Merge pull request #3600 from NixOS/auto-uid-allocation

Automatic UID allocation

doc/manual/src/release-notes/rl-next.md

@@ -10,3 +10,45 @@
   This avoids a lot of spurious errors where some benign strings end-up having
   a context just because they are read from a store path
   ([#7260](https://github.com/NixOS/nix/pull/7260)).
+
+* Nix can now automatically pick UIDs for builds, removing the need to
+  create `nixbld*` user accounts. These UIDs are allocated starting at
+  872415232 (0x34000000) on Linux and 56930 on macOS.
+
+  This is an experimental feature. To enable it, add the following to
+  `nix.conf`:
+
+  ```
+  extra-experimental-features = auto-allocate-uids
+  auto-allocate-uids = true
+  ```
+
+* On Linux, Nix can now run builds in a user namespace where the build
+  runs as root (UID 0) and has 65,536 UIDs available. This is
+  primarily useful for running containers such as `systemd-nspawn`
+  inside a Nix build. For an example, see
+  https://github.com/NixOS/nix/blob/67bcb99700a0da1395fa063d7c6586740b304598/tests/systemd-nspawn.nix.
+
+  A build can enable this by requiring the `uid-range` system feature,
+  i.e. by setting the derivation attribute
+
+  ```
+  requiredSystemFeatures = [ "uid-range" ];
+  ```
+
+  The `uid-range` system feature requires the `auto-allocate-uids`
+  setting to be enabled (see above).
+
+* On Linux, Nix has experimental support for running builds inside a
+  cgroup. It can be enabled by adding
+
+  ```
+  extra-experimental-features = cgroups
+  use-cgroups = true
+  ```
+
+  to `nix.conf`. Cgroups are required for derivations that require the
+  `uid-range` system feature.
+
+* `nix build --json` now prints some statistics about top-level
+  derivations, such as CPU statistics when cgroups are enabled.

flake.nix

@@ -506,6 +506,12 @@
           overlay = self.overlays.default;
         });
 
+        tests.containers = (import ./tests/containers.nix rec {
+          system = "x86_64-linux";
+          inherit nixpkgs;
+          overlay = self.overlays.default;
+        });
+
         tests.setuid = nixpkgs.lib.genAttrs
           ["i686-linux" "x86_64-linux"]
           (system:

src/libcmd/installables.cc

@@ -931,7 +931,7 @@ std::vector<std::pair<std::shared_ptr<Installable>, BuiltPathWithResult>> Instal
 
     case Realise::Outputs: {
         if (settings.printMissing)
-          printMissing(store, pathsToBuild, lvlInfo);
+            printMissing(store, pathsToBuild, lvlInfo);
 
         for (auto & buildResult : store->buildPathsWithResults(pathsToBuild, bMode, evalStore)) {
             if (!buildResult.success())

src/libstore/build-result.hh

@@ -5,7 +5,7 @@
 
 #include <string>
 #include <chrono>
-
+#include <optional>
 
 namespace nix {
 
@@ -78,6 +78,9 @@ struct BuildResult
        was repeated). */
     time_t startTime = 0, stopTime = 0;
 
+    /* User and system CPU time the build took. */
+    std::optional<std::chrono::microseconds> cpuUser, cpuSystem;
+
     bool success()
     {
         return status == Built || status == Substituted || status == AlreadyValid || status == ResolvesToAlreadyValid;

src/libstore/build/derivation-goal.cc

@@ -886,6 +886,14 @@ void DerivationGoal::buildDone()
 
     cleanupPostChildKill();
 
+    if (buildResult.cpuUser && buildResult.cpuSystem) {
+        debug("builder for '%s' terminated with status %d, user CPU %.3fs, system CPU %.3fs",
+            worker.store.printStorePath(drvPath),
+            status,
+            ((double) buildResult.cpuUser->count()) / 1000000,
+            ((double) buildResult.cpuSystem->count()) / 1000000);
+    }
+
     bool diskFull = false;
 
     try {

src/libstore/build/local-derivation-goal.cc

@@ -14,6 +14,7 @@
 #include "topo-sort.hh"
 #include "callback.hh"
 #include "json-utils.hh"
+#include "cgroup.hh"
 
 #include <regex>
 #include <queue>
@@ -129,26 +130,44 @@ void LocalDerivationGoal::killChild()
     if (pid != -1) {
         worker.childTerminated(this);
 
-        if (buildUser) {
+        /* If we're using a build user, then there is a tricky race
-            /* If we're using a build user, then there is a tricky
+           condition: if we kill the build user before the child has
-               race condition: if we kill the build user before the
+           done its setuid() to the build user uid, then it won't be
-               child has done its setuid() to the build user uid, then
+           killed, and we'll potentially lock up in pid.wait().  So
-               it won't be killed, and we'll potentially lock up in
+           also send a conventional kill to the child. */
-               pid.wait().  So also send a conventional kill to the
+        ::kill(-pid, SIGKILL); /* ignore the result */
-               child. */
-            ::kill(-pid, SIGKILL); /* ignore the result */
-            buildUser->kill();
-            pid.wait();
-        } else
-            pid.kill();
 
-        assert(pid == -1);
+        killSandbox(true);
+
+        pid.wait();
     }
 
     DerivationGoal::killChild();
 }
 
 
+void LocalDerivationGoal::killSandbox(bool getStats)
+{
+    if (cgroup) {
+        #if __linux__
+        auto stats = destroyCgroup(*cgroup);
+        if (getStats) {
+            buildResult.cpuUser = stats.cpuUser;
+            buildResult.cpuSystem = stats.cpuSystem;
+        }
+        #else
+        abort();
+        #endif
+    }
+
+    else if (buildUser) {
+        auto uid = buildUser->getUID();
+        assert(uid != 0);
+        killUser(uid);
+    }
+}
+
+
 void LocalDerivationGoal::tryLocalBuild() {
     unsigned int curBuilds = worker.getNrLocalBuilds();
     if (curBuilds >= settings.maxBuildJobs) {
@@ -158,28 +177,46 @@ void LocalDerivationGoal::tryLocalBuild() {
         return;
     }
 
-    /* If `build-users-group' is not empty, then we have to build as
+    /* Are we doing a chroot build? */
-       one of the members of that group. */
+    {
-    if (settings.buildUsersGroup != "" && getuid() == 0) {
+        auto noChroot = parsedDrv->getBoolAttr("__noChroot");
-#if defined(__linux__) || defined(__APPLE__)
+        if (settings.sandboxMode == smEnabled) {
-        if (!buildUser) buildUser = std::make_unique<UserLock>();
+            if (noChroot)
+                throw Error("derivation '%s' has '__noChroot' set, "
+                    "but that's not allowed when 'sandbox' is 'true'", worker.store.printStorePath(drvPath));
+#if __APPLE__
+            if (additionalSandboxProfile != "")
+                throw Error("derivation '%s' specifies a sandbox profile, "
+                    "but this is only allowed when 'sandbox' is 'relaxed'", worker.store.printStorePath(drvPath));
+#endif
+            useChroot = true;
+        }
+        else if (settings.sandboxMode == smDisabled)
+            useChroot = false;
+        else if (settings.sandboxMode == smRelaxed)
+            useChroot = derivationType.isSandboxed() && !noChroot;
+    }
 
