nixpkgs/pkgs/build-support/build-fhs-userenv/chrootenv/chrootenv.c
Nikolay Amiantov 06f27dc2e9 chrootenv: propagate nested chrootenv /host
To avoid symlink loops to /host in nested chrootenvs we need to remove
one level of indirection. This is also what's generally expected of
/host contents.
2019-05-30 15:34:01 +03:00

160 lines
4.3 KiB
C

#define _GNU_SOURCE
#include <glib.h>
#include <glib/gstdio.h>
#include <errno.h>
#include <sched.h>
#include <unistd.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/syscall.h>
#define fail(s, err) g_error("%s: %s: %s", __func__, s, g_strerror(err))
#define fail_if(expr) \
if (expr) \
fail(#expr, errno);
const gchar *bind_blacklist[] = {"bin", "etc", "host", "real-host", "usr", "lib", "lib64", "lib32", "sbin", NULL};
int pivot_root(const char *new_root, const char *put_old) {
return syscall(SYS_pivot_root, new_root, put_old);
}
void mount_tmpfs(const gchar *target) {
fail_if(mount("none", target, "tmpfs", 0, NULL));
}
void bind_mount(const gchar *source, const gchar *target) {
fail_if(g_mkdir(target, 0755));
fail_if(mount(source, target, NULL, MS_BIND | MS_REC, NULL));
}
const gchar *create_tmpdir() {
gchar *prefix =
g_build_filename(g_get_tmp_dir(), "chrootenvXXXXXX", NULL);
fail_if(!g_mkdtemp_full(prefix, 0755));
return prefix;
}
void pivot_host(const gchar *guest) {
g_autofree gchar *point = g_build_filename(guest, "host", NULL);
fail_if(g_mkdir(point, 0755));
fail_if(pivot_root(guest, point));
}
void bind_mount_item(const gchar *host, const gchar *guest, const gchar *name) {
g_autofree gchar *source = g_build_filename(host, name, NULL);
g_autofree gchar *target = g_build_filename(guest, name, NULL);
if (G_LIKELY(g_file_test(source, G_FILE_TEST_IS_DIR)))
bind_mount(source, target);
}
void bind(const gchar *host, const gchar *guest) {
mount_tmpfs(guest);
pivot_host(guest);
g_autofree gchar *host_dir = g_build_filename("/host", host, NULL);
g_autoptr(GError) err = NULL;
g_autoptr(GDir) dir = g_dir_open(host_dir, 0, &err);
if (err != NULL)
fail("g_dir_open", errno);
const gchar *item;
while ((item = g_dir_read_name(dir)))
if (!g_strv_contains(bind_blacklist, item))
bind_mount_item(host_dir, "/", item);
}
void spit(const char *path, char *fmt, ...) {
va_list args;
va_start(args, fmt);
FILE *f = g_fopen(path, "w");
if (f == NULL)
fail("g_fopen", errno);
g_vfprintf(f, fmt, args);
fclose(f);
}
int main(gint argc, gchar **argv) {
const gchar *self = *argv++;
if (argc < 2) {
g_message("%s command [arguments...]", self);
return 1;
}
g_autofree const gchar *prefix = create_tmpdir();
pid_t cpid = fork();
if (cpid < 0)
fail("fork", errno);
else if (cpid == 0) {
uid_t uid = getuid();
gid_t gid = getgid();
if (unshare(CLONE_NEWNS | CLONE_NEWUSER) < 0) {
int unshare_errno = errno;
g_message("Requires Linux version >= 3.19 built with CONFIG_USER_NS");
if (g_file_test("/proc/sys/kernel/unprivileged_userns_clone",
G_FILE_TEST_EXISTS))
g_message("Run: sudo sysctl -w kernel.unprivileged_userns_clone=1");
fail("unshare", unshare_errno);
}
spit("/proc/self/setgroups", "deny");
spit("/proc/self/uid_map", "%d %d 1", uid, uid);
spit("/proc/self/gid_map", "%d %d 1", gid, gid);
// If there is a /host directory, assume this is nested chrootenv and use it as host instead.
gboolean nested_host = g_file_test("/host", G_FILE_TEST_EXISTS | G_FILE_TEST_IS_DIR);
g_autofree const gchar *host = nested_host ? "/host" : "/";
bind(host, prefix);
// Replace /host by an actual (inner) /host.
if (nested_host) {
fail_if(g_mkdir("/real-host", 0755));
fail_if(mount("/host/host", "/real-host", NULL, MS_BIND | MS_REC, NULL));
// For some reason umount("/host") returns EBUSY even immediately after
// pivot_root. We detach it at least to keep `/proc/mounts` from blowing
// up in nested cases.
fail_if(umount2("/host", MNT_DETACH));
fail_if(mount("/real-host", "/host", NULL, MS_MOVE, NULL));
fail_if(rmdir("/real-host"));
}
fail_if(chdir("/"));
fail_if(execvp(*argv, argv));
}
else {
int status;
fail_if(waitpid(cpid, &status, 0) != cpid);
fail_if(rmdir(prefix));
if (WIFEXITED(status))
return WEXITSTATUS(status);
else if (WIFSIGNALED(status))
kill(getpid(), WTERMSIG(status));
return 1;
}
}