian¹ was a principal designer and engineer of Mach². There was no NeXTSTEP-based path where the Mach-derived XNU would not be at the core of Apple's new OS family.

¹ https://en.wikipedia.org/wiki/Avie_Tevanian ² https://en.wikipedia.org/wiki/Mach_(kernel)

kergonath2• 1 year ago

Apple had no money or time to invest in rewriting NeXTStep for a completely new kernel they had no experience in.

I broadly agree, but it is more nuanced than that. They actually had experience with Linux. Shortly before acquiring NeXT, they did the opposite of what you mentioned and ported Linux to the Mach microkernel for their MkLinux OS. It was cancelled at some point, but had things turned a bit differently, it could have ended up more important than it actually did.

bigger_cheese• 1 year ago

1) We are talking about the late 90s, well before Ubuntu, where Desktop Linux was pretty poor in terms of features and polish.

I think it's hard to understate how much traction Linux had in the late 90's/ early 2000's. It felt like ground breaking stuff was happening pretty much all the time, major things were changing rapidly every release and it felt exciting and genuinely revolutionary to download updates and try out all the new things it really felt like you were on the bleeding edge, your system would break all the time but it was fun and exciting.

I remember reading Slashdot daily being excited to try out every new distribution I'd see on distrowatch, I'd download and build kernels fairly regularly etc.

Things I can remember from back in those days:

- LILO to GRUB boot loader changes

- Going from EXT2 to EXT3 and all the other experimental filesystems that kept coming out.

- Sound system changing from OSS to ASLA

- Introduction of /sys

- Gentoo and all the memes (funroll-loops website)

- Udev and being able to hotplug usb devices

- Signalfd

- Splice/VMsplice

- Early wireless support and the cursed "ndiswrapper"

Nowadays Linux is pretty stable and dare I say it "boring" (in a good way). It's probably mostly because I've gotten older and have way less free time to spend living on the bleeding edge. It feels like Linux has gone from something you had to wrestle with constantly to have a working system to a spot where nowadays everything "mostly works" out of the box. I can't remember last time I've had to cntrl + alt + backspace my desktop for example.

Last major thing I can remember hearing about and being excited for was io_uring.

wtallis• 1 year ago

There was never a right time for Apple to make such a switch. NeXTSTEP predates Linux, and when it was adapted into Mac OS X, Apple couldn't afford a wholesale kernel replacement project on top of everything else, and Linux in the late 1990s was far from being an obviously superior choice. Once they were a few versions in to OS X and solidly established as the most successful UNIX-like OS for consumer PCs, switching to a Linux base would have been an expensive risk with very little short-term upside.

Maybe if Apple had been able to keep classic MacOS going five years longer, or Linux had matured five years earlier, the OS X transition could have been very different. But throwing out XNU in favor of a pre-2.6 Linux kernel wouldn't have made much sense.

swatson7412• 1 year ago

I agree with all of this. Moreover depending on what Torvalds chooses to do Apple may have ended up with a more expensive XNU in the end which would have been a disaster. Although I think Apple can deal with Torvalds just fine who really knows how that would have played out.

lunarlull3• 1 year ago

Making a switch is one thing, but using Linux from the start for OS X would have made more sense. The only reason that didn't happen is because of Jobs' attachment to his other baby. It wasn't a bad choice, but it was a choice made from vanity and ego over technical merit.

GianFabien• 1 year ago

Back in the days when Apple acquired NeXT, Linux was undergoing lots of development and wasn't well established. Linux being a monolithic kernel didn't offer the levels of compartmentalization that Mach did.

As things now stand, FreeBSD represents many of the benefits of Darwin and the open source nature of Linux. If you seek a more secure environment without Apple's increasing levels of lock-in, then FreeBSD (and the other BSDs) merit consideration for deployment.

finnjohnsen2• 1 year ago

Is the driver support fit for using FreeBSD as a desktop OS these days?

Last I tried (~10 years ago) I gave up and I assumed FreeBSD was a Server OS, because I couldn't for the life of me get Nvidia drivers working in native resolution. I don't recall specifics but Bluetooth was problematic also.

laurencerowe2• 1 year ago

Isn’t FreeBSD a monolithic kernel? I don’t believe it provides the compartmentalisation that you talk about.

