daemon: add and use spawn.cc and spawn.hh.

This adds a mechanism for manipulating and running "spawn phases" similarly to
how builder-side code manipulates "build phases".  The main difference is that
spawn phases take a (reference to a) single structure that they can both read
from and write to, with their writes being visible to subsequent phases.  The
base structure type for this is SpawnContext.

It also adds some predefined phase sequences, namely basicSpawnPhases and
cloneSpawnPhases, and exposes each of the actions performed by these phases.

Finally, it modifies build.cc to replace runChild() with use of this new code.

* nix/libutil/util.cc (keepOnExec, waitForMessage): new functions.
* nix/libutil.util.hh (keepOnExec, waitForMessage): add prototypes.
* nix/libutil/spawn.cc, nix/libutil/spawn.hh: new files.
  (addPhaseAfter, addPhaseBefore, prependPhase, appendPhase, deletePhase,
  replacePhase, reset_writeToStderrAction, restoreAffinityAction,
  setsidAction, earlyIOSetupAction, dropAmbientCapabilitiesAction,
  chrootAction, chdirAction, closeMostFDsAction, setPersonalityAction,
  oomSacrificeAction, setIDsAction, restoreSIGPIPEAction, setupSuccessAction,
  execAction, getBasicSpawnPhases, usernsInitSyncAction, usernsSetIDsAction,
  initLoopbackAction, setHostAndDomainAction, makeFilesystemsPrivateAction,
  makeChrootSeparateFilesystemAction, statfsToMountFlags, bindMount,
  mountIntoChroot, mountIntoChrootAction, mountProcAction, mountDevshmAction,
  mountDevptsAction, pivotRootAction, lockMountsAction, getCloneSpawnPhases,
  runChildSetup, runChildSetupEntry, cloneChild, idMapToIdentityMap,
  unshareAndInitUserns): new procedures.
* nix/local.mk (libutil_a_SOURCES): add spawn.cc.
  (libutil_headers): add spawn.hh.
* nix/libstore/build.cc (restoreSIGPIPE, DerivationGoal::runChild,
  childEntry): removed procedures.
  (DerivationGoal::{dirsInChroot,env,readiness}): removed.
  (execBuilderOrBuiltin, execBuilderOrBuiltinAction,
  clearRootWritePermsAction): new procedures.
  (DerivationGoal::startBuilder): modified to use a CloneSpawnContext if
  chroot builds are available, otherwise a SpawnContext.

Change-Id: Ifd50110de077378ee151502eda62b99973d083bf

Change-Id: I76e10d3f928cc30566e1e6ca79077196972349f8

spawn.cc, util.cc, util.hh changes

Change-Id: I287320e63197cb4f65665ee5b3fdb3a0e125ebac
Signed-off-by: John Kehayias <john.kehayias@protonmail.com>

1 files changed, 829 insertions, 0 deletions

diff --git a/nix/libutil/spawn.cc b/nix/libutil/spawn.cc
new file mode 100644
index 00000000000..93bab9f59e4
--- /dev/null
+++ b/nix/libutil/spawn.cc
@@ -0,0 +1,829 @@
+/* GNU Guix --- Functional package management for GNU
+   Copyright (C) 2025 Caleb Ristvedt <reepca@russelstein.xyz>
+
+   This file is part of GNU Guix.
+
+   GNU Guix is free software; you can redistribute it and/or modify it
+   under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or (at
+   your option) any later version.
