Use `dmidecode` with the nixbld user

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1

Hi :wave:

I would like to build the hostname of a machine based on its serial-number.
Before nixpkgs 24.11 was released, i used to read file from sysfs to obtain the requiered values ( e.g. /sys/class/dmi/id/chassis_serial). However, with the new version of nixpkgs this is no longer possible and i’m searching for an alternative solution.

I’m considering using dmidecode. To achieve this, I created a file in nix store and that can be accessed from my NixOS config. However dmidecode requires read access to the /dev/mem file which is restricted to users in the kmem group. During the build process, i noticed that the nixbld user doesn’t belong to this group.
It is possible to add nixbld user to kmem group ?

FYI : I tried to do this by overriding all nixbldX user with the following config :

{ config, lib, ... }:
...
  # Add nixbuild to kmem groups
  users.extraUsers = builtins.listToAttrs ( lib.lists.forEach ( ( builtins.genList (x: x+1) config.nix.nrBuildUsers ) ++ [""] ) 
    (i:{ 
       name = "nixbld${builtins.toString i}"; 
       value = { 
	uid = if i == "" then 1000 else null;
         group = "nixbld"; 
         isSystemUser = true; 
         extraGroups = ["kmem"]; 
       }; 
    })
  );

Unfortunately, this approach did not work. The build user nixbld with uid 1000, only belongs to the nixbld group.
Thanks in advance! How can I resolve this?

2

Just don’t. Stop trying to work around the sandbox, it doesn’t work for good reason.

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