As I understand it Mach was based on BSD and was effectively a hybrid with much of the existing BSD kernel running as a single big task under the microkernel. Darwin has since updated the BSD kernel under microkernel with the current developments from FreeBSD.

inkyoto9• 1 year ago

As things now stand, FreeBSD represents many of the benefits of Darwin and the open source nature of Linux.

No. FreeBSD has committed the original sin of UNIX by deliberately dropping support for all non-Intel architectures, intending to focus on optimising FreeBSD for the Intel ISA and platforms. UNIX portability and support for a diverse range of CPU's and hardware platforms are ingrained in the DNA of UNIX, however.

I would argue that FreeBSD has paid the price for this decision – FreeBSD has faded into irrelevance today (despite having introduced some of the most outstanding and brilliant innovations in UNIX kernel design) – because the FreeBSD core team bet heavily on Intel remaining the only hardware platform in existence, and they missed the turn (ARM, RISC-V, and marginally MIPS in embdedded). Linux stepped in and filled in the niche very quickly, and it now runs everywhere. FreeBSD is faster but Linux is better.

And it does not matter that Netflix still runs FreeBSD on its servers serving up the content at the theoretical speed of light – it is a sad living proof of FreeBSD having become a niche within a niche.

P.S. I would also argue that the BSD core teams (Free/Net/Open) were a major factor in the downfall of all BSD's, due to their insular nature and, especially in the early days, a near-hostile attitude towards outsiders. «Customers» voted with their feet – and chose Linux.

linguae2• 1 year ago

Interestingly enough, Apple did contribute to porting Linux to PowerPC Macs in the mid-1990s under the MkLinux project, which started in 1996 before Apple’s purchase of NeXT later that year:

https://en.m.wikipedia.org/wiki/MkLinux

I don’t think there was any work done on bringing the Macintosh GUI and application ecosystem to Linux. However, until the purchase of NeXT, Apple already had the Macintosh environment running on top of Unix via A/UX (for 68k Macs) and later the Macintosh Application Environment for Solaris and HP-UX; the latter ran Mac OS as a Unix process. If I remember correctly, the work Apple did for creating the Macintosh Application Environment laid the groundwork for Rhapsody’s Blue Box, which later became Mac OS X’s Classic environment. It is definitely possible to imagine the Macintosh Application Environment being ported to MkLinux. The modern FOSS BSDs were also available in 1996, since this was after the settlement of the lawsuit affecting the BSDs.

Of course, running the classic Mac OS as a process on top of Linux, FreeBSD, BeOS, Windows NT, or some other contemporary OS was not a viable consumer desktop OS strategy in the mid 1990s, since this required workstation-level resources at a time when Apple was still supporting 68k Macs (Mac OS 8 ran on some 68030 and 68040 machines). This idea would’ve been more viable in the G3/G4 era, and by the 2000s it would have be feasible to give each classic Macintosh program its own Mac OS process running on top of a modern OS, but I don’t think Apple would have made it past 1998 without Jobs’ return, not to mention that the NeXT purchase brought other important components to the Mac such as Cocoa, IOKit, Quartz (the successor to Display PostScript) and other now-fundamental technologies.

CharlesW2• 1 year ago

I don’t think there was any work done on bringing the Macintosh GUI and application ecosystem to Linux.

QTML (which became the foundation of the Carbon API) was OS agnostic. The Windows versions of QuickTime and iTunes used QTML, and in an alternate universe Apple could've empowered developers to bring Mac OS apps to Windows and Linux with a more mature version of that technology.

threeseed2• 1 year ago

Completely forget about MkLinux. The timing is fascinating.

MkLinux was released in February 1996 whilst Copland got officially cancelled in August 1996.

So it's definitely conceivable that internally they were considering to just give up on the Copland microkernel and run it all on Linux. And maybe this was a legitimate third option to BeOS and NeXT that was never made public.

askvictor• 1 year ago

Diverse systems are more resilient. It's probably a good thing for IT in a general sense, even if it's not the most efficient

toast02• 1 year ago

Based on how often they pull in updated bits from FreeBSD (pretty much never), an Apple fork of Linux would be more or less Linux 2.4 today.

I don't know what the loss that open source suffers is in this context?

I don't think Apple would need to spend less time or money on their kernel grafted ontop of Linux 2.4 vs their kernel grafted on top of FreeBSD 4.4

WD-422• 1 year ago

Because presumably the GPL would force them to release their modifications. Apple gets/got away with leeching off the BSDs because of the permissive license.

surajrmal• 1 year ago

Why would we want more of a monoculture? We've put so many eggs in one basket already. I hope we see more diversity in kernels, not further consolidation.

