macOS 15 Sequoia clobbers _nixbld1-4 users

[abathur]

Note: I keep this first comment up-to-date. It includes fixes. You do not have to read the thread unless you are having trouble with its suggestions!

The macOS 15 Sequoia update takes 4 UIDs in the range we've been using, clobbering any _nixbldN users in the way (typically _nixbld1-4).

This manifests as:

• On existing installs, build errors like: error: the user '_nixbld1' in the group 'nixbld' does not exist

  To fix this, run our migration script (which relocates/replaces _nixbld users) before or after taking the macOS 15 Sequoia update:

  curl --proto '=https' --tlsv1.2 -sSf -L https://github.com/NixOS/nix/raw/master/scripts/sequoia-nixbld-user-migration.sh | bash -
  

  Caution: If you installed Nix with a third-party installer, you should check with them for additional/different instructions.

• On fresh installs (with unpatched installer versions), the following error creating user _nixbld1:

  <main> attribute status: eDSRecordAlreadyExists
  <dscl_cmd> DS Error: -14135 (eDSRecordAlreadyExists)
  

  While the 2.24.6+ installers are fixed, older installers don't all work at the moment. (The installer fixes for this have been backported for every release back to the 2.18 series--but these aren't all quite released yet.)

  If you're trying to install versions from 2.20.0 and 2.24.5, you can explicitly override the starting UID:

  NIX_FIRST_BUILD_UID="351" sh <(curl -L <whatever release-specific installer URL you need>)
  

  If you run into this error with versions older than 2.18, you'll need to download the installer tarball for your platform, unpack it, and update the first UID in install-darwin-multi-user.sh to 351.

More background/context on the issue

Edit: As the macOS release is near, I'm tucking context away to focus the first comment on how users can fix broken installs.

Reports are percolating about the upcoming macOS Sequoia 15 (from people trying the beta out) using 4 UIDs in the range we've been using:

• nix install breaks on UID clash #6153 (comment)
• nix install breaks on UID clash #6153 (comment)
• https://mastodon.social/@ryanbooker/112597391954278800

History on our previous change and ID range selection is in:

• fix nixbld user name/uid for macOS/darwin #4532
• macOS update "booted to Recovery" because "an error occurred migrating user data" on Big Sur/Monterey w/ multi-user Nix present  #4531

PRs to address:

• installer fix: install-darwin: fix _nixbld uids for macOS sequoia #10919
• migration script to fix existing installs w/o reinstalling: add UID migration script for macOS Sequoia 15 #11075

[fbettag]

cat /etc/passwd on sequoia shows

_aonsensed:*:300:300:Always On Sense Daemon:/var/db/aonsensed:/usr/bin/false
_modelmanagerd:*:301:301:Model Manager:/var/db/modelmanagerd:/usr/bin/false
_reportsystemmemory:*:302:302:ReportSystemMemory:/var/empty:/usr/bin/false
_swtransparencyd:*:303:303:Software Transparency Services:/var/db/swtransparencyd:/usr/bin/false
_naturallanguaged:*:304:304:Natural Language Services:/var/db/com.apple.naturallanguaged:/usr/bin/false
_oahd:*:441:441:OAH Daemon:/var/empty:/usr/bin/false

[ryanbooker]

Perhaps count down from 400? That's what I've done locally to fix the immediate issue. FYI, everything works with an arbitrary range, e.g. 3001–3032.

[abathur]

No personal objection to counting down as a strategy, but:

• IIRC the code that does this is in the general installer so we'd be changing it (and the "first" UID defaults) for linux as well.
• Since the starting UIDs are overridable via environment variables, a naive flip in the order would technically be a breaking interface change for anyone actually doing scripted installs that pre-set UIDs. (No clue how common this is in the wild.) We could try to preserve the semantics/behavior of the current env by doing math on the value, but it may turn into a bit of a stumbling block over time if people get used to it going in reverse order and expect the variable to control the number it counts down from?

[roberth]

Also affects nix-darwin

• macOS Sequoia replaces _nixbld{1,2,3,4} with system users causing darwin-rebuild to fail LnL7/nix-darwin#970

[stepbrobd]

Question 1:
Would it be possible to workaround this without reinstalling Nix on macOS systems with ids option (exposed but not in docs, and changing the settings doesn't change anyting)?

