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Use chroots for all derivations
If ‘build-use-chroot’ is set to ‘true’, fixed-output derivations are now also chrooted. However, unlike normal derivations, they don't get a private network namespace, so they can still access the network. Also, the use of the ‘__noChroot’ derivation attribute is no longer allowed. Setting ‘build-use-chroot’ to ‘relaxed’ gives the old behaviour.
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@ -227,24 +227,32 @@ flag, e.g. <literal>--option gc-keep-outputs false</literal>.</para>
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<varlistentry><term><literal>build-use-chroot</literal></term>
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<listitem><para>If set to <literal>true</literal>, builds will be
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performed in a <emphasis>chroot environment</emphasis>, i.e., the
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build will be isolated from the normal file system hierarchy and
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will only see its dependencies in the Nix store, the temporary
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build directory, private versions of <filename>/proc</filename>,
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performed in a <emphasis>chroot environment</emphasis>, i.e.,
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they’re isolated from the normal file system hierarchy and will
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only see their dependencies in the Nix store, the temporary build
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directory, private versions of <filename>/proc</filename>,
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<filename>/dev</filename>, <filename>/dev/shm</filename> and
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<filename>/dev/pts</filename>, and the paths configured with the
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<link linkend='conf-build-chroot-dirs'><literal>build-chroot-dirs</literal>
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option</link>. This is useful to prevent undeclared dependencies
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on files in directories such as
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<filename>/usr/bin</filename>.</para>
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on files in directories such as <filename>/usr/bin</filename>. In
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addition, on Linux, builds run in rivate PID, mount, network, IPC
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and UTS namespaces to isolate them from other processes in the
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system (except that fixed-output derivations do not run in private
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network namespace to ensure they can access the network).</para>
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<para>The use of a chroot requires that Nix is run as root (so you
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should use the <link linkend='conf-build-users-group'>“build
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users” feature</link> to perform the actual builds under different
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users than root). Currently, chroot builds only work on Linux
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because Nix uses “bind mounts” to make the Nix store and other
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directories available inside the chroot. Kernel version 3.13 or later
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is needed.</para>
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<para>Currently, chroots only work on Linux and Mac OS X. The use
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of a chroot requires that Nix is run as root (so you should use
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the <link linkend='conf-build-users-group'>“build users”
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feature</link> to perform the actual builds under different users
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than root).</para>
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<para>If this option is set to <literal>relaxed</literal>, then
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fixed-output derivations and derivations that have the
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<varname>__noChroot</varname> attribute set to
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<literal>true</literal> do not run in chroots.</para>
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<para>The default is <literal>false</literal>.</para>
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</listitem>
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@ -1768,12 +1768,20 @@ void DerivationGoal::startBuilder()
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functions like fetchurl (which needs a proper /etc/resolv.conf)
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work properly. Purity checking for fixed-output derivations
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is somewhat pointless anyway. */
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useChroot = settings.useChroot;
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if (fixedOutput) useChroot = false;
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/* Hack to allow derivations to disable chroot builds. */
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if (get(drv.env, "__noChroot") == "1") useChroot = false;
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{
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string x = settings.get("build-use-chroot", string("false"));
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if (x != "true" && x != "false" && x != "relaxed")
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throw Error("option ‘build-use-chroot’ must be set to one of ‘true’, ‘false’ or ‘relaxed’");
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if (x == "true") {
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if (get(drv.env, "__noChroot") == "1")
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throw Error(format("derivation ‘%1%’ has ‘__noChroot’ set, but that's not allowed when ‘build-use-chroot’ is ‘true’") % drvPath);
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useChroot = true;
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}
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else if (x == "false")
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useChroot = false;
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else if (x == "relaxed")
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useChroot = !fixedOutput && get(drv.env, "__noChroot") != "1";
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}
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if (useChroot) {
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/* Allow a user-configurable set of directories from the
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@ -1856,7 +1864,8 @@ void DerivationGoal::startBuilder()
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% (buildUser.enabled() ? buildUser.getGID() : getgid())).str());
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/* Create /etc/hosts with localhost entry. */
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writeFile(chrootRootDir + "/etc/hosts", "127.0.0.1 localhost\n");
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if (!fixedOutput)
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writeFile(chrootRootDir + "/etc/hosts", "127.0.0.1 localhost\n");
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/* Make the closure of the inputs available in the chroot,
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rather than the whole Nix store. This prevents any access
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@ -1964,7 +1973,9 @@ void DerivationGoal::startBuilder()
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- The private network namespace ensures that the builder
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cannot talk to the outside world (or vice versa). It
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only has a private loopback interface.