Taken a different way, it feels similar to suggesting Apple should rebase safari on chromium.

hypercube33• 1 year ago

Keep in mind they were also looking at BeOS which is more real time and notably not unix/Linux. I wish I lived in the timeline that they went with it as I'm a huge Be fan.

palata• 1 year ago

Seen differently, I think it's great that there is yet another kernel being maintained out there.

Imagine if Apple decided to open source Darwin: wouldn't that be a big win for open source?

chuckadams• 1 year ago

The core of it always has been open source: https://github.com/apple-oss-distributions/xnu

phendrenad2• 1 year ago

Control is important. Apple has never had to fight with Torvalds or IBM or Microsoft over getting something added to the kernel. Just look at the fiasco when Microsoft wanted to add a driver for their virtualization system to the kernel.

Also, one thing you'll notice about big companies - they know that not only is time valuable, worst-case time is important too. If someone in an open-source ecosystem CAN delay your project, that's almost as bad as if they regularly DO delay your project. This is why big companies like Google tend to invent everything themselves, I.E. Google may have "invented Kubernetes" (really, an engineer at Google uninvolved with the progenitor of K8s - Borg - invented it based on Borg), but they still use Borg, which everyone Xoogler here likes to say is "not as good as k8s". Yet they still use it. Because it gives them full control, and no possibility of outsiders slowing them down.

DeathArrow2• 1 year ago

Whenever I see the Darwin kernel brought into the discussion I can't help but wonder how different things could have been if Apple had just forked Linux and ran their OS services on top of that.

They have a long history with XNU and BSD. And Linux has s GPL license which might not suit Apple.

Especially when I think about how committed they are to Darwin it really paints a poor image in my mind. The loss that open source suffers from that, and the time and money Apple has to dedicate to this with a disproportionate return.

They share a lot of code with FreeBSD, NetBSD and OpenBSD. Which are open source. And Darwin is open source, too. So there's no loss that open source suffers.

piuantiderp• 1 year ago

The world is better with multiple flavors instead of one bloated one.

mycall• 1 year ago

One must consider the loss of control moving to Linux would bring. Even Google is reconsidering with fuchsia inline to replace Linux on Android.

95014_refugee2• 1 year ago

It would never have worked. So many of the things that owning xnu has made possible would never have happened on top of Linux. The things you can do when you know each and every customer of the stack, and you all belong to the same business with common objectives and leadership direction just can’t be done in the open-source context.

emchammer• 1 year ago

Couldn't Apple have used ZFS instead of inventing APFS? Maybe modifying it to use less physical memory?

inkyoto9• 1 year ago

Supporting ZFS in a UNIX kernel requires excessively extensive modifications to the design and implementation of the VMM, namely:

  1. Integration of the kernel's VM with ZFS's adaptive replacement cache which runs in user space – memory pressure cooperation, page accounting and unified memory management. It also requires extensive VM modifications to support ZFS's controlled page eviction, fine-grained dirty page tracking, plus other stuff.

  2. VMM alignment with the ZFS transactional semantics and intent logs – delayed write optimisations, efficient page syncing.

  3. Support for large memory pages and proper memory page alignment – support for superpages (to reduce the TLB pressure and to efficiently map large ZFS blocks efficiently) and I/O alignment awareness (to ensure proper alignment of memory pages to avoid unnecessary copies).

  4. Memory-mapped I/O: different implementation of mmap and support for lazy checksumming for mmap pages.

  5. Integration with kernel thread management and scheduling, co-opertation with VMM memmory allocators.

  6. … and the list goes on and on.

ZFS is just not the right answer for consumer facing and mobile/portable devices due being a heavyweight server design with vastly different design provisions and due to being the answer to a entirely different question.

AndrewDavis• 1 year ago

Supporting ZFS in a UNIX kernel requires excessively extensive modifications to the design and implementation of the VMM, namely:

FYI: Apple did a bunch of that work. They ported ZFS to OSX shortly after it was open sourced. With with only support landing in 10.5. With it being listed as an upcoming feature in 10.6.