-        if (buildUser->findFreeUser()) {
+    auto & localStore = getLocalStore();
-            /* Make sure that no other processes are executing under this
+    if (localStore.storeDir != localStore.realStoreDir.get()) {
-               uid. */
+        #if __linux__
-            buildUser->kill();
+            useChroot = true;
-        } else {
+        #else
+            throw Error("building using a diverted store is not supported on this platform");
+        #endif
+    }
+
+    if (useBuildUsers()) {
+        if (!buildUser)
+            buildUser = acquireUserLock(parsedDrv->useUidRange() ? 65536 : 1, useChroot);
+
+        if (!buildUser) {
             if (!actLock)
                 actLock = std::make_unique<Activity>(*logger, lvlWarn, actBuildWaiting,
                     fmt("waiting for UID to build '%s'", yellowtxt(worker.store.printStorePath(drvPath))));
             worker.waitForAWhile(shared_from_this());
             return;
         }
-#else
-        /* Don't know how to block the creation of setuid/setgid
-           binaries on this platform. */
-        throw Error("build users are not supported on this platform for security reasons");
-#endif
     }
 
     actLock.reset();
@@ -270,7 +307,7 @@ void LocalDerivationGoal::cleanupPostChildKill()
        malicious user from leaving behind a process that keeps files
        open and modifies them after they have been chown'ed to
        root. */
-    if (buildUser) buildUser->kill();
+    killSandbox(true);
 
     /* Terminate the recursive Nix daemon. */
     stopDaemon();
@@ -363,6 +400,60 @@ static void linkOrCopy(const Path & from, const Path & to)
 
 void LocalDerivationGoal::startBuilder()
 {
+    if ((buildUser && buildUser->getUIDCount() != 1)
+        #if __linux__
+        || settings.useCgroups
+        #endif
+        )
+    {
+        #if __linux__
+        settings.requireExperimentalFeature(Xp::Cgroups);
+
+        auto ourCgroups = getCgroups("/proc/self/cgroup");
+        auto ourCgroup = ourCgroups[""];
+        if (ourCgroup == "")
+            throw Error("cannot determine cgroup name from /proc/self/cgroup");
+
+        auto ourCgroupPath = canonPath("/sys/fs/cgroup/" + ourCgroup);
+
+        if (!pathExists(ourCgroupPath))
+            throw Error("expected cgroup directory '%s'", ourCgroupPath);
+
+        static std::atomic<unsigned int> counter{0};
+
+        cgroup = buildUser
+            ? fmt("%s/nix-build-uid-%d", ourCgroupPath, buildUser->getUID())
+            : fmt("%s/nix-build-pid-%d-%d", ourCgroupPath, getpid(), counter++);
+
+        debug("using cgroup '%s'", *cgroup);
+
+        /* When using a build user, record the cgroup we used for that
+           user so that if we got interrupted previously, we can kill
+           any left-over cgroup first. */
+        if (buildUser) {
+            auto cgroupsDir = settings.nixStateDir + "/cgroups";
+            createDirs(cgroupsDir);
+
+            auto cgroupFile = fmt("%s/%d", cgroupsDir, buildUser->getUID());
+
+            if (pathExists(cgroupFile)) {
+                auto prevCgroup = readFile(cgroupFile);
+                destroyCgroup(prevCgroup);
+            }
+
+            writeFile(cgroupFile, *cgroup);
+        }
+
+        #else
+        throw Error("cgroups are not supported on this platform");
+        #endif
+    }
+
+    /* Make sure that no other processes are executing under the
+       sandbox uids. This must be done before any chownToBuilder()
+       calls. */
+    killSandbox(false);
+
     /* Right platform? */
     if (!parsedDrv->canBuildLocally(worker.store))
         throw Error("a '%s' with features {%s} is required to build '%s', but I am a '%s' with features {%s}",
@@ -376,35 +467,6 @@ void LocalDerivationGoal::startBuilder()
     additionalSandboxProfile = parsedDrv->getStringAttr("__sandboxProfile").value_or("");
 #endif
 
-    /* Are we doing a chroot build? */
-    {
-        auto noChroot = parsedDrv->getBoolAttr("__noChroot");
-        if (settings.sandboxMode == smEnabled) {
-            if (noChroot)
-                throw Error("derivation '%s' has '__noChroot' set, "
-                    "but that's not allowed when 'sandbox' is 'true'", worker.store.printStorePath(drvPath));
-#if __APPLE__
-            if (additionalSandboxProfile != "")
-                throw Error("derivation '%s' specifies a sandbox profile, "
-                    "but this is only allowed when 'sandbox' is 'relaxed'", worker.store.printStorePath(drvPath));
-#endif
-            useChroot = true;
-        }
-        else if (settings.sandboxMode == smDisabled)
-            useChroot = false;
-        else if (settings.sandboxMode == smRelaxed)
-            useChroot = derivationType.isSandboxed() && !noChroot;
-    }
-
-    auto & localStore = getLocalStore();
-    if (localStore.storeDir != localStore.realStoreDir.get()) {
-        #if __linux__
-            useChroot = true;
-        #else
-            throw Error("building using a diverted store is not supported on this platform");
-        #endif
-    }
-
     /* Create a temporary directory where the build will take
        place. */
     tmpDir = createTempDir("", "nix-build-" + std::string(drvPath.name()), false, false, 0700);
@@ -580,10 +642,11 @@ void LocalDerivationGoal::startBuilder()
 
         printMsg(lvlChatty, format("setting up chroot environment in '%1%'") % chrootRootDir);
 
-        if (mkdir(chrootRootDir.c_str(), 0750) == -1)
+        // FIXME: make this 0700
+        if (mkdir(chrootRootDir.c_str(), buildUser && buildUser->getUIDCount() != 1 ? 0755 : 0750) == -1)
             throw SysError("cannot create '%1%'", chrootRootDir);
 
-        if (buildUser && chown(chrootRootDir.c_str(), 0, buildUser->getGID()) == -1)
+        if (buildUser && chown(chrootRootDir.c_str(), buildUser->getUIDCount() != 1 ? buildUser->getUID() : 0, buildUser->getGID()) == -1)
             throw SysError("cannot change ownership of '%1%'", chrootRootDir);
 
         /* Create a writable /tmp in the chroot.  Many builders need
@@ -597,6 +660,10 @@ void LocalDerivationGoal::startBuilder()
            nobody account.  The latter is kind of a hack to support
            Samba-in-QEMU. */
         createDirs(chrootRootDir + "/etc");
+        chownToBuilder(chrootRootDir + "/etc");
+
+        if (parsedDrv->useUidRange() && (!buildUser || buildUser->getUIDCount() < 65536))
+            throw Error("feature 'uid-range' requires the setting '%s' to be enabled", settings.autoAllocateUids.name);
 
         /* Declare the build user's group so that programs get a consistent
            view of the system (e.g., "id -gn"). */
@@ -647,12 +714,28 @@ void LocalDerivationGoal::startBuilder()
                 dirsInChroot.erase(worker.store.printStorePath(*i.second.second));
         }
 
