gnu: build: Add Linux container module.

* gnu/build/linux-container.scm: New file.
* gnu-system.am (GNU_SYSTEM_MODULES): Add it.
* .dir-locals.el: Add Scheme indent rules for 'call-with-container', and
  'container-excursion'.
* tests/containers.scm: New file.
* Makefile.am (SCM_TESTS): Add it.

1 files changed, 260 insertions, 0 deletions

diff --git a/gnu/build/linux-container.scm b/gnu/build/linux-container.scm
new file mode 100644
index 0000000000..7a03a29d2c
--- /dev/null
+++ b/gnu/build/linux-container.scm
@@ -0,0 +1,260 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2015 David Thompson <davet@gnu.org>
+;;;
+;;; 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/>.
+
+(define-module (gnu build linux-container)
+  #:use-module (ice-9 format)
+  #:use-module (ice-9 match)
+  #:use-module (srfi srfi-98)
+  #:use-module (guix utils)
+  #:use-module (guix build utils)
+  #:use-module (guix build syscalls)
+  #:use-module ((gnu build file-systems) #:select (mount-file-system))
+  #:export (%namespaces
+            run-container
+            call-with-container
+            container-excursion))
+
+(define %namespaces
+  '(mnt pid ipc uts user net))
+
+(define (call-with-clean-exit thunk)
+  "Apply THUNK, but exit with a status code of 1 if it fails."
+  (dynamic-wind
+    (const #t)
+    thunk
+    (lambda ()
+      (primitive-exit 1))))
+
+(define (purify-environment)
+  "Unset all environment variables."
+  (for-each unsetenv
+            (match (get-environment-variables)
+              (((names . _) ...) names))))
+
+;; The container setup procedure closely resembles that of the Docker
+;; specification:
+;; https://raw.githubusercontent.com/docker/libcontainer/master/SPEC.md
+(define* (mount-file-systems root mounts #:key mount-/sys? mount-/proc?)
+  "Mount the essential file systems and the those in the MOUNTS list relative
+to ROOT, then make ROOT the new root directory for the process."
+  (define (scope dir)
+    (string-append root dir))
+
+  (define (bind-mount src dest)
+    (mount src dest "none" MS_BIND))
+
+  ;; Like mount, but creates the mount point if it doesn't exist.
+  (define* (mount* source target type #:optional (flags 0) options
+                   #:key (update-mtab? #f))
+    (mkdir-p target)
+    (mount source target type flags options #:update-mtab? update-mtab?))
+
+  ;; The container's file system is completely ephemeral, sans directories
+  ;; bind-mounted from the host.
+  (mount "none" root "tmpfs")
+
+  ;; A proc mount requires a new pid namespace.
+  (when mount-/proc?
+    (mount* "none" (scope "/proc") "proc"
+            (logior MS_NOEXEC MS_NOSUID MS_NODEV)))
+
+  ;; A sysfs mount requires the user to have the CAP_SYS_ADMIN capability in
+  ;; the current network namespace.
+  (when mount-/sys?
+    (mount* "none" (scope "/sys") "sysfs"
+            (logior MS_NOEXEC MS_NOSUID MS_NODEV MS_RDONLY)))
+
+  (mount* "none" (scope "/dev") "tmpfs"
+          (logior MS_NOEXEC MS_STRICTATIME)
+          "mode=755")
+
+  ;; Create essential device nodes via bind-mounting them from the
+  ;; host, because a process within a user namespace cannot create
+  ;; device nodes.
+  (for-each (lambda (device)
+              (when (file-exists? device)
+                ;; Create the mount point file.
+                (call-with-output-file (scope device)
+                  (const #t))
+                (bind-mount device (scope device))))
+            '("/dev/null"
+              "/dev/zero"
+              "/dev/full"
+              "/dev/random"
+              "/dev/urandom"
+              "/dev/tty"
+              "/dev/ptmx"
+              "/dev/fuse"))
+
+  ;; Setup standard input/output/error.
+  (symlink "/proc/self/fd"   (scope "/dev/fd"))
+  (symlink "/proc/self/fd/0" (scope "/dev/stdin"))
+  (symlink "/proc/self/fd/1" (scope "/dev/stdout"))
+  (symlink "/proc/self/fd/2" (scope "/dev/stderr"))
+
+  ;; Mount user-specified file systems.
+  (for-each (lambda (spec)
+              (mount-file-system spec #:root root))
+            mounts)
+
+  ;; Jail the process inside the container's root file system.
+  (let ((put-old (string-append root "/real-root")))
+    (mkdir put-old)
+    (pivot-root root put-old)
+    (chdir "/")
+    (umount "real-root" MNT_DETACH)
+    (rmdir "real-root")))
+
+(define (initialize-user-namespace pid)
+  "Configure the user namespace for PID."
+  (define proc-dir
+    (string-append "/proc/" (number->string pid)))
+
+  (define (scope file)
+    (string-append proc-dir file))
+
+  ;; Only root can map more than a single uid/gid.  A range of 65536 uid/gids
+  ;; is used to cover 16 bits worth of users and groups, which is sufficient
+  ;; for most cases.
+  ;;
+  ;; See also: http://www.freedesktop.org/software/systemd/man/systemd-nspawn.html#--private-users=
+  (let* ((uid       (getuid))
+         (gid       (getgid))
+         (uid-range (if (zero? uid) 65536 1))
+         (gid-range (if (zero? gid) 65536 1)))
+
+    ;; Only root can write to the gid map without first disabling the