But something happened and they abandoned it. The rumour is a sun exec let the cat out of the bag about it being the next main filesystem for osx (ie not just support for non root drives) and this annoyed Jobs so much he canned the whole project.

cosmic_cheese• 1 year ago

IIRC that was something they had been working on, but it got axed when ZFS changed hands and licensing became potentially thorny. My memory may be failing me though.

linguae2• 1 year ago

I remember reading back in 2007-2008 that Apple was interested in bringing ZFS support to Mac OS X, but discussions ended once Oracle purchased Sun. This was a bummer; I would’ve loved ZFS on a Mac.

After a cursory Google search, I found this article:

https://www.zdnet.com/article/zfs-on-snow-leopard-forget-abo...

Lammy• 1 year ago

There were a couple of (iirc read-only) beta ZFS PKGs from Apple in the 10.5 era: https://macintoshgarden.org/forum/looking-zfs-beta-seeds-ye-...

leoh2• 1 year ago

Kind of surprising that the Oracle deal would have killed it given that Jobs and Ellison were such close friends.

wpm3• 1 year ago

Around the time of Snow Leopard, it was rumored. I assume the Oracle buyout of Sun around the same time had a big part in killing that particular idea.

agentkilo• 1 year ago

The article states that "pager daemons" that manage swap files runs in user space, and the kernel memory can also get swapped out, but never explained how a user space daemon swaps out kernel memory. Do they have hard-coded exceptions for special daemons, or use special system calls? Where can I find out more details about the user space memory management specifically?

jjtheblunt• 1 year ago

https://github.com/apple-oss-distributions/xnu

comex4• 1 year ago

The claim is inaccurate and mixes together multiple different things:

- The Mach microkernel originally supported true userland paging, like mmap but with an arbitrary daemon in place of the filesystem. You can see the interface here:

https://web.mit.edu/darwin/src/modules/xnu/osfmk/man/memory_...

But I'm not sure if Darwin ever used this functionality; it certainly hasn't used it for the last ~20 years.

- dynamic_pager never used this interface. It used a different, much more limited Mach interface where xnu could alert it when it was low on swap; dynamic_pager would create swap files, and pass them back into the kernel using macx_swapon and macx_swapoff syscalls. But the actual swapping was done by the kernel. Here is what dynamic_pager used to look like:

https://github.com/apple-oss-distributions/system_cmds/blob/...

But that functionality has since moved into the kernel, so now dynamic_pager does basically nothing:

https://github.com/apple-oss-distributions/system_cmds/blob/...

- The vast majority of kernel memory is wired and cannot be paged out. But the kernel can explicitly ask for pageable memory (e.g. with IOMallocPageable), and yes, that memory can be swapped to disk. It's just rarely used.

Still, any code that does this needs to be careful to avoid deadlocks. Even though userland is no longer involved in "paging" per se, it's still possible and in fact common for userland to get involved one or two layers down. You can have userland filesystems with FSKit (or third-party FUSE). You can have filesystems mounted on disk images which rely on userland to convert reads and writes to the virtual block device into reads and writes to the underlying dmg file (see `man hdiutil`). You can have NFS or SMB connections going through userland networking extensions. There are probably other cases I'm not thinking of.

EDIT: Actually, I may be wrong about that last bit. You can definitely have filesystems that block on userspace, but it may not be supported to put swap on those filesystems.

krackers• 1 year ago

xnu could alert it when it was low on swap; dynamic_pager would create swap files, and pass them back into the kernel

What's the benefit of this indirection through userspace for swap file creation? Can't the kernel create the swap file itself?

fithisux• 1 year ago

They should have fostered a better FOSS community around XNU, now that they moved to ARM there should have been a runnable distribution for x64

larusso2• 1 year ago

I’m not sure if I/O kit was written in this c++ subset just for speed. There was this controversial at the time. Apple announced MacOS X and said that it won’t be compatible with current software. All partners would need to rewrite the software in Objective-C. This didn’t go over well. Apple back paddelt and introduced “carbon”. An API layer for cpp applications as well as “Core Foundation” an underpinning to the objective-c base Framework “Foundation”. Also the reason why we have Obj-c++. The interesting part is that they managed to get the memory management toll free. Means an object allocated in the c/cpp world can be passed to obj-c without extra overhead.

comex4• 1 year ago

IOKit C++ is running in the kernel, so it's not really related to any of the technologies you mentioned which are all userland-only.

dcrazy4• 1 year ago

Being able to port your existing C++ driver to IOKit instead of rewriting it in Objective-C is a selling point. For some reason people a lot of people seem to dislike writing an Objective-C shell around their C++.

larusso2• 1 year ago

You may underestimate how many drivers had to be shipped and developed by external companies compared to today. For software / hardware companies that was a huge deal.

comex4• 1 year ago

At the risk of nitpicking, there are a bunch of things that are not quite right. Nonexhaustive list:

- Discussion of paging mixes together some concepts as I described in [1].

