1 files changed, 139 insertions, 0 deletions
diff --git a/pkgs/build-support/build-fhs-userenv/chrootenv/chrootenv.c b/pkgs/build-support/build-fhs-userenv/chrootenv/chrootenv.c
new file mode 100644
index 000000000000..c03a1710f451
--- /dev/null
+++ b/pkgs/build-support/build-fhs-userenv/chrootenv/chrootenv.c
@@ -0,0 +1,139 @@
+#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>
+
+const gchar *bind_blacklist[] = {"bin", "etc", "host", "usr", 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 bind_mount_host(const gchar *host, const gchar *guest) {
+  g_autofree gchar *point = g_build_filename(guest, "host", NULL);
+  bind_mount(host, 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) {
+  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);
+
+  bind_mount_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;
+  }
+
+  if (g_getenv("NIX_CHROOTENV"))
+    g_warning("chrootenv doesn't stack!");
+  else
+    g_setenv("NIX_CHROOTENV", "", TRUE);
+
+  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(chroot(prefix));
+    fail_if(execvp(*argv, argv));
+  }
+
+  else {
+    int status;
+
+    fail_if(waitpid(cpid, &status, 0) != cpid);
+    fail_if(nftw(prefix, nftw_remove, getdtablesize(),
+                 FTW_DEPTH | FTW_MOUNT | FTW_PHYS));
+
+    if (WIFEXITED(status))
+      return WEXITSTATUS(status);
+
+    else if (WIFSIGNALED(status))
+      kill(getpid(), WTERMSIG(status));
+
+    return 1;
+  }
+}