+
+   GNU Guix is distributed in the hope that it will be useful, but
+   WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* Process spawning and setup code.  */
+
+#include <spawn.hh>
+#include <util.hh>
+#include <affinity.hh>
+#include <stddef.h>
+#include <unistd.h>
+#include <grp.h>
+#include <limits.h>
+#include <sys/wait.h>
+#include <cstring>
+#include <cstdlib>
+
+#if HAVE_SYS_MOUNT_H
+#include <sys/mount.h>
+#endif
+
+#if HAVE_SCHED_H
+#include <sched.h>
+#endif
+
+#if HAVE_STATVFS
+#include <sys/statvfs.h>
+#endif
+
+#if HAVE_SYS_SYSCALL_H
+#include <sys/syscall.h>
+#endif
+
+#if HAVE_SYS_PRCTL_H
+#include <sys/prctl.h>
+#endif
+
+#ifdef __linux__
+#include <sys/personality.h>
+#endif
+
+#if defined(SYS_pivot_root)
+#define pivot_root(new_root, put_old) (syscall(SYS_pivot_root, new_root,put_old))
+#endif
+
+
+#define CLONE_ENABLED defined(CLONE_NEWNS)
+
+#if CLONE_ENABLED
+#include <sys/ioctl.h>
+#include <net/if.h>
+#include <netinet/in.h>
+#endif
+
+namespace nix {
+
+
+void addPhaseAfter(Phases & phases, string afterLabel, string addLabel, Action addAction)
+{
+    for(auto i = phases.begin(); i != phases.end(); i++)
+        if((*i).label == afterLabel) {
+            i++; /* std::vector::insert inserts before, not after */
+            Phase p;
+            p.label = addLabel;
+            p.action = addAction;
+            phases.insert(i, p);
+            return;
+        }
+    throw Error(format("label `%1%' not found in phases") % afterLabel);
+}
+
+
+void addPhaseBefore(Phases & phases, string beforeLabel, string addLabel, Action addAction)
+{
+    for(auto i = phases.begin(); i != phases.end(); i++)
+        if((*i).label == beforeLabel) {
+            Phase p;
+            p.label = addLabel;
+            p.action = addAction;
+            phases.insert(i, p);
+            return;
+        }
+    throw Error(format("label `%1%' not found in phases") % beforeLabel);
+}
+
+
+void prependPhase(Phases & phases, string addLabel, Action addAction)
+{
+    Phase p;
+    p.label = addLabel;
+    p.action = addAction;
+    phases.insert(phases.begin(), p);
+}
+
+
+void appendPhase(Phases & phases, string addLabel, Action addAction)
+{
+    Phase p;
+    p.label = addLabel;
+    p.action = addAction;
+    phases.push_back(p);
+}
+
+
+void deletePhase(Phases & phases, string delLabel)
+{
+    for(auto i = phases.begin(); i != phases.end(); i++)
+        if((*i).label == delLabel) {
+            phases.erase(i);
+            return;
+        }
+    throw Error(format("label `%1%' not found in phases") % delLabel);
+}
+
+
+void replacePhase(Phases & phases, string replaceLabel, Action newAction)
+{
+    for(auto i = phases.begin(); i != phases.end(); i++)
+        if((*i).label == replaceLabel) {
+            (*i).action = newAction;
+            return;
+        }
+    throw Error(format("label `%1' not found in phases") % replaceLabel);
+}
+
+
+/* A curated selection of predefined actions */
+
+void reset_writeToStderrAction(SpawnContext & ctx)
+{
+    _writeToStderr = 0;
+}
+
+
+void restoreAffinityAction(SpawnContext & ctx)
+{
+    restoreAffinity();
+}
+
+
+void setsidAction(SpawnContext & ctx)
+{
+    /* Puts the current process in a separate session, which implies a
+       separate process group, so it doesn't receive group-directed signals
+       sent at the parent.  The new session initially has no controlling
+       terminal, so it also doesn't receive terminal signals and can't open
+       /dev/tty. */
+    if(ctx.setsid && setsid() == (pid_t)-1)
+        throw SysError("creating a new session");
+}
+
+
+void earlyIOSetupAction(SpawnContext & ctx)
+{
+    for(auto i = ctx.earlyCloseFDs.begin(); i != ctx.earlyCloseFDs.end(); i++)
+        if(close(*i) == -1)
+            throw SysError("closing fd");
+
+    if(ctx.logFD != -1) {
+        if(dup2(ctx.logFD, STDOUT_FILENO) == -1)
+            throw SysError("cannot dup2 log fd into stdout fd");