- Mach port "rights" are not directly related to entitlements. Port rights are port of the original Mach design; entitlements are part of a very different, Apple-specific security system grafted on much later. They are connected in the sense that Mach IPC lets the receiver get an "audit token" describing the process that sent them, which it can then use to look up entitlements.

- All IOKit calls go through Mach IPC, not just asynchronous events.

- "kmem" (assuming this refers to the kmem_* functions) is not really a “general-purpose kernel malloc”; that would be kalloc. The kmem_* functions are sometimes used for allocations, but they’re closer to a “kernel mmap” in the sense that they always allocate new whole pages.

- It’s true that xnu can map the same physical pages into multiple tasks read-only, but that’s nothing special. Every OS does that if you use mmap or similar APIs. What does make the shared cache special is that it can also share physical page tables between tasks.

- The discussion about “shared address space” is mixing things up.

The current 64-bit behavior is the same as the traditional 32-bit behavior: the lower half of the address space is reserved for the current user process, and the upper half is reserved for the kernel. This is typically called a shared address space, in the sense that the kernel page tables are always loaded, and only page permissions prevent userland from accessing kernel memory. Though you could also think of it as a 'separate' address space in the sense that userland and kernel stick to separate addresses. Anyway, this approach is more efficient (because you don't have to swap page tables for every syscall) and it's the standard thing kernels do.

What was tricky and unusual was the intermediate 32-bit behavior where the kernel and user page tables actually were completely independent (so the same address would mean one thing in user mode and another thing in kernel mode). This allowed 32-bit user processes to use more memory (4GB rather than 2GB), but at the cost of making syscalls more expensive.

Even weirder, in the same era, xnu could even run 64-bit processes while itself being 32-bit! [2]

- The part about Secure Enclave / Exclaves does not explain the main difference between them: the Secure Enclave is its own CPU, while Exclaves are running on the main CPU, just in a more-trusted context.

- Probably shouldn't describe dispatch queues as a "new technique". They're more than 15 years old, and now they're sort of being phased out, at least as a programming model you interact with directly, in favor of Swift Concurrency. To be fair, Swift Concurrency uses libdispatch as a backend.

[1] https://news.ycombinator.com/item?id=43599230

[2] https://superuser.com/questions/23214/why-does-my-mac-os-x-1...

saagarjha3• 1 year ago

Also,

As of iOS 15, Apple even allows virtualization on iOS (to run Linux inside an iPad app, for example, which some developers have demoed), indicating the XNU hypervisor is capable on mobile as well, though subject to entitlement.

Apple definitely does not allow this; in fact the hypervisor code has been removed from the kernel as of late.

tansanrao4• 1 year ago

These are very helpful, thank you! A lot of good corrections and suggestions were provided in the comments, I’ll verify and correct the portions in the post for future readers.

opengears• 1 year ago

Does this give us any indication on when Apple will be ditching x86 support?

SG-• 1 year ago

any day now, usually after 6-7 years after hardware releases.

icedchai4• 1 year ago

We probably have a few more years to go then. Consider that they were still selling Mac Pro 2019 Intel systems as late as June, 2023.

snovymgodym• 1 year ago

Late 2027 or 2028 most likely. MacOS version lifespan is about 3 years give or take so we'll have a better idea once they announce whether or not MacOS 16 (Sequoia's successor) will support Intel Macs.

I have a hunch we'll get one more MacOS version with Intel support since they were still making Mac Minis and Pros with Intel chips in the first half of 2023.

devmtk• 1 year ago

oh interesting

pjmlp9• 1 year ago

Lots of love and work went into this article, as someone around for most of this history, ported code from NeXTSTEP into Windows, dived into the GNUStep attempts to clone the experience, remembers YellowBox and OpenStep, read the internals books, regular consumer of WWDC content, I would say the article matches pretty much my recollection on most of the systems have evolved.

lapcat2• 1 year ago

Question for the author, who is here in the comments: for clarification, to what extent is the article a deep dive into the OS itself (e.g., reverse engineering) vs. a deep dive into the extant literature on the OS?

tansanrao4• 1 year ago

I suppose this would depend on the context. As a PhD student, I primarily focus on building an understanding of systems through existing literature, so I would define a deep dive as a sufficiently thorough explanation of the system, its motivation, and it’s history.