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only has a private loopback interface. (Fixed-output
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derivations are not run in a private network namespace
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to allow functions like fetchurl to work.)
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- The IPC namespace prevents the builder from communicating
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with outside processes using SysV IPC mechanisms (shared
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@ -1983,8 +1994,9 @@ void DerivationGoal::startBuilder()
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*/
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Pid helper = startProcess([&]() {
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char stack[32 * 1024];
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pid_t child = clone(childEntry, stack + sizeof(stack) - 8,
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CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWNET | CLONE_NEWIPC | CLONE_NEWUTS | CLONE_PARENT | SIGCHLD, this);
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int flags = CLONE_NEWPID | CLONE_NEWNS | CLONE_NEWIPC | CLONE_NEWUTS | CLONE_PARENT | SIGCHLD;
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if (!fixedOutput) flags |= CLONE_NEWNET;
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pid_t child = clone(childEntry, stack + sizeof(stack) - 8, flags, this);
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if (child == -1) {
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if (errno == EINVAL)
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throw SysError("cloning builder process (Linux chroot builds require 3.13 or later)");
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@ -2081,10 +2093,10 @@ void DerivationGoal::runChild()
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/* Set up a nearly empty /dev, unless the user asked to
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bind-mount the host /dev. */
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Strings ss;
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if (dirsInChroot.find("/dev") == dirsInChroot.end()) {
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createDirs(chrootRootDir + "/dev/shm");
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createDirs(chrootRootDir + "/dev/pts");
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Strings ss;
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ss.push_back("/dev/full");
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#ifdef __linux__
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if (pathExists("/dev/kvm"))
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@ -2095,13 +2107,24 @@ void DerivationGoal::runChild()
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ss.push_back("/dev/tty");
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ss.push_back("/dev/urandom");
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ss.push_back("/dev/zero");
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foreach (Strings::iterator, i, ss) dirsInChroot[*i] = *i;
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createSymlink("/proc/self/fd", chrootRootDir + "/dev/fd");
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createSymlink("/proc/self/fd/0", chrootRootDir + "/dev/stdin");
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createSymlink("/proc/self/fd/1", chrootRootDir + "/dev/stdout");
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createSymlink("/proc/self/fd/2", chrootRootDir + "/dev/stderr");
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}
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/* Fixed-output derivations typically need to access the
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network, so give them access to /etc/resolv.conf and so
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on. */
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if (fixedOutput) {
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ss.push_back("/etc/resolv.conf");
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ss.push_back("/etc/nsswitch.conf");
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ss.push_back("/etc/services");
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ss.push_back("/etc/hosts");
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}
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for (auto & i : ss) dirsInChroot[i] = i;
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/* Bind-mount all the directories from the "host"
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filesystem that we want in the chroot
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environment. */
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@ -47,7 +47,6 @@ Settings::Settings()
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syncBeforeRegistering = false;
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useSubstitutes = true;
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buildUsersGroup = getuid() == 0 ? "nixbld" : "";
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useChroot = false;
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useSshSubstituter = true;
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impersonateLinux26 = false;
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keepLog = true;
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@ -158,7 +157,6 @@ void Settings::update()
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_get(syncBeforeRegistering, "sync-before-registering");
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_get(useSubstitutes, "build-use-substitutes");
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_get(buildUsersGroup, "build-users-group");
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_get(useChroot, "build-use-chroot");
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_get(impersonateLinux26, "build-impersonate-linux-26");
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_get(keepLog, "build-keep-log");
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_get(compressLog, "build-compress-log");
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@ -145,9 +145,6 @@ struct Settings {
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/* The Unix group that contains the build users. */
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string buildUsersGroup;
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/* Whether to build in chroot. */
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bool useChroot;
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/* Set of ssh connection strings for the ssh substituter */
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Strings sshSubstituterHosts;
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