+        if(dup2(ctx.logFD, STDERR_FILENO) == -1)
+            throw SysError("cannot dup2 log fd into stderr fd");
+    }
+
+    if(ctx.setStdin) {
+        if(ctx.stdinFD != -1) {
+            if(dup2(ctx.stdinFD, STDIN_FILENO) == -1)
+                throw SysError("cannot dup2 fd into stdin fd");
+        }
+        else {
+            /* Doesn't make sense for it to be writable, but compatibility... */
+            AutoCloseFD fd = open(ctx.stdinFile.c_str(), O_RDWR);
+            if(fd == -1)
+                throw SysError(format("cannot open `%1%'") % ctx.stdinFile);
+            if(dup2(fd, STDIN_FILENO) == -1)
+                throw SysError("cannot dup2 fd into stdin fd");
+        }
+    }
+}
+
+
+void dropAmbientCapabilitiesAction(SpawnContext & ctx)
+{
+  /* Drop ambient capabilities such as CAP_CHOWN that might have been granted
+     when starting guix-daemon.  */
+    if(ctx.dropAmbientCapabilities)
+#if HAVE_SYS_PRCTL_H
+        prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_CLEAR_ALL, 0, 0, 0);
+#else
+        throw Error("dropping ambient capabilities is not supported on this system");
+#endif
+}
+
+
+void chrootAction(SpawnContext & ctx)
+{
+    if(ctx.doChroot)
+#if HAVE_CHROOT
+        if(chroot(ctx.chrootRootDir.c_str()) == -1)
+            throw SysError(format("cannot change root directory to '%1%'") % ctx.chrootRootDir);
+#else
+    throw Error("chroot is not supported on this system");
+#endif
+}
+
+
+void chdirAction(SpawnContext & ctx)
+{
+    if(ctx.setcwd)
+        if(chdir(ctx.cwd.c_str()) == -1)
+            throw SysError(format("changing into `%1%'") % ctx.cwd);
+}
+
+
+void closeMostFDsAction(SpawnContext & ctx)
+{
+    if(ctx.closeMostFDs) closeMostFDs(ctx.preserveFDs);
+    for(auto i = ctx.preserveFDs.begin(); i != ctx.preserveFDs.end(); i++)
+        keepOnExec(*i);
+}
+
+
+void setPersonalityAction(SpawnContext & ctx)
+{
+  if(ctx.setPersona)
+#ifdef __linux__
+    if(personality(ctx.persona) == -1)
+      throw SysError("cannot set personality");
+#else
+    throw Error("setting the personality is not supported on this system");
+#endif
+}
+
+
+void oomSacrificeAction(SpawnContext & ctx)
+{
+#ifdef __linux__
+    if(ctx.oomSacrifice)
+        /* Ask the kernel to eagerly kill us & our children if it runs out of
+           memory, regardless of blame, to preserve ‘real’ user data &
+           state. */
+        try {
+            writeFile("/proc/self/oom_score_adj", "1000"); // 100%
+        } catch(...) { ignoreException(); }
+#endif
+}
+
+
+void setIDsAction(SpawnContext & ctx)
+{
+    if(ctx.setSupplementaryGroups)
+        if(setgroups(ctx.supplementaryGroups.size(),
+                     ctx.supplementaryGroups.data()) == -1)
+            throw SysError("cannot set supplementary groups");
+
+    if(ctx.setgid)
+        if(setgid(ctx.group) == -1 ||
+           getgid() != ctx.group ||
+           getegid() != ctx.group)
+            throw SysError("setgid failed");
+
+    if(ctx.setuid)
+        if(setuid(ctx.user) == -1 ||
+           getuid() != ctx.user ||
+           geteuid() != ctx.user)
+            throw SysError("setuid failed");
+}
+
+
+void restoreSIGPIPEAction(SpawnContext & ctx)
+{
+    /* Restore default handling of SIGPIPE, otherwise some programs will
+       randomly say "Broken pipe". */
+    struct sigaction act, oact;
+    act.sa_handler = SIG_DFL;
+    act.sa_flags = 0;
+    sigemptyset(&act.sa_mask);
+    if (sigaction(SIGPIPE, &act, &oact)) throw SysError("resetting SIGPIPE");
+}
+
+
+void setupSuccessAction(SpawnContext & ctx)
+{
+    if(ctx.signalSetupSuccess)
+        writeFull(STDERR_FILENO, "\n");
+}
+
+
+void execAction(SpawnContext & ctx)
+{
+    Strings envStrs;
+    std::vector<char *> envPtrs;
+    char **env;
+    if(ctx.inheritEnv) {
+        for(auto i = ctx.env.begin(); i != ctx.env.end(); i++)
+            if(setenv(i->first.c_str(), i->second.c_str(), 1) == -1)