Q1 RFC:
In this file, nixbld = 300; is set but this is not used anywhere. Perhaps we can add an idempotent shell script to add/remove nixbld group and nixbld* users on every rebuild?

Question 2:
For NixOS systems, if I'm understanding the docs correctly, nixbld* users are not needed to perform builds when auto-allocate-uids or cgroups is enabled, is there anything equivalent to these on macOS systems?

[abathur]

@michaelvanstraten noted in #6153 (comment) that you can get unblocked on new installs for testing with:

To repeat @ikuz's solution again for a quick fix on macOS 15 install/reinstall with:

 NIX_FIRST_BUILD_UID="351" sh <(curl -L https://nixos.org/nix/install)

[emilazy]

Some thoughts on where to reassign the IDs:

We need to be below (or equal to?) UID 400, and Apple has now used up to UID 304. Clearly we should expect they might keep adding users to the low end of this range occasionally. However, running right up against the 400 limit doesn’t seem safe to me either; /etc/group on my Sonoma machine contains groups from 395 to 400, so it seems like Apple considers the upper end of the system range to be its for the taking as well. The natural place to go, then, would be in the middle of the range.

We default to 32 build users currently. The main reason you might want more is that it limits the number of concurrent build jobs, and the number of those you might want to run is proportional to the cores/threads on your machine. The highest number of cores on a currently-shipping Mac is the 24‐core M2 Ultra, they used to ship Xeons with 48 threads (24 cores × 2 threads/core), and there are rumours of a 32‐core M3 Ultra and even a 64‐core M3 Ultimate. We can’t fit more than 96 users at the absolute maximum as of Sequoia, and that would obviously be risky, so let’s say we want to plan to have space for around 64 to 80 build users.

I suggest we start at 331 (if we want to keep the last digit matching the build user number) or 330 (if we don’t). That gives us enough margin for Apple to add ~26 new users before we run into problems again, 38 empty spaces above the top UID for the current default of 32 users, and just enough space to squeeze in 64 users before hitting the 395 ID that Apple has already used for a group.

The other good candidate would be 321/320; that reduces the margin on the low end to ~16 new Apple users, but increases the margin on the upper end, to (if using 320) just barely allow squeezing in 80 users in the available range. Personally I feel like the release of an 80‐core Mac would make me scared for 96 to 128‐core Macs that we have no realistic way of adding enough users for with our current approach anyway, and we have precedent that Apple is happy to add users on the low end of the range, so I lean towards 331 to give us more margin on that side. But I’m ambivalent if people have a strong preference for more margin to add and think that Apple will continue adding system users at a restrained enough pace compared to core count inflation. 331 spreads our users around the middle of the available range for 32 users, 321 does the same for 64 users.

This would also be a good opportunity to change the group ID; the current default of 30000 has the unfortunate effect of making the group show up in System Settings, unlike system groups. I would suggest using a related ID of 330 or 320 depending on the choice, and perhaps renaming it to _nixbld for consistency with the user names and other system groups (unless there’s any reason not to?).

Since we have to coordinate this with two installers and nix-darwin and get a migration plan in place before the release of Sequoia, I hope we can commit to a set of IDs ASAP to enable this to go smoothly.

[emilazy]

Incidentally I note that _oahd has UID 441 which makes me wonder if Apple has secretly expanded the system user range without updating the meagre documentation of it? But I remember it being a pain to test and reproduce the issues that led us to use the system UID range in the first place, so it’d need careful verification of the bounds if we wanted to see if we could go beyond 400.

[emilazy]

Some more investigation:

It seems like groups with GID < 500 don’t show up in System Settings. This may imply that the system UID range has also expanded, but I don’t know a convenient way to check, and anyway considering that Apple is using the middle of that range with 441 it might be awkward real estate to occupy; who knows what values Apple might pick in future. (There are also groups with higher GIDs that don’t show up in System Settings (e.g. com.apple.sharepoint.group.1/“<name>’s Public Folder”) that have dsAttrTypeNative:IsHidden: 1 in dscl(1) and Directory Utility, but setting that for the nixbld group didn’t seem to help.)

