354 lines
14 KiB
Scheme
354 lines
14 KiB
Scheme
;;; GNU Guix --- Functional package management for GNU
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;;; Copyright © 2015 David Thompson <davet@gnu.org>
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;;; Copyright © 2017, 2018 Ludovic Courtès <ludo@gnu.org>
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;;;
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;;; This file is part of GNU Guix.
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;;;
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;;; GNU Guix is free software; you can redistribute it and/or modify it
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;;; under the terms of the GNU General Public License as published by
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;;; the Free Software Foundation; either version 3 of the License, or (at
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;;; your option) any later version.
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;;;
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;;; GNU Guix is distributed in the hope that it will be useful, but
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;;; WITHOUT ANY WARRANTY; without even the implied warranty of
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;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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;;; GNU General Public License for more details.
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;;;
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;;; You should have received a copy of the GNU General Public License
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;;; along with GNU Guix. If not, see <http://www.gnu.org/licenses/>.
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(define-module (gnu build linux-container)
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#:use-module (ice-9 format)
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#:use-module (ice-9 match)
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#:use-module (ice-9 rdelim)
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#:use-module (srfi srfi-98)
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#:use-module (guix utils)
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#:use-module (guix build utils)
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#:use-module (guix build syscalls)
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#:use-module (gnu system file-systems) ;<file-system>
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#:use-module ((gnu build file-systems) #:select (mount-file-system))
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#:export (user-namespace-supported?
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unprivileged-user-namespace-supported?
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setgroups-supported?
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%namespaces
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run-container
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call-with-container
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container-excursion
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container-excursion*))
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(define (user-namespace-supported?)
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"Return #t if user namespaces are supported on this system."
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(file-exists? "/proc/self/ns/user"))
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(define (unprivileged-user-namespace-supported?)
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"Return #t if user namespaces can be created by unprivileged users."
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(let ((userns-file "/proc/sys/kernel/unprivileged_userns_clone"))
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(if (file-exists? userns-file)
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(eqv? #\1 (call-with-input-file userns-file read-char))
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#t)))
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(define (setgroups-supported?)
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"Return #t if the setgroups proc file, introduced in Linux-libre 3.19,
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exists."
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(file-exists? "/proc/self/setgroups"))
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(define %namespaces
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'(mnt pid ipc uts user net))
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(define (call-with-clean-exit thunk)
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"Apply THUNK, but exit with a status code of 1 if it fails."
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(dynamic-wind
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(const #t)
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(lambda ()
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(thunk)
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;; XXX: Somehow we sometimes get EBADF from write(2) or close(2) upon
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;; exit (coming from fd finalizers) when used by the Shepherd. To work
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;; around that, exit forcefully so fd finalizers don't have a chance to
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;; run and fail.
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(primitive-_exit 0))
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(lambda ()
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(primitive-_exit 1))))
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(define (purify-environment)
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"Unset all environment variables."
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(for-each unsetenv
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(match (get-environment-variables)
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(((names . _) ...) names))))
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;; The container setup procedure closely resembles that of the Docker
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;; specification:
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;; https://raw.githubusercontent.com/docker/libcontainer/master/SPEC.md
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(define* (mount-file-systems root mounts #:key mount-/sys? mount-/proc?)
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"Mount the essential file systems and the those in MOUNTS, a list of
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<file-system> objects, relative to ROOT; then make ROOT the new root directory
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for the process."
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(define (scope dir)
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(string-append root dir))
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(define (touch file-name)
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(call-with-output-file file-name (const #t)))
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(define (bind-mount src dest)
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(mount src dest "none" MS_BIND))
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;; Like mount, but creates the mount point if it doesn't exist.
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(define* (mount* source target type #:optional (flags 0) options
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#:key (update-mtab? #f))
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(mkdir-p target)
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(mount source target type flags options #:update-mtab? update-mtab?))
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;; The container's file system is completely ephemeral, sans directories
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;; bind-mounted from the host.
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(mount "none" root "tmpfs")
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;; A proc mount requires a new pid namespace.
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(when mount-/proc?
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(mount* "none" (scope "/proc") "proc"
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(logior MS_NOEXEC MS_NOSUID MS_NODEV)))
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;; A sysfs mount requires the user to have the CAP_SYS_ADMIN capability in
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;; the current network namespace.