+                throw SysError("setenv");
+        env = environ;
+    } else {
+        for(auto i = ctx.env.begin(); i != ctx.env.end(); i++)
+            envStrs.push_back(i->first + "=" + i->second);
+        /* Need to keep the envPtrs vector alive as long as its .data()! */
+        envPtrs = stringsToCharPtrs(envStrs);
+        env = envPtrs.data();
+    }
+    if(execvpe(ctx.program.c_str(), stringsToCharPtrs(ctx.args).data(), env) == -1)
+        throw SysError(format("executing `%1%'") % ctx.program);
+}
+
+
+Phases getBasicSpawnPhases()
+{
+    return { { "reset_writeToStderr",     reset_writeToStderrAction },
+             { "restoreAffinity",         restoreAffinityAction },
+             { "setsid",                  setsidAction },
+             { "earlyIOSetup",            earlyIOSetupAction },
+             { "dropAmbientCapabilities", dropAmbientCapabilitiesAction },
+             { "chroot",                  chrootAction },
+             { "chdir",                   chdirAction },
+             { "closeMostFDs",            closeMostFDsAction },
+             { "setPersonality",          setPersonalityAction },
+             { "oomSacrifice",            oomSacrificeAction },
+             { "setIDs",                  setIDsAction },
+             { "restoreSIGPIPE",          restoreSIGPIPEAction },
+             { "setupSuccess",            setupSuccessAction },
+             { "exec",                    execAction } };
+}
+
+
+void usernsInitSyncAction(SpawnContext & sctx)
+{
+#if CLONE_ENABLED
+    CloneSpawnContext & ctx = (CloneSpawnContext &) sctx;
+    if((ctx.cloneFlags & CLONE_NEWUSER) != 0) {
+        /* Close the earlyCloseFDs before we try reading anything */
+        for(auto i = ctx.earlyCloseFDs.begin(); i != ctx.earlyCloseFDs.end(); i++)
+            if(close(*i) == -1)
+                throw SysError("closing fd");
+        /* Don't try closing them again later */
+        ctx.earlyCloseFDs.clear();
+        /* Wait for the parent process to initialize the UID/GID mapping of
+           our user namespace.  */
+        waitForMessage(ctx.setupFD, "go\n");
+    }
+#endif
+}
+
+
+void usernsSetIDsAction(SpawnContext & sctx)
+{
+#if CLONE_ENABLED
+    CloneSpawnContext & ctx = (CloneSpawnContext &) sctx;
+    if((ctx.cloneFlags & CLONE_NEWUSER) != 0) {
+        /* Note: 'man capabilities' says that a transition from zero to
+           nonzero uids causes capabilities to be lost, but doesn't say what
+           happens when a transition from an unmapped (possibly zero) uid to a
+           nonzero uid happens. */
+        if(ctx.usernsSetuid)
+            /* Since we presumably have CAP_SETUID, this sets the real,
+               effective, saved, and filesystem uids */
+            if(setuid(ctx.usernsUser) != 0)
+                throw SysError("setuid");
+        if(ctx.usernsSetgid)
+            /* Ditto but with gids */
+            if(setgid(ctx.usernsGroup) != 0)
+                throw SysError("setgid");
+    }
+#endif
+}
+
+
+void initLoopbackAction(SpawnContext & sctx)
+{
+#if CLONE_ENABLED
+    CloneSpawnContext & ctx = (CloneSpawnContext &) sctx;
+    if(((ctx.cloneFlags & CLONE_NEWNET) != 0) && ctx.initLoopback) {
+        AutoCloseFD fd(socket(PF_INET, SOCK_DGRAM, IPPROTO_IP));
+        if (fd == -1) throw SysError("cannot open IP socket");
+
+        struct ifreq ifr;
+        strcpy(ifr.ifr_name, "lo");
+        ifr.ifr_flags = IFF_UP | IFF_LOOPBACK | IFF_RUNNING;
+        if (ioctl(fd, SIOCSIFFLAGS, &ifr) == -1)
+            throw SysError("cannot set loopback interface flags");
+
+        fd.close();
+    }
+#endif
+}
+
+
+void setHostAndDomainAction(SpawnContext & sctx)
+{
+#if CLONE_ENABLED
+    CloneSpawnContext & ctx = (CloneSpawnContext &) sctx;
+    if((ctx.cloneFlags & CLONE_NEWUTS) != 0) {
+        if (sethostname(ctx.hostname.c_str(),
+                        strlen(ctx.hostname.c_str())) == -1)