+    ;; setgroups syscall.
+    (unless (and (zero? uid) (zero? gid))
+      (call-with-output-file (scope "/setgroups")
+        (lambda (port)
+          (display "deny" port))))
+
+    ;; Map the user/group that created the container to the root user
+    ;; within the container.
+    (call-with-output-file (scope "/uid_map")
+      (lambda (port)
+        (format port "0 ~d ~d" uid uid-range)))
+    (call-with-output-file (scope "/gid_map")
+      (lambda (port)
+        (format port "0 ~d ~d" gid gid-range)))))
+
+(define (namespaces->bit-mask namespaces)
+  "Return the number suitable for the 'flags' argument of 'clone' that
+corresponds to the symbols in NAMESPACES."
+  (apply logior SIGCHLD
+         (map (match-lambda
+               ('mnt  CLONE_NEWNS)
+               ('uts  CLONE_NEWUTS)
+               ('ipc  CLONE_NEWIPC)
+               ('user CLONE_NEWUSER)
+               ('pid  CLONE_NEWPID)
+               ('net  CLONE_NEWNET))
+              namespaces)))
+
+(define (run-container root mounts namespaces thunk)
+  "Run THUNK in a new container process and return its PID.  ROOT specifies
+the root directory for the container.  MOUNTS is a list of file system specs
+that specify the mapping of host file systems into the container.  NAMESPACES
+is a list of symbols that correspond to the possible Linux namespaces: mnt,
+ipc, uts, user, and net."
+  ;; The parent process must initialize the user namespace for the child
+  ;; before it can boot.  To negotiate this, a pipe is used such that the
+  ;; child process blocks until the parent writes to it.
+  (match (pipe)
+    ((in . out)
+     (let ((flags (namespaces->bit-mask namespaces)))
+       (match (clone flags)
+         (0
+          (call-with-clean-exit
+           (lambda ()
+             (close out)
+             ;; Wait for parent to set things up.
+             (read in)
+             (close in)
+             (purify-environment)
+             (when (memq 'mnt namespaces)
+               (mount-file-systems root mounts
+                                   #:mount-/proc? (memq 'pid namespaces)
+                                   #:mount-/sys?  (memq 'net namespaces)))
+             ;; TODO: Manage capabilities.
+             (thunk))))
+         (pid
+          (when (memq 'user namespaces)
+            (initialize-user-namespace pid))
+          ;; TODO: Initialize cgroups.
+          (close in)
+          (write 'ready out)
+          (close out)
+          pid))))))
+
+(define* (call-with-container mounts thunk #:key (namespaces %namespaces))
+  "Run THUNK in a new container process and return its exit status.
+MOUNTS is a list of file system specs that specify the mapping of host file
+systems into the container.  NAMESPACES is a list of symbols corresponding to
+the identifiers for Linux namespaces: mnt, ipc, uts, pid, user, and net.  By
+default, all namespaces are used.
+
+Note that if THUNK needs to load any additional Guile modules, the relevant
+module files must be present in one of the mappings in MOUNTS and the Guile
+load path must be adjusted as needed."
+  (call-with-temporary-directory
+   (lambda (root)
+     (let ((pid (run-container root mounts namespaces thunk)))
+       ;; Catch SIGINT and kill the container process.
+       (sigaction SIGINT
+         (lambda (signum)
+           (false-if-exception
+            (kill pid SIGKILL))))
+
+       (match (waitpid pid)
+         ((_ . status) status))))))
+
+(define (container-excursion pid thunk)
+  "Run THUNK as a child process within the namespaces of process PID and
+return the exit status."
+  (define (namespace-file pid namespace)
+    (string-append "/proc/" (number->string pid) "/ns/" namespace))
+
+  (match (primitive-fork)
+    (0
+     (call-with-clean-exit
+      (lambda ()
+        (for-each (lambda (ns)
+                    (call-with-input-file (namespace-file (getpid) ns)
+                      (lambda (current-ns-port)
+                        (call-with-input-file (namespace-file pid ns)
+                          (lambda (new-ns-port)
+                            ;; Joining the namespace that the process
+                            ;; already belongs to would throw an error.
+                            (unless (= (port->fdes current-ns-port)
+                                       (port->fdes new-ns-port))
+                              (setns (port->fdes new-ns-port) 0)))))))
+                  ;; It's important that the user namespace is joined first,
+                  ;; so that the user will have the privileges to join the
+                  ;; other namespaces.  Furthermore, it's important that the
+                  ;; mount namespace is joined last, otherwise the /proc mount
+                  ;; point would no longer be accessible.
+                  '("user" "ipc" "uts" "net" "pid" "mnt"))
+        (purify-environment)
+        (chdir "/")
+        (thunk))))
+    (pid
+     (match (waitpid pid)
+       ((_ . status)
+        (status:exit-val status))))))