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(when mount-/sys?
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(mount* "none" (scope "/sys") "sysfs"
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(logior MS_NOEXEC MS_NOSUID MS_NODEV MS_RDONLY)))
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(mount* "none" (scope "/dev") "tmpfs"
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(logior MS_NOEXEC MS_STRICTATIME)
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"mode=755")
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;; Create essential device nodes via bind-mounting them from the
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;; host, because a process within a user namespace cannot create
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;; device nodes.
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(for-each (lambda (device)
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(when (file-exists? device)
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;; Create the mount point file.
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(touch (scope device))
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(bind-mount device (scope device))))
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'("/dev/null"
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"/dev/zero"
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"/dev/full"
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"/dev/random"
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"/dev/urandom"
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"/dev/tty"
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"/dev/ptmx"
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"/dev/fuse"))
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;; Setup the container's /dev/console by bind mounting the pseudo-terminal
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;; associated with standard input when there is one.
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(let* ((in (current-input-port))
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(tty (catch 'system-error
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(lambda ()
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;; This call throws if IN does not correspond to a tty.
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;; This is more reliable than 'isatty?'.
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(ttyname in))
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(const #f)))
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(console (scope "/dev/console")))
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(when tty
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(touch console)
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(chmod console #o600)
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(bind-mount tty console)))
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;; Setup standard input/output/error.
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(symlink "/proc/self/fd" (scope "/dev/fd"))
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(symlink "/proc/self/fd/0" (scope "/dev/stdin"))
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(symlink "/proc/self/fd/1" (scope "/dev/stdout"))
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(symlink "/proc/self/fd/2" (scope "/dev/stderr"))
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;; Mount user-specified file systems.
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(for-each (lambda (file-system)
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(mount-file-system file-system #:root root))
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mounts)
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;; Jail the process inside the container's root file system.
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(let ((put-old (string-append root "/real-root")))
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(mkdir put-old)
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(pivot-root root put-old)
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(chdir "/")
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(umount "real-root" MNT_DETACH)
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(rmdir "real-root")))
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(define (initialize-user-namespace pid host-uids)
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"Configure the user namespace for PID. HOST-UIDS specifies the number of
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host user identifiers to map into the user namespace."
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(define proc-dir
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(string-append "/proc/" (number->string pid)))
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(define (scope file)
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(string-append proc-dir file))
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(let ((uid (getuid))
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(gid (getgid)))
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;; Only root can write to the gid map without first disabling the
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;; setgroups syscall.
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(unless (and (zero? uid) (zero? gid))
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(call-with-output-file (scope "/setgroups")
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(lambda (port)
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(display "deny" port))))
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;; Map the user/group that created the container to the root user
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;; within the container.
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(call-with-output-file (scope "/uid_map")
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(lambda (port)
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(format port "0 ~d ~d" uid host-uids)))
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(call-with-output-file (scope "/gid_map")
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(lambda (port)
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(format port "0 ~d ~d" gid host-uids)))))
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(define (namespaces->bit-mask namespaces)
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"Return the number suitable for the 'flags' argument of 'clone' that
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corresponds to the symbols in NAMESPACES."
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;; Use the same flags as fork(3) in addition to the namespace flags.
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(apply logior SIGCHLD
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(map (match-lambda
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('mnt CLONE_NEWNS)
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('uts CLONE_NEWUTS)
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('ipc CLONE_NEWIPC)
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('user CLONE_NEWUSER)
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('pid CLONE_NEWPID)
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('net CLONE_NEWNET))
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namespaces)))
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(define (run-container root mounts namespaces host-uids thunk)
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"Run THUNK in a new container process and return its PID. ROOT specifies
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the root directory for the container. MOUNTS is a list of <file-system>
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objects that specify file systems to mount inside the container. NAMESPACES
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is a list of symbols that correspond to the possible Linux namespaces: mnt,
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ipc, uts, user, and net. HOST-UIDS specifies the number of
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host user identifiers to map into the user namespace."
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;; The parent process must initialize the user namespace for the child
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;; before it can boot. To negotiate this, a pipe is used such that the
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;; child process blocks until the parent writes to it.
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(match (socketpair PF_UNIX SOCK_STREAM 0)
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((child . parent)
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(let ((flags (namespaces->bit-mask namespaces)))
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(match (clone flags)
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(0
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(call-with-clean-exit
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(lambda ()
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(close-port parent)
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;; Wait for parent to set things up.
