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guix/gnu/packages/aux-files/run-in-namespace.c
Ludovic Courtès 30da3173d5
pack: Relocatable wrapper now properly maps the current UID/GID.
* gnu/packages/aux-files/run-in-namespace.c (write_id_map)
(disallow_setgroups): New functions.
(main): Use 'clone' via 'syscall' instead of 'fork' followed by
'unshare'.  Add calls to 'disallow_setgroups' and 'write_id_map' in the
parent process.
2018-05-11 18:42:42 +02:00

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/* GNU Guix --- Functional package management for GNU
Copyright (C) 2018 Ludovic Courtès <ludo@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/>. */
/* Make the given @WRAPPED_PROGRAM@ relocatable by executing it in a separate
mount namespace where the store is mounted in its right place.
We would happily do that in Scheme using 'call-with-container'. However,
this very program needs to be relocatable, so it needs to be statically
linked, which complicates things (Guile's modules can hardly be "linked"
into a single executable.) */
#define _GNU_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sched.h>
#include <sys/mount.h>
#include <errno.h>
#include <libgen.h>
#include <limits.h>
#include <string.h>
#include <assert.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <dirent.h>
#include <sys/syscall.h>
/* Concatenate DIRECTORY, a slash, and FILE. Return the result, which the
caller must eventually free. */
static char *
concat (const char *directory, const char *file)
{
char *result = malloc (strlen (directory) + 2 + strlen (file));
assert (result != NULL);
strcpy (result, directory);
strcat (result, "/");
strcat (result, file);
return result;
}
static void
mkdir_p (const char *directory)
{
if (strcmp (directory, "/") != 0)
{
char *parent = dirname (strdupa (directory));
mkdir_p (parent);
int err = mkdir (directory, 0700);
if (err < 0 && errno != EEXIST)
assert_perror (errno);
}
}
static void
rm_rf (const char *directory)
{
DIR *stream = opendir (directory);
for (struct dirent *entry = readdir (stream);
entry != NULL;
entry = readdir (stream))
{
if (strcmp (entry->d_name, ".") == 0
|| strcmp (entry->d_name, "..") == 0)
continue;
char *full = concat (directory, entry->d_name);
int err = unlink (full);
if (err < 0)
{
if (errno == EISDIR)
/* Recurse (we expect a shallow directory structure so there's
little risk of stack overflow.) */
rm_rf (full);
else
assert_perror (errno);
}
free (full);
}
closedir (stream);
int err = rmdir (directory);
if (err < 0 && errno != ENOENT)
assert_perror (errno);
}
/* Bind mount all the top-level entries in SOURCE to TARGET. */
static void
bind_mount (const char *source, const char *target)
{
DIR *stream = opendir (source);
for (struct dirent *entry = readdir (stream);
entry != NULL;
entry = readdir (stream))
{
/* XXX: Some file systems may not report a useful 'd_type'. Ignore them
for now. */
assert (entry->d_type != DT_UNKNOWN);
if (strcmp (entry->d_name, ".") == 0
|| strcmp (entry->d_name, "..") == 0)
continue;
char *abs_source = concat (source, entry->d_name);
char *new_entry = concat (target, entry->d_name);
if (entry->d_type == DT_LNK)
{
char target[PATH_MAX];
ssize_t result = readlink (abs_source, target, sizeof target - 1);
if (result > 0)
{
target[result] = '\0';
int err = symlink (target, new_entry);
if (err < 0)
assert_perror (errno);
}
}
else
{
/* Create the mount point. */
if (entry->d_type == DT_DIR)
{
int err = mkdir (new_entry, 0700);
if (err != 0)
assert_perror (errno);
}
else
close (open (new_entry, O_WRONLY | O_CREAT));
int err = mount (abs_source, new_entry, "none",
MS_BIND | MS_REC | MS_RDONLY, NULL);
/* It used to be that only directories could be bind-mounted. Thus,
keep going if we fail to bind-mount a non-directory entry.