+            throw SysError("cannot set host name");
+        if (setdomainname(ctx.domainname.c_str(),
+                          strlen(ctx.domainname.c_str())) == -1)
+            throw SysError("cannot set domain name");
+    }
+#endif
+}
+
+
+void makeFilesystemsPrivateAction(SpawnContext & sctx)
+{
+#if CLONE_ENABLED && HAVE_SYS_MOUNT_H && defined(MS_REC) && defined(MS_PRIVATE)
+    CloneSpawnContext & ctx = (CloneSpawnContext &) sctx;
+    if((ctx.cloneFlags & CLONE_NEWNS) != 0) {
+        if(mount(0, "/", 0, MS_REC|MS_PRIVATE, 0) == -1)
+            throw SysError("unable to make `/' private mount");
+    }
+#endif
+}
+
+
+void makeChrootSeparateFilesystemAction(SpawnContext & sctx)
+{
+#if CLONE_ENABLED && HAVE_SYS_MOUNT_H && defined(MS_BIND)
+    CloneSpawnContext & ctx = (CloneSpawnContext &) sctx;
+    if(((ctx.cloneFlags & CLONE_NEWNS) != 0) && ctx.doChroot) {
+        /* Bind-mount chroot directory to itself, to treat it as a different
+           filesystem from /, as needed for pivot_root.  Alternatively, mount
+           a tmpfs on it. */
+        if(ctx.mountTmpfsOnChroot) {
+            if(mount("none", ctx.chrootRootDir.c_str(), "tmpfs", 0, 0) == -1)
+                throw SysError(format("unable to mount tmpfs on `%1%'") % ctx.chrootRootDir);
+        }
+        else {
+            if(mount(ctx.chrootRootDir.c_str(), ctx.chrootRootDir.c_str(), 0, MS_BIND, 0) == -1)
+                throw SysError(format("unable to bind mount ‘%1%’") % ctx.chrootRootDir);
+        }
+    }
+#endif
+}
+
+
+static int statfsToMountFlags(int f_flags)
+{
+#if HAVE_SYS_MOUNT_H && HAVE_STATVFS
+    int ret = 0;
+#if defined(ST_RDONLY) && defined(MS_RDONLY)
+    if((f_flags & ST_RDONLY) != 0)     ret |= MS_RDONLY;
+#endif
+#if defined(ST_NOSUID) && defined(MS_NOSUID)
+    if((f_flags & ST_NOSUID) != 0)     ret |= MS_NOSUID;
+#endif
+#if defined(ST_NODEV) && defined(MS_NODEV)
+    if((f_flags & ST_NODEV) != 0)      ret |= MS_NODEV;
+#endif
+#if defined(ST_NOEXEC) && defined(MS_NOEXEC)
+    if((f_flags & ST_NOEXEC) != 0)     ret |= MS_NOEXEC;
+#endif
+#if defined(ST_NOATIME) && defined(MS_NOATIME)
+    if((f_flags & ST_NOATIME) != 0)    ret |= MS_NOATIME;
+#endif
+#if defined(ST_NODIRATIME) && defined(MS_NODIRATIME)
+    if((f_flags & ST_NODIRATIME) != 0) ret |= MS_NODIRATIME;
+#endif
+#if defined(ST_RELATIME) && defined(MS_RELATIME)
+    if((f_flags & ST_RELATIME) != 0)   ret |= MS_RELATIME;
+#endif
+    return ret;
+#else
+    throw Error("statfsToMountFlags not supported on this platform");
+#endif
+}
+
+
+void bindMount(Path source, Path target, bool readOnly)
+{
+#if HAVE_SYS_MOUNT_H && defined(MS_BIND)
+    struct stat st;
+    if (lstat(source.c_str(), &st) == -1)
+        throw SysError(format("getting attributes of path `%1%'") % source);
+
+    if(S_ISDIR(st.st_mode))
+        createDirs(target);
+    else if(S_ISLNK(st.st_mode)) {
+        /* bind-mounts follow symlinks, thus representing their target and not
+           the symlink itself.  Create a copy of the symlink instead.*/
+        createDirs(dirOf(target));
+        createSymlink(readLink(source), target);
+        return;
+    }
+    else {
+        createDirs(dirOf(target));
+        writeFile(target, "");
+    }
+
+    /* This may fail with EINVAL unless we specify MS_REC, specifically if we
+       are in an unprivileged mount namespace and not specifying MS_REC would
+       reveal subtrees that had been covered up. */
+    if (mount(source.c_str(), target.c_str(), 0, MS_BIND|MS_REC, 0) == -1)
+        throw SysError(format("bind mount from `%1%' to `%2%' failed") % source % target);
+    if(readOnly) {
+#if defined(MS_REMOUNT) && defined(MS_RDONLY)
+        /* Extra flags passed with MS_BIND are ignored, hence the extra
+           MS_REMOUNT.  */
+        unsigned long mount_flags = MS_BIND | MS_REMOUNT | MS_RDONLY;
+        /* MS_BIND | MS_REMOUNT sets all mountpoint flags, so we may get EPERM