-#elif __APPLE__
+        if (cgroup) {
-        /* We don't really have any parent prep work to do (yet?)
+            if (mkdir(cgroup->c_str(), 0755) != 0)
-           All work happens in the child, instead. */
+                throw SysError("creating cgroup '%s'", *cgroup);
+            chownToBuilder(*cgroup);
+            chownToBuilder(*cgroup + "/cgroup.procs");
+            chownToBuilder(*cgroup + "/cgroup.threads");
+            //chownToBuilder(*cgroup + "/cgroup.subtree_control");
+        }
+
 #else
-        throw Error("sandboxing builds is not supported on this platform");
+        if (parsedDrv->useUidRange())
+            throw Error("feature 'uid-range' is not supported on this platform");
+        #if __APPLE__
+            /* We don't really have any parent prep work to do (yet?)
+               All work happens in the child, instead. */
+        #else
+            throw Error("sandboxing builds is not supported on this platform");
+        #endif
 #endif
+    } else {
+        if (parsedDrv->useUidRange())
+            throw Error("feature 'uid-range' is only supported in sandboxed builds");
     }
 
     if (needsHashRewrite() && pathExists(homeDir))
@@ -913,14 +996,16 @@ void LocalDerivationGoal::startBuilder()
                the calling user (if build users are disabled). */
             uid_t hostUid = buildUser ? buildUser->getUID() : getuid();
             uid_t hostGid = buildUser ? buildUser->getGID() : getgid();
+            uid_t nrIds = buildUser ? buildUser->getUIDCount() : 1;
 
             writeFile("/proc/" + std::to_string(pid) + "/uid_map",
-                fmt("%d %d 1", sandboxUid(), hostUid));
+                fmt("%d %d %d", sandboxUid(), hostUid, nrIds));
 
-            writeFile("/proc/" + std::to_string(pid) + "/setgroups", "deny");
+            if (!buildUser || buildUser->getUIDCount() == 1)
+                writeFile("/proc/" + std::to_string(pid) + "/setgroups", "deny");
 
             writeFile("/proc/" + std::to_string(pid) + "/gid_map",
-                fmt("%d %d 1", sandboxGid(), hostGid));
+                fmt("%d %d %d", sandboxGid(), hostGid, nrIds));
         } else {
             debug("note: not using a user namespace");
             if (!buildUser)
@@ -947,6 +1032,10 @@ void LocalDerivationGoal::startBuilder()
                 throw SysError("getting sandbox user namespace");
         }
 
+        /* Move the child into its own cgroup. */
+        if (cgroup)
+            writeFile(*cgroup + "/cgroup.procs", fmt("%d", (pid_t) pid));
+
         /* Signal the builder that we've updated its user namespace. */
         writeFull(userNamespaceSync.writeSide.get(), "1");
 
@@ -1779,6 +1868,13 @@ void LocalDerivationGoal::runChild()
             if (mount("none", (chrootRootDir + "/proc").c_str(), "proc", 0, 0) == -1)
                 throw SysError("mounting /proc");
 
+            /* Mount sysfs on /sys. */
+            if (buildUser && buildUser->getUIDCount() != 1) {
+                createDirs(chrootRootDir + "/sys");
+                if (mount("none", (chrootRootDir + "/sys").c_str(), "sysfs", 0, 0) == -1)
+                    throw SysError("mounting /sys");
+            }
+
             /* Mount a new tmpfs on /dev/shm to ensure that whatever
                the builder puts in /dev/shm is cleaned up automatically. */
             if (pathExists("/dev/shm") && mount("none", (chrootRootDir + "/dev/shm").c_str(), "tmpfs", 0,
@@ -1821,6 +1917,12 @@ void LocalDerivationGoal::runChild()
             if (unshare(CLONE_NEWNS) == -1)
                 throw SysError("unsharing mount namespace");
 
+            /* Unshare the cgroup namespace. This means
+               /proc/self/cgroup will show the child's cgroup as '/'
+               rather than whatever it is in the parent. */
+            if (cgroup && unshare(CLONE_NEWCGROUP) == -1)
+                throw SysError("unsharing cgroup namespace");
+
             /* Do the chroot(). */
             if (chdir(chrootRootDir.c_str()) == -1)
                 throw SysError("cannot change directory to '%1%'", chrootRootDir);
@@ -1906,9 +2008,8 @@ void LocalDerivationGoal::runChild()
         if (setUser && buildUser) {
             /* Preserve supplementary groups of the build user, to allow
                admins to specify groups such as "kvm".  */
-            if (!buildUser->getSupplementaryGIDs().empty() &&
+            auto gids = buildUser->getSupplementaryGIDs();
-                setgroups(buildUser->getSupplementaryGIDs().size(),
+            if (setgroups(gids.size(), gids.data()) == -1)
-                          buildUser->getSupplementaryGIDs().data()) == -1)
                 throw SysError("cannot set supplementary groups of build user");
 
             if (setgid(buildUser->getGID()) == -1 ||
@@ -2237,7 +2338,10 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
         /* Canonicalise first.  This ensures that the path we're
            rewriting doesn't contain a hard link to /etc/shadow or
            something like that. */
-        canonicalisePathMetaData(actualPath, buildUser ? buildUser->getUID() : -1, inodesSeen);
+        canonicalisePathMetaData(
+            actualPath,
+            buildUser ? std::optional(buildUser->getUIDRange()) : std::nullopt,
+            inodesSeen);
 
         debug("scanning for references for output '%s' in temp location '%s'", outputName, actualPath);
 
@@ -2330,6 +2434,10 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
                 sink.s = rewriteStrings(sink.s, outputRewrites);
                 StringSource source(sink.s);
                 restorePath(actualPath, source);
+
+                /* FIXME: set proper permissions in restorePath() so
+                   we don't have to do another traversal. */
+                canonicalisePathMetaData(actualPath, {}, inodesSeen);
             }
         };
 
@@ -2492,7 +2600,7 @@ DrvOutputs LocalDerivationGoal::registerOutputs()
 
         /* FIXME: set proper permissions in restorePath() so
             we don't have to do another traversal. */
-        canonicalisePathMetaData(actualPath, -1, inodesSeen);
+        canonicalisePathMetaData(actualPath, {}, inodesSeen);
 
         /* Calculate where we'll move the output files. In the checking case we
            will leave leave them where they are, for now, rather than move to

src/libstore/build/local-derivation-goal.hh

@@ -15,6 +15,9 @@ struct LocalDerivationGoal : public DerivationGoal
     /* The process ID of the builder. */
     Pid pid;
 
+    /* The cgroup of the builder, if any. */
+    std::optional<Path> cgroup;
+
     /* The temporary directory. */
     Path tmpDir;
 
@@ -92,8 +95,8 @@ struct LocalDerivationGoal : public DerivationGoal
        result. */
     std::map<Path, ValidPathInfo> prevInfos;
 
-    uid_t sandboxUid() { return usingUserNamespace ? 1000 : buildUser->getUID(); }
+    uid_t sandboxUid() { return usingUserNamespace ? (!buildUser || buildUser->getUIDCount() == 1 ? 1000 : 0) : buildUser->getUID(); }
-    gid_t sandboxGid() { return usingUserNamespace ?  100 : buildUser->getGID(); }
+    gid_t sandboxGid() { return usingUserNamespace ? (!buildUser || buildUser->getUIDCount() == 1 ? 100  : 0) : buildUser->getGID(); }
 
     const static Path homeDir;
 