In the case of XNU and Darwin, a lot of the sources are also blog posts from reverse engineering efforts by security researchers and jailbreaking communities so it blurs the lines.

naves• 1 year ago

Jobs tried to hire Torvalds to work on Mac OS X and Linus declined: https://www.macrumors.com/2012/03/22/steve-jobs-tried-to-hir...

tux3• 1 year ago

Hard to imagine Torvalds working on a microkernel, of all people

mike_hearn3• 1 year ago

That's a good history, but it skips over a lot of the nice security work that really distinguishes Apple's operating systems from Linux or Windows. There's a lack of appreciation out there for just how far ahead Apple now is when it comes to security. I sometimes wonder if one day awareness of this will grow and people working in sensitive contexts will be required to use a Mac by their CISO.

The keystone is the code signing system. It's what allows apps to be granted permissions, or to be sandboxed, and for that to actually stick. Apple doesn't use ELF like most UNIXs do, they use a format called Mach-O. The differences between ELF and Mach-O aren't important except for one: Mach-O supports an extra section containing a signed code directory. The code directory contains a series of hashes over code pages. The kernel has some understanding of this data structure and dyld can associate it with the binary or library as it gets loaded. XNU checks the signature over the code directory and the VMM subsystem then hashes code pages as they are loaded on demand, verifying the hashes match the signed hash in the directory. The hash of the code directory therefore can act as a unique identifier for any program in the Apple ecosystem. There's a bug here: the association hangs off the Mach vnode structure so if you overwrite a signed binary and then run it the kernel gets upset and kills the process, even if the new file has a valid signature. You have to actually replace the file as a whole for it to recognize the new situation.

On top of this foundation Apple adds code requirements. These are programs written in a small expression language that specifies constraints over aspects of a code signature. You can write a requirement like, "this binary must be signed by Apple" or "this binary can be of any version signed by an entity whose identity is X according to certificate authority Y" or "this binary must have a cdhash of Z" (i.e. be that exact binary). Binaries can also expose a designated requirement, which is the requirement by which they'd like to be known by other parties. This system initially looks like overkill but enables programs to evolve whilst retaining a stable and unforgeable identity.

The kernel exposes the signing identity of tasks to other tasks via ports. Requirements can then be imposed on those ports using a userspace library that interprets the constraint language. For example, if a program stores a key in the system keychain (which is implemented in user space) the keychain daemon examines the designated requirement of the program sending the RPC and ensures it matches future requests to use the key.

This system is abstracted by entitlements. These are key=value pairs that express permissions. Entitlements are an open system and apps can define their own. However, most entitlements are defined by Apple. Some are purely opt-in: you obtain the permission merely by asking for it and the OS grants it automatically and silently. These seem useless at first, but allow the App Store to explain what an app will do up front, and more generally enable a least-privilege stance where apps don't have access to things unless they need them. Some require additional evidence like a provisioning profile: this is a signed CMS data structure provided by Apple that basically says "apps with designated requirement X are allowed to use restricted entitlement Y", and so you must get Apple's permission to use them. And some are basically abused as a generic signed flags system; they aren't security related at all.

The system is then extended further, again through cooperation of userspace and XNU. Binaries being signable is a start but many programs have data files too. At this point the Apple security system becomes a bit hacky IMHO: the kernel isn't involved in checking the integrity of data files. Instead a plist is included at a special place in the slightly ad-hoc bundle directory layout format, the plist contains hashes of every data file in the bundle (at file not page granularity), the hash of the plist is placed in the code signature, and finally the whole thing is checked by Gatekeeper on first run. Gatekeeper is asked by the kernel if it's willing to let a program run and it decides based on the presence of extended attributes that are placed on files and then propagated by GUI tools like web browsers and decompression utilities. The userspace OS code like Finder invokes Gatekeeper to check out a program when it's been first downloaded, and Gatekeeper hashes every file in the bundle to ensure it matches what's signed in the binaries. This is why macOS has this slow "Verifying app" dialog that pops up on first run. Presumably it's done this way to avoid causing apps to stall when they open large data files without using mmap, but it's a pity because on fast networks the unoptimized Gatekeeper verification can actually be slower than the download itself. Apple doesn't care because they view out-of-store distribution as legacy tech.