+           unless we preserve the other flags (for example because it would
+           result in trying to clear the nosuid flag). */
+#if HAVE_STATVFS
+        struct statvfs stvfs;
+        if(statvfs(target.c_str(), &stvfs) == -1)
+            throw SysError(format("statvfs of `%1%'") % target);
+        mount_flags |= statfsToMountFlags(stvfs.f_flag);
+#endif
+
+        if (mount(source.c_str(), target.c_str(), 0, mount_flags, 0) == -1)
+            throw SysError(format("read-only remount of `%1%' failed") % target);
+#else
+        throw Error("remounting read-only is not supported on this platform");
+#endif
+    }
+#endif
+}
+
+
+void mountIntoChroot(std::map<Path, Path> filesInChroot,
+                     set<Path> readOnlyFiles,
+                     Path chrootRootDir)
+{
+#if HAVE_SYS_MOUNT_H && defined(MS_BIND)
+    for(auto i = filesInChroot.begin(); i != filesInChroot.end(); i++) {
+        Path source = i->second;
+        Path target = chrootRootDir + i->first;
+        bool readOnly = readOnlyFiles.find(i->first) != readOnlyFiles.end();
+        bindMount(source, target, readOnly);
+    }
+#else
+    throw Error("bind mounting not supported on this platform");
+#endif
+}
+
+
+void mountIntoChrootAction(SpawnContext & sctx)
+{
+#if CLONE_ENABLED && HAVE_SYS_MOUNT_H && defined(MS_BIND)
+    CloneSpawnContext & ctx = (CloneSpawnContext &) sctx;
+    if((ctx.cloneFlags & CLONE_NEWNS) != 0 && ctx.doChroot) {
+        mountIntoChroot(ctx.filesInChroot, ctx.readOnlyFilesInChroot, ctx.chrootRootDir);
+    }
+#endif
+}
+
+
+void mountProcAction(SpawnContext & sctx)
+{
+#if CLONE_ENABLED && HAVE_SYS_MOUNT_H
+    CloneSpawnContext & ctx = (CloneSpawnContext &) sctx;
+    if((ctx.cloneFlags & CLONE_NEWNS) != 0 && ctx.mountProc) {
+        Path target = (ctx.doChroot ? ctx.chrootRootDir : "") + "/proc";
+        createDirs(target);
+        if(mount("none", target.c_str(), "proc", 0, 0) == -1)
+            throw SysError(format("mounting `%1%'") % target);
+    }
+#endif
+}
+
+
+void mountDevshmAction(SpawnContext & sctx)
+{
+#if CLONE_ENABLED && HAVE_SYS_MOUNT_H
+    CloneSpawnContext & ctx = (CloneSpawnContext &) sctx;
+    if((ctx.cloneFlags & CLONE_NEWNS) != 0 && ctx.mountDevshm) {
+        Path target = (ctx.doChroot ? ctx.chrootRootDir : "") + "/dev/shm";
+        createDirs(target);
+        if(mount("none", target.c_str(), "tmpfs", 0, 0) == -1)
+            throw SysError(format("mounting `%1%'") % target);
+    }
+#endif
+}
+
+
+void mountDevptsAction(SpawnContext & sctx)
+{
+#if CLONE_ENABLED && HAVE_SYS_MOUNT_H
+    CloneSpawnContext & ctx = (CloneSpawnContext &) sctx;
+    if((ctx.cloneFlags & CLONE_NEWNS) != 0 && ctx.maybeMountDevpts) {
+        Path chroot = (ctx.doChroot ? ctx.chrootRootDir : "");
+        Path target = chroot + "/dev/pts";
+        if(pathExists(chroot + "/dev/ptmx")) return;
+        createDirs(target);
+        if(mount("none", target.c_str(), "devpts", 0, "newinstance,mode=0620") == -1)
+            throw SysError(format("mounting `%1%'") % target);
+        createSymlink("/dev/pts/ptmx", chroot + "/dev/ptmx");
+        /* Make sure /dev/pts/ptmx is world-writable.  With some Linux
+           versions, it is created with permissions 0.  */
+        Path targetPtmx = chroot + "/dev/pts/ptmx";
+        if (chmod(targetPtmx.c_str(), 0666) == -1)
+            throw SysError(format("setting permissions on `%1%'") % targetPtmx);
+    }
+#endif
+}
+
+
+void pivotRootAction(SpawnContext & sctx)
+{
+#if CLONE_ENABLED && HAVE_SYS_MOUNT_H
+    CloneSpawnContext & ctx = (CloneSpawnContext &) sctx;
+    if((ctx.cloneFlags & CLONE_NEWNS) != 0 && ctx.doChroot) {
+        if (chdir(ctx.chrootRootDir.c_str()) == -1)
+            throw SysError(format("cannot change directory to '%1%'") % ctx.chrootRootDir);
+
+        if (mkdir("real-root", 0) == -1)
+            throw SysError("cannot create real-root directory");
+
+        if (pivot_root(".", "real-root") == -1)