@@ -197,6 +200,10 @@ struct LocalDerivationGoal : public DerivationGoal
     /* Forcibly kill the child process, if any. */
     void killChild() override;
 
+    /* Kill any processes running under the build user UID or in the
+       cgroup of the build. */
+    void killSandbox(bool getStats);
+
     /* Create alternative path calculated from but distinct from the
        input, so we can avoid overwriting outputs (or other store paths)
        that already exist. */

src/libstore/cgroup.cc

@@ -0,0 +1,131 @@
+#if __linux__
+
+#include "cgroup.hh"
+#include "util.hh"
+
+#include <chrono>
+#include <cmath>
+#include <regex>
+#include <unordered_set>
+#include <thread>
+
+#include <dirent.h>
+
+namespace nix {
+
+// FIXME: obsolete, check for cgroup2
+std::map<std::string, std::string> getCgroups(const Path & cgroupFile)
+{
+    std::map<std::string, std::string> cgroups;
+
+    for (auto & line : tokenizeString<std::vector<std::string>>(readFile(cgroupFile), "\n")) {
+        static std::regex regex("([0-9]+):([^:]*):(.*)");
+        std::smatch match;
+        if (!std::regex_match(line, match, regex))
+            throw Error("invalid line '%s' in '%s'", line, cgroupFile);
+
+        std::string name = hasPrefix(std::string(match[2]), "name=") ? std::string(match[2], 5) : match[2];
+        cgroups.insert_or_assign(name, match[3]);
+    }
+
+    return cgroups;
+}
+
+static CgroupStats destroyCgroup(const Path & cgroup, bool returnStats)
+{
+    if (!pathExists(cgroup)) return {};
+
+    auto procsFile = cgroup + "/cgroup.procs";
+
+    if (!pathExists(procsFile))
+        throw Error("'%s' is not a cgroup", cgroup);
+
+    /* Use the fast way to kill every process in a cgroup, if
+       available. */
+    auto killFile = cgroup + "/cgroup.kill";
+    if (pathExists(killFile))
+        writeFile(killFile, "1");
+
+    /* Otherwise, manually kill every process in the subcgroups and
+       this cgroup. */
+    for (auto & entry : readDirectory(cgroup)) {
+        if (entry.type != DT_DIR) continue;
+        destroyCgroup(cgroup + "/" + entry.name, false);
+    }
+
+    int round = 1;
+
+    std::unordered_set<pid_t> pidsShown;
+
+    while (true) {
+        auto pids = tokenizeString<std::vector<std::string>>(readFile(procsFile));
+
+        if (pids.empty()) break;
+
+        if (round > 20)
+            throw Error("cannot kill cgroup '%s'", cgroup);
+
+        for (auto & pid_s : pids) {
+            pid_t pid;
+            if (auto o = string2Int<pid_t>(pid_s))
+                pid = *o;
+            else
+                throw Error("invalid pid '%s'", pid);
+            if (pidsShown.insert(pid).second) {
+                try {
+                    auto cmdline = readFile(fmt("/proc/%d/cmdline", pid));
+                    using namespace std::string_literals;
+                    warn("killing stray builder process %d (%s)...",
+                        pid, trim(replaceStrings(cmdline, "\0"s, " ")));
+                } catch (SysError &) {
+                }
+            }
+            // FIXME: pid wraparound
+            if (kill(pid, SIGKILL) == -1 && errno != ESRCH)
+                throw SysError("killing member %d of cgroup '%s'", pid, cgroup);
+        }
+
+        auto sleep = std::chrono::milliseconds((int) std::pow(2.0, std::min(round, 10)));
+        if (sleep.count() > 100)
+            printError("waiting for %d ms for cgroup '%s' to become empty", sleep.count(), cgroup);
+        std::this_thread::sleep_for(sleep);
+        round++;
+    }
+
+    CgroupStats stats;
+
+    if (returnStats) {
+        auto cpustatPath = cgroup + "/cpu.stat";
+
+        if (pathExists(cpustatPath)) {
+            for (auto & line : tokenizeString<std::vector<std::string>>(readFile(cpustatPath), "\n")) {
+                std::string_view userPrefix = "user_usec ";
+                if (hasPrefix(line, userPrefix)) {
+                    auto n = string2Int<uint64_t>(line.substr(userPrefix.size()));
+                    if (n) stats.cpuUser = std::chrono::microseconds(*n);
+                }
+
+                std::string_view systemPrefix = "system_usec ";
+                if (hasPrefix(line, systemPrefix)) {
+                    auto n = string2Int<uint64_t>(line.substr(systemPrefix.size()));
+                    if (n) stats.cpuSystem = std::chrono::microseconds(*n);
+                }
+            }
+        }
+
+    }
+
+    if (rmdir(cgroup.c_str()) == -1)
+        throw SysError("deleting cgroup '%s'", cgroup);
+
+    return stats;
+}
+
+CgroupStats destroyCgroup(const Path & cgroup)
+{
+    return destroyCgroup(cgroup, true);
+}
+
+}
+
+#endif

src/libstore/cgroup.hh

@@ -0,0 +1,27 @@
+#pragma once
+
+#if __linux__
+
+#include <chrono>
+#include <optional>
+
+#include "types.hh"
+
+namespace nix {
+
+std::map<std::string, std::string> getCgroups(const Path & cgroupFile);
+
+struct CgroupStats
+{
+    std::optional<std::chrono::microseconds> cpuUser, cpuSystem;
+};
+
+/* Destroy the cgroup denoted by 'path'. The postcondition is that
+   'path' does not exist, and thus any processes in the cgroup have
+   been killed. Also return statistics from the cgroup just before
+   destruction. */
+CgroupStats destroyCgroup(const Path & cgroup);
+
+}
+
+#endif

src/libstore/globals.cc

@@ -130,6 +130,10 @@ StringSet Settings::getDefaultSystemFeatures()
        actually require anything special on the machines. */
     StringSet features{"nixos-test", "benchmark", "big-parallel"};
 
+    #if __linux__
+    features.insert("uid-range");
+    #endif
+
     #if __linux__
     if (access("/dev/kvm", R_OK | W_OK) == 0)
         features.insert("kvm");

src/libstore/globals.hh

@@ -46,6 +46,14 @@ struct PluginFilesSetting : public BaseSetting<Paths>
     void set(const std::string & str, bool append = false) override;
 };
 
+const uint32_t maxIdsPerBuild =
+    #if __linux__
+    1 << 16
+    #else
+    1
+    #endif
+    ;
+
 class Settings : public Config {
 
     unsigned int getDefaultCores();
@@ -275,6 +283,38 @@ public:
           multi-user settings with untrusted users.
         )"};
 
