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#define _GNU_SOURCE
#include <glib.h>
#include <glib/gstdio.h>
#include <errno.h>
#include <sched.h>
#include <unistd.h>
#define fail(s, err) g_error("%s: %s: %s", __func__, s, g_strerror(err))
#define fail_if(expr) \
if (expr) \
fail(#expr, errno);
#include <ftw.h>
#include <sys/mount.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/syscall.h>
int min(int a, int b) {
return a > b ? b : a;
}
const gchar *bind_blacklist[] = {"bin", "etc", "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, "bind", MS_BIND | MS_REC, NULL));
}
void pivot_host(const gchar *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);
g_autoptr(GError) err = NULL;
g_autoptr(GDir) dir = g_dir_open(host, 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, guest, item);
pivot_host(host, guest);
}
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 nftw_remove(const char *path, const struct stat *sb, int type,
struct FTW *ftw) {
return remove(path);
}
int main(gint argc, gchar **argv) {
const gchar *self = *argv++;
if (argc < 2) {
g_message("%s command [arguments...]", self);
return 1;
}
g_autofree gchar *prefix =
g_build_filename(g_get_tmp_dir(), "chrootenvXXXXXX", NULL);
fail_if(!g_mkdtemp_full(prefix, 0755));
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);
bind("/", prefix);
fail_if(chdir("/"));
fail_if(execvp(*argv, argv));
}
else {
int status;
fail_if(waitpid(cpid, &status, 0) != cpid);
// glibc 2.27 (and possibly other versions) can't handle
// an nopenfd value larger than 2^19
fail_if(nftw(prefix, nftw_remove, min(getdtablesize(), 1<<19),
FTW_DEPTH | FTW_MOUNT | FTW_PHYS));
if (WIFEXITED(status))
return WEXITSTATUS(status);
else if (WIFSIGNALED(status))
kill(getpid(), WTERMSIG(status));
return 1;
}
}
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