+            throw SysError(format("cannot pivot old root directory onto '%1%'") % (ctx.chrootRootDir + "/real-root"));
+
+        if (chroot(".") == -1)
+            throw SysError(format("cannot change root directory to '%1%'") % ctx.chrootRootDir);
+
+        if (umount2("real-root", MNT_DETACH) == -1)
+            throw SysError("cannot unmount real root filesystem");
+
+        if (rmdir("real-root") == -1)
+            throw SysError("cannot remove real-root directory");
+    }
+#endif
+}
+
+
+string idMapToIdentityMap(const string & map)
+{
+    std::vector<string> mapLines =
+        tokenizeString<std::vector<string> >(map, "\n");
+    string out;
+
+    for(auto & i : mapLines) {
+        std::vector<string> elements =
+            tokenizeString<std::vector<string> >(i, " ");
+        out.append(elements.at(0) + " " + elements.at(0) + " " + elements.at(2) + "\n");
+    }
+    return out;
+}
+
+
+/* Initializing a user namespace with more than one id mapped requires
+ * capabilities in the *parent* user namespace, which may not even have any
+ * processes in it after unshare is called.  So fork a child and have it do
+ * the initialization. */
+void unshareAndInitUserns(int flags, const string & uidMap,
+                          const string & gidMap, bool allowSetgroups)
+{
+#if CLONE_ENABLED
+    pid_t pid_ = getpid();
+    string pid = std::to_string(pid_);
+    Pipe toChild;
+    Pipe fromChild;
+    toChild.create();
+    fromChild.create();
+    pid_t child = fork();
+    if(child == -1)
+        throw SysError("creating child process");
+    if(child == 0) {
+        try {
+            toChild.writeSide.close();
+            fromChild.readSide.close();
+            waitForMessage(toChild.readSide, "ready\n");
+            writeFile("/proc/" + pid + "/uid_map", uidMap);
+            writeFile("/proc/" + pid + "/setgroups",
+                      allowSetgroups ? "allow" : "deny");
+            writeFile("/proc/" + pid + "/gid_map", gidMap);
+            writeFull(fromChild.writeSide, (unsigned char*)"go\n", 3);
+        } catch(...) {
+            /* Don't unwind the stack in case of exception, halt
+             * immediately. */
+            _exit(1);
+        }
+        _exit(EXIT_SUCCESS);
+    } else {
+        toChild.readSide.close();
+        fromChild.writeSide.close();
+        if(unshare(flags) == -1)
+            throw SysError("unshare");
+        writeFull(toChild.writeSide, (unsigned char*)"ready\n", 6);
+        waitForMessage(fromChild.readSide, "go\n");
+        int status;
+        while(waitpid(child, &status, 0) == -1) {
+            if(errno != EINTR)
+                throw SysError("reaping userns init process");
+        }
+        if(!(WIFEXITED(status) != 0 && WEXITSTATUS(status) == EXIT_SUCCESS))
+            throw Error(format("userns init child exited with status %1%") % WEXITSTATUS(status));
+    }
+#endif
+}
+
+
+void lockMountsAction(SpawnContext & sctx)
+{
+#if CLONE_ENABLED && HAVE_SYS_MOUNT_H
+    CloneSpawnContext & ctx = (CloneSpawnContext &) sctx;
+    if(ctx.lockMounts) {
+        string uidMap;
+        string gidMap;
+        if(ctx.lockMountsMapAll) {
+            string oldUidMap = readFile("/proc/self/uid_map", true);
+            string oldGidMap = readFile("/proc/self/gid_map", true);
+            uidMap = idMapToIdentityMap(oldUidMap);
+            gidMap = idMapToIdentityMap(oldGidMap);
+        } else {
+            string uid = std::to_string(getuid());
+            string gid = std::to_string(getgid());
+            uidMap = uid + " " + uid + " 1";
+            gidMap = gid + " " + gid + " 1";
+        }
+        unshareAndInitUserns(CLONE_NEWNS | CLONE_NEWUSER,
+                             uidMap, gidMap, ctx.lockMountsAllowSetgroups);
+        /* Check that mounts inherited in our new mount namespace are "locked"
+           together and cannot be separated from within our mount namespace.