+    Setting<bool> autoAllocateUids{this, false, "auto-allocate-uids",
+        "Whether to allocate UIDs for builders automatically."};
+
+    Setting<uint32_t> startId{this,
+        #if __linux__
+        0x34000000,
+        #else
+        56930,
+        #endif
+        "start-id",
+        "The first UID and GID to use for dynamic ID allocation."};
+
+    Setting<uint32_t> uidCount{this,
+        #if __linux__
+        maxIdsPerBuild * 128,
+        #else
+        128,
+        #endif
+        "id-count",
+        "The number of UIDs/GIDs to use for dynamic ID allocation."};
+
+    #if __linux__
+    Setting<bool> useCgroups{
+        this, false, "use-cgroups",
+        R"(
+          Whether to execute builds inside cgroups. Cgroups are
+          enabled automatically for derivations that require the
+          `uid-range` system feature.
+        )"
+    };
+    #endif
+
     Setting<bool> impersonateLinux26{this, false, "impersonate-linux-26",
         "Whether to impersonate a Linux 2.6 machine on newer kernels.",
         {"build-impersonate-linux-26"}};
@@ -563,10 +603,10 @@ public:
           cache) must have a signature by a trusted key. A trusted key is one
           listed in `trusted-public-keys`, or a public key counterpart to a
           private key stored in a file listed in `secret-key-files`.
-          
+
           Set to `false` to disable signature checking and trust all
           non-content-addressed paths unconditionally.
-          
+
           (Content-addressed paths are inherently trustworthy and thus
           unaffected by this configuration option.)
         )"};

src/libstore/local-store.cc

@@ -583,7 +583,10 @@ void canonicaliseTimestampAndPermissions(const Path & path)
 }
 
 
-static void canonicalisePathMetaData_(const Path & path, uid_t fromUid, InodesSeen & inodesSeen)
+static void canonicalisePathMetaData_(
+    const Path & path,
+    std::optional<std::pair<uid_t, uid_t>> uidRange,
+    InodesSeen & inodesSeen)
 {
     checkInterrupt();
 
@@ -630,7 +633,7 @@ static void canonicalisePathMetaData_(const Path & path, uid_t fromUid, InodesSe
        However, ignore files that we chown'ed ourselves previously to
        ensure that we don't fail on hard links within the same build
        (i.e. "touch $out/foo; ln $out/foo $out/bar"). */
-    if (fromUid != (uid_t) -1 && st.st_uid != fromUid) {
+    if (uidRange && (st.st_uid < uidRange->first || st.st_uid > uidRange->second)) {
         if (S_ISDIR(st.st_mode) || !inodesSeen.count(Inode(st.st_dev, st.st_ino)))
             throw BuildError("invalid ownership on file '%1%'", path);
         mode_t mode = st.st_mode & ~S_IFMT;
@@ -663,14 +666,17 @@ static void canonicalisePathMetaData_(const Path & path, uid_t fromUid, InodesSe
     if (S_ISDIR(st.st_mode)) {
         DirEntries entries = readDirectory(path);
         for (auto & i : entries)
-            canonicalisePathMetaData_(path + "/" + i.name, fromUid, inodesSeen);
+            canonicalisePathMetaData_(path + "/" + i.name, uidRange, inodesSeen);
     }
 }
 
 
-void canonicalisePathMetaData(const Path & path, uid_t fromUid, InodesSeen & inodesSeen)
+void canonicalisePathMetaData(
+    const Path & path,
+    std::optional<std::pair<uid_t, uid_t>> uidRange,
+    InodesSeen & inodesSeen)
 {
-    canonicalisePathMetaData_(path, fromUid, inodesSeen);
+    canonicalisePathMetaData_(path, uidRange, inodesSeen);
 
     /* On platforms that don't have lchown(), the top-level path can't
        be a symlink, since we can't change its ownership. */
@@ -683,10 +689,11 @@ void canonicalisePathMetaData(const Path & path, uid_t fromUid, InodesSeen & ino
 }
 
 
-void canonicalisePathMetaData(const Path & path, uid_t fromUid)
+void canonicalisePathMetaData(const Path & path,
+    std::optional<std::pair<uid_t, uid_t>> uidRange)
 {
     InodesSeen inodesSeen;
-    canonicalisePathMetaData(path, fromUid, inodesSeen);
+    canonicalisePathMetaData(path, uidRange, inodesSeen);
 }
 
 
@@ -1331,7 +1338,7 @@ void LocalStore::addToStore(const ValidPathInfo & info, Source & source,
 
             autoGC();
 
-            canonicalisePathMetaData(realPath, -1);
+            canonicalisePathMetaData(realPath, {});
 
             optimisePath(realPath, repair); // FIXME: combine with hashPath()
 
@@ -1444,7 +1451,7 @@ StorePath LocalStore::addToStoreFromDump(Source & source0, std::string_view name
                 narHash = narSink.finish();
             }
 
-            canonicalisePathMetaData(realPath, -1); // FIXME: merge into restorePath
+            canonicalisePathMetaData(realPath, {}); // FIXME: merge into restorePath
 
             optimisePath(realPath, repair);
 
@@ -1486,7 +1493,7 @@ StorePath LocalStore::addTextToStore(
 
             writeFile(realPath, s);
 
-            canonicalisePathMetaData(realPath, -1);
+            canonicalisePathMetaData(realPath, {});
 
             StringSink sink;
             dumpString(s, sink);

src/libstore/local-store.hh

@@ -310,9 +310,18 @@ typedef std::set<Inode> InodesSeen;
    - the permissions are set of 444 or 555 (i.e., read-only with or
      without execute permission; setuid bits etc. are cleared)
    - the owner and group are set to the Nix user and group, if we're
-     running as root. */
+     running as root.
-void canonicalisePathMetaData(const Path & path, uid_t fromUid, InodesSeen & inodesSeen);
+   If uidRange is not empty, this function will throw an error if it
-void canonicalisePathMetaData(const Path & path, uid_t fromUid);
+   encounters files owned by a user outside of the closed interval
+   [uidRange->first, uidRange->second].
+*/
+void canonicalisePathMetaData(
+    const Path & path,
+    std::optional<std::pair<uid_t, uid_t>> uidRange,
+    InodesSeen & inodesSeen);
+void canonicalisePathMetaData(
+    const Path & path,
+    std::optional<std::pair<uid_t, uid_t>> uidRange);
 
 void canonicaliseTimestampAndPermissions(const Path & path);
 

src/libstore/lock.cc

@@ -2,105 +2,197 @@
 #include "globals.hh"
 #include "pathlocks.hh"
 
-#include <grp.h>
 #include <pwd.h>
-
+#include <grp.h>
-#include <fcntl.h>
-#include <unistd.h>
 
 namespace nix {
 
-UserLock::UserLock()
+struct SimpleUserLock : UserLock
 {
-    assert(settings.buildUsersGroup != "");
+    AutoCloseFD fdUserLock;
-    createDirs(settings.nixStateDir + "/userpool");
+    uid_t uid;
-}
+    gid_t gid;
+    std::vector<gid_t> supplementaryGIDs;
 
-bool UserLock::findFreeUser() {
+    uid_t getUID() override { assert(uid); return uid; }
-    if (enabled()) return true;
+    uid_t getUIDCount() override { return 1; }
+    gid_t getGID() override { assert(gid); return gid; }
 
-    /* Get the members of the build-users-group. */
+    std::vector<gid_t> getSupplementaryGIDs() override { return supplementaryGIDs; }
-    struct group * gr = getgrnam(settings.buildUsersGroup.get().c_str());
-    if (!gr)
-        throw Error("the group '%1%' specified in 'build-users-group' does not exist",
-            settings.buildUsersGroup);
-    gid = gr->gr_gid;
 
