The minimum build directory length on Darwin with default settings
when we were still using `/tmp` was 22 bytes. Deriving build directory
names from my local store, the median and mode were 43, the mean was
around 44½, and the maximum was 127.
The switch to `/nix/var/nix/builds` over `/tmp` added a 15 byte
penalty, and the additional `/b` directory added another 2.
Now that we use opaque build directory names, the length is 48, so
we’re still at a ~3½–5 byte penalty over the previous status
quo. This change brings us down to 43, matching the previous median.
Note that these calculations do not take into account the fact that
`/tmp` is a symbolic link to `/private/tmp` on Darwin. Anything
that was canonicalizing paths would have had an additional 8 byte
penalty in the previous status quo that is not applicable here,
so we may already be ahead even without this change. If the more
opaque directory name here is undesirable, then that factor could
potentially help us squeeze by without. Alternatively, in combination
with dropping the `/b` on Darwin we could use `/nix/var/nix/bld` or
similar, but I feel that the paths in general are sufficiently opaque
that it should be okay to go with the shorter option here. Given that
some projects already had to reduce filename lengths to avoid this
limit even before the recent changes, I think it is best to try and
improve on the previous status quo.
Note that `/nix/var/nix/builds` will unfortunately not be cleaned up
on Darwin. However, we don’t clean up the directories inside it on
Darwin anyway, so hopefully that’s okay for now?
Closes: #913
Change-Id: I6a6a6964bffce7194bcddcaefb4c4a37569c7df5
if a build directory is accessible to other users it is possible to
smuggle data in and out of build directories. usually this ins only
a build purity problem, but in combination with other issues it can
be used to break out of a build sandbox. to prevent this we default
to using a subdirectory of nixStateDir (which is more restrictive).
Fixes CVE-2025-52991.
Change-Id: Iacfc9b50534de158618c815f9fb99d7dae1be4d0
nix-daemon.socket is used to socket-activate nix-daemon.service when
/nix/var/nix/daemon-socket/socket is accessed.
In container usecases, sometimes /nix/var/nix/daemon-socket is
bind-mounted read-only into the container.
In these cases, we want to skip starting nix-daemon.socket.
However, since systemd 250, `ConditionPathIsReadWrite` is also not met
if /nix/var/nix/daemon-socket doesn't exist at all. This means, a
regular NixOS system will skip starting nix-daemon.socket:
> [ 237.187747] systemd[1]: Nix Daemon Socket was skipped because of a failed condition check (ConditionPathIsReadWrite=/nix/var/nix/daemon-socket).
To prevent this from happening, ship a tmpfiles file that'll cause the
directory to be created if it doesn't exist already.
In the case of NixOS, we can just add Nix to `systemd.tmpfiles.packages`
and have these files picked up automatically.