That's OK because regular files in the root file system are
usually uninteresting. */
if (err != 0 && entry->d_type != DT_DIR)
assert_perror (errno);
free (new_entry);
free (abs_source);
}
}
closedir (stream);
}
/* Write the user/group ID map for PID to FILE, mapping ID to itself. See
user_namespaces(7). */
static void
write_id_map (pid_t pid, const char *file, int id)
{
char id_map_file[100];
snprintf (id_map_file, sizeof id_map_file, "/proc/%d/%s", pid, file);
char id_map[100];
/* Map root and the current user. */
int len = snprintf (id_map, sizeof id_map, "%d %d 1\n", id, id);
int fd = open (id_map_file, O_WRONLY);
if (fd < 0)
assert_perror (errno);
int n = write (fd, id_map, len);
if (n < 0)
assert_perror (errno);
close (fd);
}
/* Disallow setgroups(2) for PID. */
static void
disallow_setgroups (pid_t pid)
{
char file[100];
snprintf (file, sizeof file, "/proc/%d/setgroups", pid);
int fd = open (file, O_WRONLY);
if (fd < 0)
assert_perror (errno);
int err = write (fd, "deny", 5);
if (err < 0)
assert_perror (errno);
close (fd);
}
int
main (int argc, char *argv[])
{
ssize_t size;
char self[PATH_MAX];
size = readlink ("/proc/self/exe", self, sizeof self - 1);
assert (size > 0);
/* SELF is something like "/home/ludo/.local/gnu/store/…-foo/bin/ls" and we
want to extract "/home/ludo/.local/gnu/store". */
size_t index = strlen (self)
- strlen ("@WRAPPED_PROGRAM@")
+ strlen ("@STORE_DIRECTORY@");
char *store = strdup (self);
store[index] = '\0';
struct stat statbuf;
/* If STORE is already at the "right" place, we can execute
@WRAPPED_PROGRAM@ right away. This is not just an optimization: it's
needed when running one of these wrappers from within an unshare'd
namespace, because 'unshare' fails with EPERM in that context. */
if (strcmp (store, "@STORE_DIRECTORY@") != 0
&& lstat ("@WRAPPED_PROGRAM@", &statbuf) != 0)
{
/* Spawn @WRAPPED_PROGRAM@ in a separate namespace where STORE is
bind-mounted in the right place. */
int err;
char *new_root = mkdtemp (strdup ("/tmp/guix-exec-XXXXXX"));
char *new_store = concat (new_root, "@STORE_DIRECTORY@");
char *cwd = get_current_dir_name ();
/* Create a child with separate namespaces and set up bind-mounts from
there. That way, bind-mounts automatically disappear when the child
exits, which simplifies cleanup for the parent. Note: clone is more
convenient than fork + unshare since the parent can directly write
the child uid_map/gid_map files. */
pid_t child = syscall (SYS_clone, SIGCHLD | CLONE_NEWNS | CLONE_NEWUSER,
NULL, NULL, NULL);
switch (child)
{
case 0:
/* Note: Due to <https://bugzilla.kernel.org/show_bug.cgi?id=183461>
we cannot make NEW_ROOT a tmpfs (which would have saved the need
for 'rm_rf'.) */
bind_mount ("/", new_root);
mkdir_p (new_store);
err = mount (store, new_store, "none", MS_BIND | MS_REC | MS_RDONLY,
NULL);
if (err < 0)
assert_perror (errno);
chdir (new_root);
err = chroot (new_root);
if (err < 0)
assert_perror (errno);
/* Change back to where we were before chroot'ing. */
chdir (cwd);
break;
case -1:
fprintf (stderr, "%s: error: 'clone' failed: %m\n", argv[0]);
fprintf (stderr, "\
This may be because \"user namespaces\" are not supported on this system.\n\
Consequently, we cannot run '@WRAPPED_PROGRAM@',\n\
unless you move it to the '@STORE_DIRECTORY@' directory.\n\
\n\
Please refer to the 'guix pack' documentation for more information.\n");
return EXIT_FAILURE;
default:
{
/* Map the current user/group ID in the child's namespace (the
default is to get the "overflow UID", i.e., the UID of
"nobody"). We must first disallow 'setgroups' for that
process. */
disallow_setgroups (child);
write_id_map (child, "uid_map", getuid ());
write_id_map (child, "gid_map", getgid ());
int status;
waitpid (child, &status, 0);
chdir ("/"); /* avoid EBUSY */
rm_rf (new_root);
free (new_root);
exit (status);
}
}
}
/* The executable is available under @STORE_DIRECTORY@, so we can now
execute it. */
int err = execv ("@WRAPPED_PROGRAM@", argv);
if (err < 0)
assert_perror (errno);
return EXIT_FAILURE;
}