-    /* Copy the result of getgrnam. */
+    static std::unique_ptr<UserLock> acquire()
-    Strings users;
+    {
-    for (char * * p = gr->gr_mem; *p; ++p) {
+        assert(settings.buildUsersGroup != "");
-        debug("found build user '%1%'", *p);
+        createDirs(settings.nixStateDir + "/userpool");
-        users.push_back(*p);
-    }
 
-    if (users.empty())
+        /* Get the members of the build-users-group. */
-        throw Error("the build users group '%1%' has no members",
+        struct group * gr = getgrnam(settings.buildUsersGroup.get().c_str());
-            settings.buildUsersGroup);
+        if (!gr)
+            throw Error("the group '%s' specified in 'build-users-group' does not exist", settings.buildUsersGroup);
 
-    /* Find a user account that isn't currently in use for another
+        /* Copy the result of getgrnam. */
-       build. */
+        Strings users;
-    for (auto & i : users) {
+        for (char * * p = gr->gr_mem; *p; ++p) {
-        debug("trying user '%1%'", i);
+            debug("found build user '%s'", *p);
-
+            users.push_back(*p);
-        struct passwd * pw = getpwnam(i.c_str());
-        if (!pw)
-            throw Error("the user '%1%' in the group '%2%' does not exist",
-                i, settings.buildUsersGroup);
-
-
-        fnUserLock = (format("%1%/userpool/%2%") % settings.nixStateDir % pw->pw_uid).str();
-
-        AutoCloseFD fd = open(fnUserLock.c_str(), O_RDWR | O_CREAT | O_CLOEXEC, 0600);
-        if (!fd)
-            throw SysError("opening user lock '%1%'", fnUserLock);
-
-        if (lockFile(fd.get(), ltWrite, false)) {
-            fdUserLock = std::move(fd);
-            user = i;
-            uid = pw->pw_uid;
-
-            /* Sanity check... */
-            if (uid == getuid() || uid == geteuid())
-                throw Error("the Nix user should not be a member of '%1%'",
-                    settings.buildUsersGroup);
-
-#if __linux__
-            /* Get the list of supplementary groups of this build user.  This
-               is usually either empty or contains a group such as "kvm".  */
-            int ngroups = 32; // arbitrary initial guess
-            supplementaryGIDs.resize(ngroups);
-
-            int err = getgrouplist(pw->pw_name, pw->pw_gid, supplementaryGIDs.data(),
-                                 &ngroups);
-
-            // Our initial size of 32 wasn't sufficient, the correct size has
-            // been stored in ngroups, so we try again.
-            if (err == -1) {
-                supplementaryGIDs.resize(ngroups);
-                err = getgrouplist(pw->pw_name, pw->pw_gid, supplementaryGIDs.data(),
-                                   &ngroups);
-            }
-
-            // If it failed once more, then something must be broken.
-            if (err == -1)
-                throw Error("failed to get list of supplementary groups for '%1%'",
-                            pw->pw_name);
-
-            // Finally, trim back the GID list to its real size
-            supplementaryGIDs.resize(ngroups);
-#endif
-
-            isEnabled = true;
-            return true;
         }
+
+        if (users.empty())
+            throw Error("the build users group '%s' has no members", settings.buildUsersGroup);
+
+        /* Find a user account that isn't currently in use for another
+           build. */
+        for (auto & i : users) {
+            debug("trying user '%s'", i);
+
+            struct passwd * pw = getpwnam(i.c_str());
+            if (!pw)
+                throw Error("the user '%s' in the group '%s' does not exist", i, settings.buildUsersGroup);
+
+            auto fnUserLock = fmt("%s/userpool/%s", settings.nixStateDir,pw->pw_uid);
+
+            AutoCloseFD fd = open(fnUserLock.c_str(), O_RDWR | O_CREAT | O_CLOEXEC, 0600);
+            if (!fd)
+                throw SysError("opening user lock '%s'", fnUserLock);
+
+            if (lockFile(fd.get(), ltWrite, false)) {
+                auto lock = std::make_unique<SimpleUserLock>();
+
+                lock->fdUserLock = std::move(fd);
+                lock->uid = pw->pw_uid;
+                lock->gid = gr->gr_gid;
+
+                /* Sanity check... */
+                if (lock->uid == getuid() || lock->uid == geteuid())
+                    throw Error("the Nix user should not be a member of '%s'", settings.buildUsersGroup);
+
+                #if __linux__
+                /* Get the list of supplementary groups of this build
+                   user.  This is usually either empty or contains a
+                   group such as "kvm".  */
+                int ngroups = 32; // arbitrary initial guess
+                std::vector<gid_t> gids;
+                gids.resize(ngroups);
+
+                int err = getgrouplist(
+                    pw->pw_name, pw->pw_gid,
+                    gids.data(),
+                    &ngroups);
+
+                /* Our initial size of 32 wasn't sufficient, the
+                   correct size has been stored in ngroups, so we try
+                   again. */
+                if (err == -1) {
+                    gids.resize(ngroups);
+                    err = getgrouplist(
+                        pw->pw_name, pw->pw_gid,
+                        gids.data(),
+                        &ngroups);
+                }
+
+                // If it failed once more, then something must be broken.
+                if (err == -1)
+                    throw Error("failed to get list of supplementary groups for '%s'", pw->pw_name);
+
+                // Finally, trim back the GID list to its real size.
+                for (auto i = 0; i < ngroups; i++)
+                    if (gids[i] != lock->gid)
+                        lock->supplementaryGIDs.push_back(gids[i]);
+                #endif
+
+                return lock;
+            }
+        }
+
+        return nullptr;
     }
+};
 