Finally there is Seatbelt, a Lisp-based programming language for expressing sandbox rules. These files are compiled in userspace to some sort of bytecode that's evaluated by the kernel. The language is quite sophisticated and lets you express arbitrary rules for how different system components interact and what they can do, all based on the code signing identities.

The above scheme has an obvious loophole that was only closed in recent releases: data files might contain code and they're only checked once. In fact for any Electron or JVM app this is true because the code is in a portable format. So, one app could potentially inject code into another by editing data files and thus subvert code signing. To block this in modern macOS Seatbelt actually sandboxes every single app running. AFAIK there is no unsandboxed code in a modern macOS. One of the policies the sandbox imposes is that apps aren't allowed to modify the data files of other apps unless they've been granted that permission. The policy is quite sophisticated: apps can modify other apps if they're signed by the same legal entity as verified by Apple, apps can allow others matching code requirements to modify them, and users can grant permission on demand. To see this in action go into Settings -> Privacy & Security -> App Management, then turn it off for Terminal.app and (re)start it. Run something like "vim /Applications/Google Chrome.app/Contents/Info.plist" and observe that although the file has rw permissions vim thinks it's read-only.

Now, I'll admit that my understanding of how this works ends here because I don't work for Apple. AFAIK the kernel doesn't understand app bundles, and I'm not sure how it decides whether an open() syscall should be converted to read only or not. My guess is that the default Seatbelt policy tells the kernel to do an upcall to a security daemon which understands the bundle format and how to read the SQLite permission database. It then compares the designated requirement of the opener against the policies expressed by the bundle and the sandbox to make the decision.

adrian_b2• 1 year ago

I do not think that "security" is the appropriate name for such features.

In my opinion "security" should always refer to the security of the computer owners or users.

These Apple features may be used for enhancing security, but the main purpose for which they have been designed is to provide enhanced control of the computer vendor on how the computer that they have sold, and which is supposed to no longer belong to them, is used by its theoretical owner, i.e. by allowing Apple to decide which programs are run by the end user.

saagarjha3• 1 year ago

I think you went for a lazy reply rather than actually reading the comment through. Most of the things mentioned here directly improve security for the computer's owner.

mike_hearn3• 1 year ago

On macOS the security system is open even though the codebase is closed. You can disable SIP and get full root access. Gatekeeper can be configured to trust some authority other than Apple, or disabled completely. You can write and load your own sandbox policies. These things aren't well known and require reading obscure man pages, but the capabilities are there.

Even in the default out-of-the-box configuration, Apple isn't exercising editorial control over what apps you can run. Out of store distribution requires only a verified identity and a notarization pass, but notarization is a fully automated malware scan. There's no human in the loop. The App Store is different, of course.

Could Apple close up the Mac? Yes. The tech is there to do so and they do it on iOS. But... people have been predicting they'd do this from the first day the unfortunately named Gatekeeper was introduced. Yet they never have.

I totally get the concern and in the beginning I shared it, but at some point you have to just stop speculating give them credit for what they've actually done. It's much easier to distribute an app Apple executives don't like to a Mac than it is to distribute an app Linux distributors don't like to Linux users, because Linux app distribution barely works if you go "out of store" (distro repositories). In theory it should be the other way around, but it's not.

jlcases• 1 year ago

What impresses me most about technical documentation like this is how it structures knowledge into comprehensible layers. This article manages to explain an extremely complex system by establishing clear relationships between components.

I've been experimenting with similar approaches for documentation in open source projects, using knowledge graphs to link concepts and architectural decisions. The biggest challenge is always keeping documentation synchronized with evolving code.

Has anyone found effective tools for maintaining this synchronization between documented architecture and implemented code? Large projects like Darwin must have established processes for this.

rollcat3• 1 year ago

Has anyone found effective tools for maintaining this synchronization between documented architecture and implemented code?

Yes, it's called structure, discipline, and iterative improvement.

Keep the documentation alongside the code. Think in BSD terms: the OS is delivered as a whole; if I modify /bin/ls to support a new flag, then I update the ls.1 man page accordingly, preferably in the same commit/PR.

The man pages are a good reference if you already have familiarity with the employed concepts, so it's good to have an intro/overview document that walks you through those basics. This core design rarely sees radical changes, it tends to evolve - so adapt the overview as you make strategic decisions. The best benchmark is always a new hire. Find out what is it that they didn't understand, and task them with improving the document.

worik• 1 year ago

Has anyone found effective tools for...

