ex-container: Make /usr/etc/{,g}shadow user-readable

For the `ex container` case, there's no security issues here; one shouldn't be
doing user management in these roots at all.

This is for work on exporting `ex container` roots to OCI as non-root. Without
this fix, libostree just tries to `openat()` the object for export to tar, and
fails.

See also https://github.com/projectatomic/rpm-ostree/issues/1045

Closes: #1046
Approved by: jlebon

src/app/rpmostree-container-builtins.c

@@ -32,6 +32,7 @@
 #include "rpmostree-util.h"
 #include "rpmostree-core.h"
 #include "rpmostree-libbuiltin.h"
+#include "rpmostree-postprocess.h"
 #include "rpmostree-rpm-util.h"
 #include "rpmostree-unpacker.h"
 
@@ -287,6 +288,9 @@ rpmostree_container_builtin_assemble (int             argc,
                                             NULL, RPMOSTREE_ASSEMBLE_TYPE_SERVER_BASE,
                                             &commit, cancellable, error))
       return EXIT_FAILURE;
+
+    if (!rpmostree_rootfs_postprocess_container (tmpdir.fd, cancellable, error))
+      return EXIT_FAILURE;
   }
 
   g_print ("Checking out %s @ %s...\n", name, commit);
@@ -476,6 +480,9 @@ rpmostree_container_builtin_upgrade (int argc, char **argv,
                                             &new_commit_checksum,
                                             cancellable, error))
       return EXIT_FAILURE;
+
+    if (!rpmostree_rootfs_postprocess_container (tmpdir.fd, cancellable, error))
+      return EXIT_FAILURE;
   }
 
   g_print ("Checking out %s @ %s...\n", name, new_commit_checksum);

src/libpriv/rpmostree-postprocess.c

@@ -1644,6 +1644,33 @@ rpmostree_prepare_rootfs_for_commit (int            src_rootfs_dfd,
   return TRUE;
 }
 
+/* Run through a standard set of postprocessing for "container"
+ * flows as used by `ex container`.  Currently:
+ *
+ *  - Make /usr/etc/{g,}shadow user readable
+ *    See https://github.com/projectatomic/rpm-ostree/issues/1045
+ */
+gboolean
+rpmostree_rootfs_postprocess_container (int           rootfs_fd,
+                                        GCancellable *cancellable,
+                                        GError       **error)
+{
+  const char *shadow_paths[] = { "usr/etc/shadow", "usr/etc/gshadow" };
+  for (guint i = 0; i < G_N_ELEMENTS (shadow_paths); i++)
+    {
+      struct stat stbuf;
+      const char *path = shadow_paths[i];
+      if (!glnx_fstatat_allow_noent (rootfs_fd, path, &stbuf, AT_SYMLINK_NOFOLLOW, error))
+        return FALSE;
+      /* Silently ignore if it's not there, or isn't a regular file for some reason */
+      if (errno == ENOENT || !S_ISREG (stbuf.st_mode))
+        continue;
+      if (fchmodat (rootfs_fd, path, stbuf.st_mode | S_IRUSR, 0) < 0)
+        return glnx_throw_errno_prefix (error, "fchmodat");
+    }
+  return TRUE;
+}
+
 struct CommitThreadData {
   volatile gint done;
   off_t n_bytes;

src/libpriv/rpmostree-postprocess.h

@@ -52,6 +52,11 @@ rpmostree_rootfs_postprocess_common (int           rootfs_fd,
                                      GCancellable *cancellable,
                                      GError       **error);
 
+gboolean
+rpmostree_rootfs_postprocess_container (int           rootfs_fd,
+                                        GCancellable *cancellable,
+                                        GError       **error);
+
 gboolean
 rpmostree_prepare_rootfs_get_sepolicy (int            dfd,
                                        OstreeSePolicy **out_sepolicy,

tests/ex-container-tests/test-bash.sh

@@ -3,6 +3,9 @@ set -xeuo pipefail
 
 cd ${test_tmpdir}
 
+dn=$(cd $(dirname $0) && pwd)
+. ${dn}/../common/libtest-core.sh
+
 cat >bash.conf <<EOF
 [tree]
 ref=bash
@@ -11,3 +14,5 @@ repos=fedora;
 EOF
 
 rpm-ostree ex container assemble bash.conf
+ostree --repo=repo ls bash /usr/etc/shadow > shadowls.txt
+assert_file_has_content shadowls.txt '^-00400 .*/usr/etc/shadow'