08/15: gnu: build: Add Linux container module.

[David Thompson]

davexunit pushed a commit to branch wip-container
in repository guix.

commit 399e0f3fe9609a2e995911a762104f802a735023
Author: David Thompson <address@hidden>
Date:   Tue Jun 2 08:48:16 2015 -0400

    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-clone', 
'with-clone',
      'call-with-container', and 'container-excursion'.
---
 .dir-locals.el                |    5 +
 gnu-system.am                 |    1 +
 gnu/build/linux-container.scm |  280 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 286 insertions(+), 0 deletions(-)

diff --git a/.dir-locals.el b/.dir-locals.el
index cbcb120..65e1c6d 100644
--- a/.dir-locals.el
+++ b/.dir-locals.el
@@ -59,6 +59,11 @@
    (eval . (put 'run-with-state 'scheme-indent-function 1))
    (eval . (put 'wrap-program 'scheme-indent-function 1))
 
+   (eval . (put 'call-with-clone 'scheme-indent-function 1))
+   (eval . (put 'with-clone 'scheme-indent-function 1))
+   (eval . (put 'call-with-container 'scheme-indent-function 1))
+   (eval . (put 'container-excursion 'scheme-indent-function 1))
+
    ;; Recognize '~', '+', and '$', as used for gexps, as quotation symbols.
    ;; This notably allows '(' in Paredit to not insert a space when the
    ;; preceding symbol is one of these.
diff --git a/gnu-system.am b/gnu-system.am
index a3c56a8..d625d9c 100644
--- a/gnu-system.am
+++ b/gnu-system.am
@@ -357,6 +357,7 @@ GNU_SYSTEM_MODULES =                                \
   gnu/build/file-systems.scm                   \
   gnu/build/install.scm                                \
   gnu/build/linux-boot.scm                     \
+  gnu/build/linux-container.scm                        \
   gnu/build/linux-initrd.scm                   \
   gnu/build/linux-modules.scm                  \
   gnu/build/vm.scm
diff --git a/gnu/build/linux-container.scm b/gnu/build/linux-container.scm
new file mode 100644
index 0000000..c79da09
--- /dev/null
+++ b/gnu/build/linux-container.scm
@@ -0,0 +1,280 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2015 David Thompson <address@hidden>
+;;;
+;;; 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)
+  #: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 (mount-flags->bit-mask flags)
+  "Return the number suitable for the 'flags' argument of 'mount' that
+corresponds to the symbols listed in FLAGS."
+  (let loop ((flags flags))
+    (match flags
+      (('read-only rest ...)
+       (logior MS_RDONLY (loop rest)))
+      (('bind-mount rest ...)
+       (logior MS_BIND (loop rest)))
+      (('no-suid rest ...)
+       (logior MS_NOSUID (loop rest)))
+      (('no-dev rest ...)
+       (logior MS_NODEV (loop rest)))
+      (('no-exec rest ...)
+       (logior MS_NOEXEC (loop rest)))
+      (()
+       0))))
+
+(define* (mount-file-system spec root)
+  "Mount the file system described by SPEC under ROOT.  SPEC must have the
+form:
+
+  (DEVICE TITLE MOUNT-POINT TYPE (FLAGS ...) OPTIONS CHECK?)
+
+DEVICE, MOUNT-POINT, and TYPE must be strings; OPTIONS can be a string or #f;
+FLAGS must be a list of symbols.  CHECK? is ignored."
+  (match spec
+    ((source title mount-point type (flags ...) options _)
+     (let ((mount-point (string-append root mount-point))
+           (flags       (mount-flags->bit-mask flags)))
+       (mkdir-p mount-point)
+       (mount source mount-point type flags options)
+
+       ;; For read-only bind mounts, an extra remount is needed, as per
+       ;; <http://lwn.net/Articles/281157/>, which still applies to Linux 4.0.
+       (when (and (= MS_BIND (logand flags MS_BIND))
+                  (= MS_RDONLY (logand flags MS_RDONLY)))
+         (let ((flags (logior MS_BIND MS_REMOUNT MS_RDONLY)))
+           (mount source mount-point type flags #f)))))))
+
+(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)
+  "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?))
+
+  (purify-environment)
+
+  ;; The container's file system is completely ephemeral, sans directories
+  ;; bind-mounted from the host.
+  (mount "none" root "tmpfs")
+
+  ;; Create essential file systems.
+  (mount* "none" (scope "/proc") "proc"
+          (logior MS_NOEXEC MS_NOSUID MS_NODEV))
+  (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))
+            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)
+             (mount-file-systems root mounts)
+             (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."
+  (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."
+  (define (namespace-file pid namespace)
+    (string-append "/proc/" pid "/ns/" namespace))
+
+  (let ((pid (number->string pid)))
+    (match (primitive-fork)
+      (0
+       (call-with-clean-exit
+        (lambda ()
+          (for-each (lambda (ns)
+                      (call-with-input-file (namespace-file "self" 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 new-ns-port)
+                                         (port->fdes current-ns-port))
+                                (setns (port->fdes new-ns-port) 0)))))))
+                    ;; It's important that the mount namespace is joined last,
+                    ;; otherwise the /proc mount point would no longer be
+                    ;; accessible.
+                    '("ipc" "net" "pid" "uts" "user" "mnt"))
+          (purify-environment)
+          (chdir "/")
+          (thunk))))
+      (child-pid (waitpid child-pid)))))