Managing management?

Code comments and documentation make no money, only features make money.

Bitter experience...

librasteve• 1 year ago

i suggest you have a look at https://podlite.org/ which encourages you to interweave docs with source code

darksaints• 1 year ago

It seems like the XNU kernel is architecturally super close to the Mach kernel, and XNU drivers architecturally work like Mach drivers, but just that they are compiled into the kernel instead of running in userspace as a separate process. And it seems like the only reason for doing so is performance.

That makes me wonder: how hard would it be to run the XNU kernel in something like a “Mach mode”, where you take the same kernel and drivers but run them separately as the Mach microkernel was intended?

I feel like from a security standpoint, a lot of situations would gladly call for giving up a little bit of performance for the process isolation security benefits that come from running a microkernel.

Is anybody here familiar enough with XNU to opine on this?

dcrazy4• 1 year ago

Many drivers are gradually moving back into userspace via DriverKit: https://developer.apple.com/documentation/driverkit

llincerd• 1 year ago

Darwin is interesting because of the pace of radical changes to its core components. From dropping syscall backwards compatibility to mandatory code signing to dyld_shared_cache eliminating individual system library files to speed up dynamic executable loading. It's a very results-oriented design approach with no nostalgia and no sacred cows. I think only a big hardware vendor like Apple could pull it off.

conradev• 1 year ago

Totally. The march continues with userspace drivers and exclaves[1]. I think it's fair to say that security is a big driver for their kernel evolution.

[1] https://www.theregister.com/2025/03/08/kernel_sanders_apple_...

mannyv• 1 year ago

I suppose with unified memory there's no real difference between the kernel and userspace; it's just different security zones.

The MMU era used separate memory spaces to enforce security, but it's probably safer in the log run to actually have secure areas instead of 'accidentslly secure areas" that aren't that secure.

dcrazy4• 1 year ago

I think you’re misunderstanding “unified memory”. That term refers to whether the GPU has its own onboard memory chips which must be populated by a DMA transfer. It doesn’t refer to whether the system has an MMU.

adolph• 1 year ago

Can someone speak to the below statement from the article? I thought Objective-C did not have a runtime like memory managed languages like C#.

avoid the runtime overhead of Objective-C in the kernel

From Apple docs[0]:

You typically don’t need to use the Objective-C runtime library directly when programming in Objective-C. This API is useful primarily for developing bridge layers between Objective-C and other languages, or for low-level debugging.

0. https://developer.apple.com/documentation/objectivec/objecti...

twoodfin• 1 year ago

Objective-C does have a runtime that maintains all the state necessary to implement the APIs in the documentation you linked.

For example, how to map class objects to string representations of their names.

dcrazy4• 1 year ago

Yep, ObjC programs call into the runtime every time they call a method.

wuming2• 1 year ago

Where is Scott Forstall mark in all of this evolution I wonder. He was responsible for adapting macOS, hence XNU, to iPhone. The most successful Apple product of all times. And where is he now?

philjohn• 1 year ago

Well, he was one of the producers[1] of the musical Hadestown, which won 8 Tony awards.

[1] https://thebroadwayproducers.com/scott-forstall-2/

ladyanita223• 1 year ago

I'm a bit disappointed at the lack of interest Apple seems to have on Rust, given their focus on performance, UX and security.

melodyogonna• 1 year ago

I believe their plan is to make Swift good enough to use in high-performance low-level scenarios. Some of their recent work has been to reduce implicit copies

pjmlp9• 1 year ago

They created Swift as replacement for C, C++ and Objective-C, why should they bother with Rust?

Even Google on Android and ChromeOS is not exposing Rust to userspace, Java, Kotlin, C, C++, Javascript, Typescript, remain the official userspace languages.

ladyanita223• 1 year ago

Low-level code is being written in Rust. Of course userspace doesn't require low-level languages (and I don't quite get why C and C++ are exposed to the userspace and Rust is not).

Swift is not a replacemente for C and C++, but rather for Objective-C.

Do you have any sources for those claims?

astrange• 1 year ago

This is likely the origin of boosts and throttles that iOS uses to ensure the foreground app gets more CPU than background apps.

The purpose of these is for daemons to inherit the priority of their client apps when doing work for them.

I don't remember what thread groups/workloads are for.

Apple’s Darwin OS and XNU Kernel Deep Dive

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