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(match (read child)
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('ready
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(purify-environment)
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(when (memq 'mnt namespaces)
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(catch #t
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(lambda ()
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(mount-file-systems root mounts
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#:mount-/proc? (memq 'pid namespaces)
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#:mount-/sys? (memq 'net
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namespaces)))
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(lambda args
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;; Forward the exception to the parent process.
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;; FIXME: SRFI-35 conditions and non-trivial objects
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;; cannot be 'read' so they shouldn't be written as is.
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(write args child)
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(primitive-exit 3))))
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;; TODO: Manage capabilities.
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(write 'ready child)
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(close-port child)
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(thunk))
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(_ ;parent died or something
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(primitive-exit 2))))))
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(pid
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(close-port child)
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(when (memq 'user namespaces)
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(initialize-user-namespace pid host-uids))
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;; TODO: Initialize cgroups.
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(write 'ready parent)
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(newline parent)
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;; Check whether the child process' setup phase succeeded.
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(let ((message (read parent)))
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(close-port parent)
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(match message
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('ready ;success
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pid)
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(((? symbol? key) args ...) ;exception
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(apply throw key args))
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(_ ;unexpected termination
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#f)))))))))
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(define* (call-with-container mounts thunk #:key (namespaces %namespaces)
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(host-uids 1))
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"Run THUNK in a new container process and return its exit status.
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MOUNTS is a list of <file-system> objects that specify file systems to mount
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inside the container. NAMESPACES is a list of symbols corresponding to
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the identifiers for Linux namespaces: mnt, ipc, uts, pid, user, and net. By
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default, all namespaces are used. HOST-UIDS is the number of host user
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identifiers to map into the container's user namespace, if there is one. By
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default, only a single uid/gid, that of the current user, is mapped into the
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container. The host user that creates the container is the root user (uid/gid
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0) within the container. Only root can map more than a single uid/gid.
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Note that if THUNK needs to load any additional Guile modules, the relevant
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module files must be present in one of the mappings in MOUNTS and the Guile
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load path must be adjusted as needed."
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(call-with-temporary-directory
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(lambda (root)
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(let ((pid (run-container root mounts namespaces host-uids thunk)))
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;; Catch SIGINT and kill the container process.
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(sigaction SIGINT
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(lambda (signum)
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(false-if-exception
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(kill pid SIGKILL))))
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(match (waitpid pid)
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((_ . status) status))))))
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(define (container-excursion pid thunk)
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"Run THUNK as a child process within the namespaces of process PID and
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return the exit status."
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(define (namespace-file pid namespace)
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(string-append "/proc/" (number->string pid) "/ns/" namespace))
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(match (primitive-fork)
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(0
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(call-with-clean-exit
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(lambda ()
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(for-each (lambda (ns)
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(let ((source (namespace-file (getpid) ns))
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(target (namespace-file pid ns)))
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;; Joining the namespace that the process already
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;; belongs to would throw an error so avoid that.
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;; XXX: This /proc interface leads to TOCTTOU.
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(unless (string=? (readlink source) (readlink target))
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(call-with-input-file source
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(lambda (current-ns-port)
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(call-with-input-file target
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(lambda (new-ns-port)
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(setns (fileno new-ns-port) 0))))))))
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;; It's important that the user namespace is joined first,
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;; so that the user will have the privileges to join the
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;; other namespaces. Furthermore, it's important that the
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;; mount namespace is joined last, otherwise the /proc mount
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;; point would no longer be accessible.
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'("user" "ipc" "uts" "net" "pid" "mnt"))
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(purify-environment)
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(chdir "/")
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(thunk))))
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(pid
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(match (waitpid pid)
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((_ . status)
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(status:exit-val status))))))
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(define (container-excursion* pid thunk)
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"Like 'container-excursion', but return the return value of THUNK."
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(match (pipe)
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((in . out)
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(match (container-excursion pid
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(lambda ()
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(close-port in)
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(write (thunk) out)
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(close-port out)))
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(0
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(close-port out)
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(let ((result (read in)))
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(close-port in)
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result))
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(_ ;maybe PID died already
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(close-port out)
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(close-port in)
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#f)))))
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