+           Since umount(2) is documented to fail with EINVAL when attempting
+           to unmount one of the mounts that are locked together, check that
+           this is what we get.  */
+        int ret = umount("/proc");
+        assert(ret == -1 && errno == EINVAL);
+    }
+#endif
+}
+
+
+Phases getCloneSpawnPhases()
+{
+#if CLONE_ENABLED
+    return { { "reset_writeToStderr",          reset_writeToStderrAction          },
+             { "usernsInitSync",               usernsInitSyncAction               },
+             { "usernsSetIDs",                 usernsSetIDsAction                 },
+             { "restoreAffinity",              restoreAffinityAction              },
+             { "setsid",                       setsidAction                       },
+             { "earlyIOSetup",                 earlyIOSetupAction                 },
+             { "dropAmbientCapabilities",      dropAmbientCapabilitiesAction      },
+             { "initLoopback",                 initLoopbackAction                 },
+             { "setHostAndDomain",             setHostAndDomainAction             },
+             { "makeFilesystemsPrivate",       makeFilesystemsPrivateAction       },
+             { "makeChrootSeparateFilesystem", makeChrootSeparateFilesystemAction },
+             { "mountIntoChroot",              mountIntoChrootAction              },
+             { "mountProc",                    mountProcAction                    },
+             { "mountDevshm",                  mountDevshmAction                  },
+             { "mountDevpts",                  mountDevptsAction                  },
+             { "chroot",                       pivotRootAction                    },
+             { "chdir",                        chdirAction                        },
+             { "closeMostFDs",                 closeMostFDsAction                 },
+             { "setPersonality",               setPersonalityAction               },
+             { "oomSacrifice",                 oomSacrificeAction                 },
+             /* Being put in a user namespace with only the current ids mapped
+                would tend to prevent switching to other ones, but if this
+                comes after setIDs then the per-process "dumpable" flag may be
+                reset, which will cause /proc/self to become root-owned,
+                making /proc/self/uid_map inaccessible.  If you need
+                lockMounts to preserve the id mappings, and you have the
+                necessary capabilities in the parent user namespace, set
+                CloneSpawnContext.lockMountsMapAll = true. */
+             { "lockMounts",                   lockMountsAction                   },
+             { "setIDs",                       setIDsAction                       },
+             { "restoreSIGPIPE",               restoreSIGPIPEAction               },
+             { "setupSuccess",                 setupSuccessAction                 },
+             { "exec",                         execAction                         }};
+#else
+    throw Error("clone not supported on this platform");
+#endif
+}
+
+
+void runChildSetup(SpawnContext & ctx)
+{
+    ctx.currentPhase = 0;
+    try {
+        /* Should not return regularly from this */
+        while(true) {
+            ctx.phases.at(ctx.currentPhase).action(ctx);
+            ctx.currentPhase++;
+        }
+    } catch (std::exception & e) {
+        try {
+            writeFull(STDERR_FILENO,
+                      "while setting up the child process: " +
+                      (ctx.currentPhase < (ssize_t)ctx.phases.size() ?
+                       "in phase " + ctx.phases[ctx.currentPhase].label + ": " : "") +
+                      string(e.what()) + "\n");
+        } catch (std::exception & e2) {
+            _exit(1);
+        }
+        _exit(1);
+    }
+    abort(); /* Should never be reached */
+}
+
+
+int runChildSetupEntry(void *data)
+{
+    runChildSetup(* (SpawnContext *)data);
+    return 1;
+}
+
+
+int cloneChild(CloneSpawnContext & ctx)
+{
+    char stack[32 * 1024];
+    /* Ensure proper alignment on the stack.  On aarch64, it has to be 16
+       bytes.  */
+    char *alignedStack = (char *)(((uintptr_t)stack + sizeof(stack) - 8) & ~(uintptr_t)0xf);
+    int ret = clone(runChildSetupEntry, alignedStack, ctx.cloneFlags, (void *) &ctx);
+    if(ret == -1)
+        throw SysError("clone");
+    return ret;
+}
+
+}