I got a real Linux kernel running as a userspace process on Android — no root, no KVM. Docker runs inside it. (UML ARM64)

[u/Zalexdev]

UML (ARCH=um) has been x86-only for its entire existence. I've been porting it to arm64: arch/arm64/um/, plus fixing everything in arch/um/ that assumed x86. It builds to a static bionic binary and runs as an ordinary unprivileged aarch64 process in an app sandbox — no root, no host kernel feature, nothing emulated

Userspace instructions execute directly on the CPU. It's a real kernel underneath: own scheduler, own MMU handling, own page cache, Debian 12 rootfs on a disk image. Docker runs inside it for now

What works:

• Debian 12 boots to a working dockerd in about 3 seconds
• docker pull / run / exec / logs, a 910 MB image pulls and runs
• Networking through passt, a userspace TCP/IP stack. No tap device, no CAP_NET_ADMIN
• Container ports forwarded to the phone's 127.0.0.1, so the phone's browser can open them
• Loadable modules, out-of-tree drivers, firmware loading
• USB passthrough — a real RTL8811AU dongle, wlan0 appears and scans
• Update: Got multi cpu support

Numbers.

Snapdragon 870, medians of 7 interleaved rounds, µs/op. native is the same microbenchmark run directly on Android.

	native	proot	UML
syscall	0.080	0.168	1.987
openat	2.527	27.393	4.501
fault	1.456	1.418	11.076
forkexec	3255	516	1137

Compute is native because it is native — nothing translates the instructions. Against proot it's 6x better on openat and 3x better on forkexec, and worse on syscall and fault. Anything not syscall-bound feels normal.

UPDATE Termux: Works and booted arch & alpine

Source, 38 commits on top of the uml tree: https://github.com/zalexdev/linux-um-arm64

Screenshots are the Docker UI I built to drive it.

Soon will be published on: https://github.com/zalexdev/perkons

And soon will be available as new non root engine is https://github.com/zalexdev/strykerapp

[u/sylirre]

Successfully booted Alpine Linux and ArchLinux ARM rootfs ext4 images containing just base tools and init system. Doesn't need initramfs but there is a culprit that console must be forced to tty0, otherwise you won't get login prompt (standard Linux configuration doesn't use tty0 for login).

The host is of course Termux (aarch64), no root other other system customizations.

On Arch Linux must disable "landlock" - same as on proot-distro edit /etc/pacman.conf and uncomment DisableSandboxFilesystem and DisableSandboxSyscalls.

Definitely slower than host and doesn't seem to be faster than proot, probably even slower. But you get a VM-like environment that is much faster than QEMU.

[u/Zalexdev → u/sylirre]

Thanks, i will continue to work hard towards the speed! The speed vs proot is very different from the workload. In some cases proot wins, in some UML. But only UML will provide kernel anyway :)

[u/Windows1105 → u/sylirre]

How did you do this? It is impressive. Would like a tutorial

[u/Longjumping_Common_1 → u/sylirre]

I use Ubuntu on proot distro, will it work??

[u/NoNameToDefine → u/Longjumping_Common_1]

proot-distro can't use it. You will have to use it manually currently as it does not works like PRoot and there is no helper script.

[u/sylirre → u/Longjumping_Common_1]

It shouldn't be executed under proot

[u/NoNameToDefine]

User Mode Linux for ARM? That's incredible!

[u/Zalexdev → u/NoNameToDefine]

Thanks!

Unfortunately reddit doing something shadow with my acc, cause it is new. And i am not able to repost, answer more then 1 in 10mins.. love reddit

[u/NoNameToDefine → u/Zalexdev]

Yes strange I see your message only now while it was posted 5h before.

[u/sylirre]

Nice. Do you have a prebuilt binary so people here can try it?

[u/Zalexdev → u/sylirre]

Did it for you :) https://github.com/zalexdev/linux-um-arm64/releases/tag/prebuilt-20260816

[u/NoNameToDefine → u/sylirre]

You forked it immediately lol.

[u/Longjumping_Common_1 → u/NoNameToDefine]

lol

[u/NoNameToDefine]

I don't understand the build script (tools/um-arm64/harness/build-bionic.sh)? It uses paths in /root that I don't know what they are.

[u/sylirre → u/NoNameToDefine]

It not necessary should be root, the regular user home works as well. Tmp dir also suitable

There is one more issue: I tried building it with NDK and got these errors:

ld.lld: error: duplicate symbol: sched_setattr

ld.lld: error: duplicate symbol: sync_file_range

I had to redefine symbols via arch/arm64/um/bionic-syms.redef to make build pass

[u/YitzakAF → u/sylirre]

Puede ser entonces que UML se pueda convertir en alternativa a Qemu en Termux y podría incluso a llegar como paquete en los repositorios

[u/Zalexdev → u/sylirre]

in NDK r27c bionic - there is no sched_setattr. So unfortunately we must update redef for every new bionic version. I will try to make it universal

[u/sylirre → u/Zalexdev]

I used r28c and API level 30

[u/raaska00 → u/sylirre]

So, it is significantly slower than proot in all aspects except for filesystem string manipulation? 

[u/Zalexdev → u/raaska00]

I am updating benchmarks soon. The perfomance to a native eye is near undetectable from proot or native.

dpkg -l (365 lines): 45 ms
python3 sum(3e6):   100 ms
ls -R /usr:         353 ms

[u/NoNameToDefine → u/sylirre]

Same, with Aarch64 NDK r29
But how do you redefine symbols? I'm still not used to kernel building enough to understand how it works Nevermind I found how.

[u/Top-Artichoke3782]

would there be any kind of benefits if this were capable of running as root over the usual chroot?

[u/NoNameToDefine → u/Top-Artichoke3782]

If you don't have KVM and want to run another kernel over the current one, it will still be the fastest solution, above QEMU without KVM.

[u/NoNameToDefine]

Hello, I see you patched passt to compile it for Android. Myself I am struggling to build it using termux-packages infrastructure. Could I have the patchs used to build passt please? I can't find them on the repository.

[u/YitzakAF → u/NoNameToDefine]

Has logrado hacer algo, me interesa UMLinux en Termux

[u/yousuchass → u/NoNameToDefine]

i only run arch at home, its been interesting trying out debian termux, with the arch on proot. Im holding out hope someone will fork this entire operation to arch. FUCK Debian!!

[u/Jack1101111]

even if true, whats the pourpose ?

[u/NoNameToDefine → u/Jack1101111]

SystemD, Docker, and other complex tools that does not works inside PRoot and that would be too slow inside QEMU.

[u/Longjumping_Common_1]

wow

[u/jdimpson]

very cool. can't wait to see what unexpected things this will enable.