-    return false;
+struct AutoUserLock : UserLock
-}
-
-void UserLock::kill()
 {
-    killUser(uid);
+    AutoCloseFD fdUserLock;
+    uid_t firstUid = 0;
+    gid_t firstGid = 0;
+    uid_t nrIds = 1;
+
+    uid_t getUID() override { assert(firstUid); return firstUid; }
+
+    gid_t getUIDCount() override { return nrIds; }
+
+    gid_t getGID() override { assert(firstGid); return firstGid; }
+
+    std::vector<gid_t> getSupplementaryGIDs() override { return {}; }
+
+    static std::unique_ptr<UserLock> acquire(uid_t nrIds, bool useChroot)
+    {
+        settings.requireExperimentalFeature(Xp::AutoAllocateUids);
+        assert(settings.startId > 0);
+        assert(settings.uidCount % maxIdsPerBuild == 0);
+        assert((uint64_t) settings.startId + (uint64_t) settings.uidCount <= std::numeric_limits<uid_t>::max());
+        assert(nrIds <= maxIdsPerBuild);
+
+        createDirs(settings.nixStateDir + "/userpool2");
+
+        size_t nrSlots = settings.uidCount / maxIdsPerBuild;
+
+        for (size_t i = 0; i < nrSlots; i++) {
+            debug("trying user slot '%d'", i);
+
+            createDirs(settings.nixStateDir + "/userpool2");
+
+            auto fnUserLock = fmt("%s/userpool2/slot-%d", settings.nixStateDir, i);
+
+            AutoCloseFD fd = open(fnUserLock.c_str(), O_RDWR | O_CREAT | O_CLOEXEC, 0600);
+            if (!fd)
+                throw SysError("opening user lock '%s'", fnUserLock);
+
+            if (lockFile(fd.get(), ltWrite, false)) {
+
+                auto firstUid = settings.startId + i * maxIdsPerBuild;
+
+                auto pw = getpwuid(firstUid);
+                if (pw)
+                    throw Error("auto-allocated UID %d clashes with existing user account '%s'", firstUid, pw->pw_name);
+
+                auto lock = std::make_unique<AutoUserLock>();
+                lock->fdUserLock = std::move(fd);
+                lock->firstUid = firstUid;
+                if (useChroot)
+                    lock->firstGid = firstUid;
+                else {
+                    struct group * gr = getgrnam(settings.buildUsersGroup.get().c_str());
+                    if (!gr)
+                        throw Error("the group '%s' specified in 'build-users-group' does not exist", settings.buildUsersGroup);
+                    lock->firstGid = gr->gr_gid;
+                }
+                lock->nrIds = nrIds;
+                return lock;
+            }
+        }
+
+        return nullptr;
+    }
+};
+
+std::unique_ptr<UserLock> acquireUserLock(uid_t nrIds, bool useChroot)
+{
+    if (settings.autoAllocateUids)
+        return AutoUserLock::acquire(nrIds, useChroot);
+    else
+        return SimpleUserLock::acquire();
+}
+
+bool useBuildUsers()
+{
+    #if __linux__
+    static bool b = (settings.buildUsersGroup != "" || settings.startId.get() != 0) && getuid() == 0;
+    return b;
+    #elif __APPLE__
+    static bool b = settings.buildUsersGroup != "" && getuid() == 0;
+    return b;
+    #else
+    return false;
+    #endif
 }
 
 }

src/libstore/lock.hh

@@ -1,37 +1,38 @@
 #pragma once
 
-#include "sync.hh"
 #include "types.hh"
-#include "util.hh"
+
+#include <optional>
+
+#include <sys/types.h>
 
 namespace nix {
 
-class UserLock
+struct UserLock
 {
-private:
+    virtual ~UserLock() { }
-    Path fnUserLock;
-    AutoCloseFD fdUserLock;
 
-    bool isEnabled = false;
+    /* Get the first and last UID. */
-    std::string user;
+    std::pair<uid_t, uid_t> getUIDRange()
-    uid_t uid = 0;
+    {
-    gid_t gid = 0;
+        auto first = getUID();
-    std::vector<gid_t> supplementaryGIDs;
+        return {first, first + getUIDCount() - 1};
+    }
 
-public:
+    /* Get the first UID. */
-    UserLock();
+    virtual uid_t getUID() = 0;
 
-    void kill();
+    virtual uid_t getUIDCount() = 0;
 
-    std::string getUser() { return user; }
+    virtual gid_t getGID() = 0;
-    uid_t getUID() { assert(uid); return uid; }
-    uid_t getGID() { assert(gid); return gid; }
-    std::vector<gid_t> getSupplementaryGIDs() { return supplementaryGIDs; }
-
-    bool findFreeUser();
-
-    bool enabled() { return isEnabled; }
 
+    virtual std::vector<gid_t> getSupplementaryGIDs() = 0;
 };
 
+/* Acquire a user lock for a UID range of size `nrIds`. Note that this
+   may return nullptr if no user is available. */
+std::unique_ptr<UserLock> acquireUserLock(uid_t nrIds, bool useChroot);
+
+bool useBuildUsers();
+
 }

src/libstore/parsed-derivations.cc

@@ -89,6 +89,7 @@ std::optional<Strings> ParsedDerivation::getStringsAttr(const std::string & name
 
 StringSet ParsedDerivation::getRequiredSystemFeatures() const
 {
+    // FIXME: cache this?
     StringSet res;
     for (auto & i : getStringsAttr("requiredSystemFeatures").value_or(Strings()))
         res.insert(i);
@@ -124,6 +125,11 @@ bool ParsedDerivation::substitutesAllowed() const
     return getBoolAttr("allowSubstitutes", true);
 }
 
+bool ParsedDerivation::useUidRange() const
+{
+    return getRequiredSystemFeatures().count("uid-range");
+}
+
 static std::regex shVarName("[A-Za-z_][A-Za-z0-9_]*");
 
 std::optional<nlohmann::json> ParsedDerivation::prepareStructuredAttrs(Store & store, const StorePathSet & inputPaths)

src/libstore/parsed-derivations.hh

@@ -38,6 +38,8 @@ public:
 
     bool substitutesAllowed() const;
 
+    bool useUidRange() const;
+
     std::optional<nlohmann::json> prepareStructuredAttrs(Store & store, const StorePathSet & inputPaths);
 };
 

src/libutil/experimental-features.cc

@@ -14,6 +14,8 @@ std::map<ExperimentalFeature, std::string> stringifiedXpFeatures = {
     { Xp::NoUrlLiterals, "no-url-literals" },
     { Xp::FetchClosure, "fetch-closure" },
     { Xp::ReplFlake, "repl-flake" },
+    { Xp::AutoAllocateUids, "auto-allocate-uids" },
+    { Xp::Cgroups, "cgroups" },
 };
 
 const std::optional<ExperimentalFeature> parseExperimentalFeature(const std::string_view & name)

src/libutil/experimental-features.hh

@@ -23,6 +23,8 @@ enum struct ExperimentalFeature
     NoUrlLiterals,
     FetchClosure,
     ReplFlake,
+    AutoAllocateUids,
+    Cgroups,
 };
 
 /**

src/libutil/filesystem.cc

@@ -1,5 +1,6 @@
 #include <sys/time.h>
 #include <filesystem>
+#include <atomic>
 
 #include "finally.hh"
 #include "util.hh"
@@ -10,7 +11,7 @@ namespace fs = std::filesystem;
 namespace nix {
 
 static Path tempName(Path tmpRoot, const Path & prefix, bool includePid,
-    int & counter)
+    std::atomic<unsigned int> & counter)
 {
     tmpRoot = canonPath(tmpRoot.empty() ? getEnv("TMPDIR").value_or("/tmp") : tmpRoot, true);
     if (includePid)
@@ -22,9 +23,9 @@ static Path tempName(Path tmpRoot, const Path & prefix, bool includePid,
 Path createTempDir(const Path & tmpRoot, const Path & prefix,
     bool includePid, bool useGlobalCounter, mode_t mode)
 {
-    static int globalCounter = 0;
+    static std::atomic<unsigned int> globalCounter = 0;
-    int localCounter = 0;
+    std::atomic<unsigned int> localCounter = 0;
-    int & counter(useGlobalCounter ? globalCounter : localCounter);
+    auto & counter(useGlobalCounter ? globalCounter : localCounter);
 
     while (1) {
         checkInterrupt();

