Previously, the .chroot directory had permission 750 or 755 (depending
on the uid-range system feature) and was owned by root/nixbld. This
makes it possible for any nixbld user (if uid-range is disabled) or
any user (if uid-range is enabled) to inspect the contents of the
chroot of an active build and maybe interfere with it (e.g. via /tmp
in the chroot, which has 1777 permission).
To prevent this, the root is now a subdirectory of .chroot, which has
permission 700 and is owned by root/root.
Instead of running the builds under
`$TMPDIR/{unique-build-directory-owned-by-the-build-user}`, run them
under `$TMPDIR/{unique-build-directory-owned-by-the-daemon}/{subdir-owned-by-the-build-user}`
where the build directory is only readable and traversable by the daemon user.
This achieves two things:
1. It prevents builders from making their build directory world-readable
(or even writeable), which would allow the outside world to interact
with them.
2. It prevents external processes running as the build user (either
because that somehow leaked, maybe as a consequence of 1., or because
`build-users` isn't in use) from gaining access to the build
directory.
the `std::filesystem::create_directories` can fail due to insufficient
permissions. We convert this error into a `SysError` and catch it
wherever required.
#2230 broadened the scope of macOS hardlink exclusion but did not change the comments. This was a little confusing for me, so I figured the comments should be updated.
This will avoid some out-of-memory issues in GitHub actions that result
from num jobs > 1 and num cores = 4. Once we only have the Meson build
system, this problem should go away, and we can reenable these jobs.
The documentation is clear about the supported formats (with at least
`builtins.fetchTarball`). The way the code was written previously it
supported all the formats that libarchive supported. That is a
surprisingly large amount of formats that are likely not on the radar
of the Nix developers and users. Before people end up relying on
this (or if they do) it is better to break it now before it becomes a
widespread "feature".
Zip file support has been retained as (at least to my knowledge)
historically that has been used to fetch nixpkgs in some shell
expressions *many* years back.
Fixes https://github.com/NixOS/nix/issues/10917