The maximum in‐use UID < 400 went from 297 to 304 in one version. I don’t know what the historical growth rate is like, but I’d definitely be more comfortable with 331 than 321 given that. If the growth continued at the rate of Sequoia (which seems unlikely, but still), we’d have to think of a new idea in 4 OS releases rather than 3. In general it seems like we’re on borrowed time here and I’m not really sure what the long‐term solution is. We may have to migrate now with the expectation of migrating again later.

If we could verify that UIDs < 500 now work fine, I suppose one solution would be to start at, say, 360 now, and hope Apple don’t eat up the lower 400s range if we start having machines that want 64 users. But I don’t remember how to test that.

[abathur]

Incidentally I note that _oahd has UID 441 which makes me wonder if Apple has secretly expanded the system user range without updating the meagre documentation of it?

Does this mean you looked at the usage info for sysadminctl and it still says 200-400?

But I remember it being a pain to test and reproduce the issues that led us to use the system UID range in the first place, so it’d need careful verification of the bounds if we wanted to see if we could go beyond 400.

Yeah. The main manifestation I recall was the system giving an obtuse adrenaline-inducing error message and booting into recovery mode during system updates that require a full reboot cycle. Looking back over the thread, it looks like that got less-scary on the next point release. The other was the build users showing up in a user list.

If we wanted to try moving outside of the current range, I suspect a workable protocol would be installing into the new range on a sub-sequoia version and then running the sequoia update and seeing if it blows up. (I don't currently have a spare mac that's eligible for this update, so someone else will have to drive...)

It can't save us from needing to fix this UID issue in the short run, but one way to address the long-run problem would be to figure out if we can get the various issues with auto-allocate-uids sorted out in order to make it the default. (The detsys folks tried defaulting to this in their installer and ran into trouble that compelled them to revert it on both macOS and Linux.)

After I hit post here I'll start drafting a feedback/radar report for Apple. Once I do, I'll also email their devrel about it, and post the FB number here in case anyone wants to refer to it from their own FB report. I'm not terribly optimistic about that helping (for example, I never got a response on the reports I opened about the big sur issue in 2021), but I guess there's an outside chance they'll improve their updater to relocate any UID/GID they trample to another valid ID.

(That would leave people with Weird installs--they might fail to fully clean up when they follow the uninstall instructions for example--but I think it would at least not break every existing macOS install made since early 2022 or whenever the multi-user default was released...)

---

For searchability, here's one real manifestation of this on an existing install (from ##10912):

...
these 13 derivations will be built:
  /nix/store/jcrd05mlpsw8wmixwd133pv3q3xbm18w-nerdfonts-3.2.1.drv
  ...
error: the user '_nixbld1' in the group 'nixbld' does not exist

[emilazy]

Does this mean you looked at the usage info for sysadminctl and it still says 200-400?

On Sonoma, which already has that _oahd user, yes; so if that’s within the system UID range and there’s not something weird going on with that user specifically and groups in the 400 to 500 range (perfectly possible! OAH is the internal name for Rosetta 2, as I understand it, so I wouldn’t be surprised if there are strange things going on there), then they expanded it without updating what passes for the “documentation”. I haven’t tried Sequoia yet, so I can’t comment on what the command says there.

Yeah. The main manifestation I recall was the system giving an obtuse adrenaline-inducing error message and booting into recovery mode during system updates that require a full reboot cycle. Looking back over the thread, it looks like that got less-scary on the next point release. The other was the build users showing up in a user list.

I think filling up the visible user list with 32 random daemon users is scary and off‐putting to users (especially in the absence of an official upstream uninstaller), so even if the more fundamental issues might be solved now I’d be reluctant to settle on that unless we can find another way to hide them.

It can't save us from needing to fix this UID issue in the short run, but one way to address the long-run problem would be to figure out if we can get the various issues with auto-allocate-uids sorted out in order to make it the default. (The detsys folks tried defaulting to this in their installer and ran into trouble that compelled them to revert it on both macOS and Linux.)

Yes, I would love this. If we can commit in the interim to e.g. UIDs starting at 331 and a GID of 330, hopefully that would give us enough runway to make something workable out of auto-allocate-uids. I remember hearing that the problems were worse on macOS than Linux, though (e.g. DeterminateSystems/nix-installer#521, DeterminateSystems/nix-installer#580 (comment) – I guess the lack of user namespaces really makes it tricky).