src/nix-store/nix-store.cc

@@ -516,7 +516,7 @@ static void registerValidity(bool reregister, bool hashGiven, bool canonicalise)
         if (!store->isValidPath(info->path) || reregister) {
             /* !!! races */
             if (canonicalise)
-                canonicalisePathMetaData(store->printStorePath(info->path), -1);
+                canonicalisePathMetaData(store->printStorePath(info->path), {});
             if (!hashGiven) {
                 HashResult hash = hashPath(htSHA256, store->printStorePath(info->path));
                 info->narHash = hash.first;

src/nix/build.cc

@@ -30,6 +30,10 @@ nlohmann::json builtPathsWithResultToJSON(const std::vector<BuiltPathWithResult>
             if (b.result) {
                 j["startTime"] = b.result->startTime;
                 j["stopTime"] = b.result->stopTime;
+                if (b.result->cpuUser)
+                    j["cpuUser"] = ((double) b.result->cpuUser->count()) / 1000000;
+                if (b.result->cpuSystem)
+                    j["cpuSystem"] = ((double) b.result->cpuSystem->count()) / 1000000;
             }
             res.push_back(j);
         }, b.path.raw());

tests/containers.nix

@@ -0,0 +1,68 @@
+# Test whether we can run a NixOS container inside a Nix build using systemd-nspawn.
+{ nixpkgs, system, overlay }:
+
+with import (nixpkgs + "/nixos/lib/testing-python.nix") {
+  inherit system;
+  extraConfigurations = [ { nixpkgs.overlays = [ overlay ]; } ];
+};
+
+makeTest ({
+  name = "containers";
+
+  nodes =
+    {
+      host =
+        { config, lib, pkgs, nodes, ... }:
+        { virtualisation.writableStore = true;
+          virtualisation.diskSize = 2048;
+          virtualisation.additionalPaths =
+            [ pkgs.stdenv
+              (import ./systemd-nspawn.nix { inherit nixpkgs; }).toplevel
+            ];
+          virtualisation.memorySize = 4096;
+          nix.binaryCaches = lib.mkForce [ ];
+          nix.extraOptions =
+            ''
+              extra-experimental-features = nix-command auto-allocate-uids cgroups
+              extra-system-features = uid-range
+            '';
+          nix.nixPath = [ "nixpkgs=${nixpkgs}" ];
+        };
+    };
+
+  testScript = { nodes }: ''
+    start_all()
+
+    host.succeed("nix --version >&2")
+
+    # Test that 'id' gives the expected result in various configurations.
+
+    # Existing UIDs, sandbox.
+    host.succeed("nix build --no-auto-allocate-uids --sandbox -L --offline --impure --file ${./id-test.nix} --argstr name id-test-1")
+    host.succeed("[[ $(cat ./result) = 'uid=1000(nixbld) gid=100(nixbld) groups=100(nixbld)' ]]")
+
+    # Existing UIDs, no sandbox.
+    host.succeed("nix build --no-auto-allocate-uids --no-sandbox -L --offline --impure --file ${./id-test.nix} --argstr name id-test-2")
+    host.succeed("[[ $(cat ./result) = 'uid=30001(nixbld1) gid=30000(nixbld) groups=30000(nixbld)' ]]")
+
+    # Auto-allocated UIDs, sandbox.
+    host.succeed("nix build --auto-allocate-uids --sandbox -L --offline --impure --file ${./id-test.nix} --argstr name id-test-3")
+    host.succeed("[[ $(cat ./result) = 'uid=1000(nixbld) gid=100(nixbld) groups=100(nixbld)' ]]")
+
+    # Auto-allocated UIDs, no sandbox.
+    host.succeed("nix build --auto-allocate-uids --no-sandbox -L --offline --impure --file ${./id-test.nix} --argstr name id-test-4")
+    host.succeed("[[ $(cat ./result) = 'uid=872415232 gid=30000(nixbld) groups=30000(nixbld)' ]]")
+
+    # Auto-allocated UIDs, UID range, sandbox.
+    host.succeed("nix build --auto-allocate-uids --sandbox -L --offline --impure --file ${./id-test.nix} --argstr name id-test-5 --arg uidRange true")
+    host.succeed("[[ $(cat ./result) = 'uid=0(root) gid=0(root) groups=0(root)' ]]")
+
+    # Auto-allocated UIDs, UID range, no sandbox.
+    host.fail("nix build --auto-allocate-uids --no-sandbox -L --offline --impure --file ${./id-test.nix} --argstr name id-test-6 --arg uidRange true")
+
+    # Run systemd-nspawn in a Nix build.
+    host.succeed("nix build --auto-allocate-uids --sandbox -L --offline --impure --file ${./systemd-nspawn.nix} --argstr nixpkgs ${nixpkgs}")
+    host.succeed("[[ $(cat ./result/msg) = 'Hello World' ]]")
+  '';
+
+})

tests/id-test.nix

@@ -0,0 +1,8 @@
+{ name, uidRange ? false }:
+
+with import <nixpkgs> {};
+
+runCommand name
+  { requiredSystemFeatures = if uidRange then ["uid-range"] else [];
+  }
+  "id; id > $out"

tests/systemd-nspawn.nix

@@ -0,0 +1,78 @@
+{ nixpkgs }:
+
+let
+
+  machine = { config, pkgs, ... }:
+    {
+      system.stateVersion = "22.05";
+      boot.isContainer = true;
+      systemd.services.console-getty.enable = false;
+      networking.dhcpcd.enable = false;
+
+      services.httpd = {
+        enable = true;
+        adminAddr = "nixos@example.org";
+      };
+
+      systemd.services.test = {
+        wantedBy = [ "multi-user.target" ];
+        after = [ "httpd.service" ];
+        script = ''
+          source /.env
+          echo "Hello World" > $out/msg
+          ls -lR /dev > $out/dev
+          ${pkgs.curl}/bin/curl -sS --fail http://localhost/ > $out/page.html
+        '';
+        unitConfig = {
+          FailureAction = "exit-force";
+          FailureActionExitStatus = 42;
+          SuccessAction = "exit-force";
+        };
+      };
+    };
+
+  cfg = (import (nixpkgs + "/nixos/lib/eval-config.nix") {
+    modules = [ machine ];
+    system = "x86_64-linux";
+  });
+
+  config = cfg.config;
+
+in
+
+with cfg._module.args.pkgs;
+
+runCommand "test"
+  { buildInputs = [ config.system.path ];
+    requiredSystemFeatures = [ "uid-range" ];
+    toplevel = config.system.build.toplevel;
+  }
+  ''
+    root=$(pwd)/root
+    mkdir -p $root $root/etc
+
+    export > $root/.env
+
+    # Make /run a tmpfs to shut up a systemd warning.
+    mkdir /run
+    mount -t tmpfs none /run
+    chmod 0700 /run
+
+    mount -t cgroup2 none /sys/fs/cgroup
+
+    mkdir -p $out
+
+    touch /etc/os-release
+    echo a5ea3f98dedc0278b6f3cc8c37eeaeac > /etc/machine-id
+
+    SYSTEMD_NSPAWN_UNIFIED_HIERARCHY=1 \
+      ${config.systemd.package}/bin/systemd-nspawn \
+      --keep-unit \
+      -M ${config.networking.hostName} -D "$root" \
+      --register=no \
+      --resolv-conf=off \
+      --bind-ro=/nix/store \
+      --bind=$out \
+      --private-network \
+      $toplevel/init
+  ''