nsresourced: re-link GID delegation file after atomic UID file write

userns_registry_remove() restores a sub-delegated UID range by writing
the previous owner's data to u<UID>.delegate with WRITE_STRING_FILE_ATOMIC.
Atomic writes go via a temp file and rename, which replaces the directory
entry with a fresh inode and severs the hardlink to g<GID>.delegate. The
stale GID side then keeps pointing at the prior inode with outdated owner
and ancestor data, so subsequent lookups via GID return wrong results.

Re-create the hardlink after the atomic write so the two views stay in
sync, matching what userns_registry_store() already does after writing
a new delegation.

diff --git a/src/nsresourced/userns-registry.c b/src/nsresourced/userns-registry.c
index 50efcd1153a5d6bfaded990d9015728210e969a2..a9e3f82e59c22643bfd8c696d6a9c27ecd1b4629 100644 (file)
--- a/src/nsresourced/userns-registry.c
+++ b/src/nsresourced/userns-registry.c
@@ -838,9 +838,11 @@ int userns_registry_remove(int dir_fd, UserNamespaceInfo *info) {
                         continue;
                 }
 
-                _cleanup_free_ char *delegate_uid_fn = NULL;
+                _cleanup_free_ char *delegate_uid_fn = NULL, *delegate_gid_fn = NULL;
                 if (asprintf(&delegate_uid_fn, "u" UID_FMT ".delegate", delegate->start_uid) < 0)
                         return log_oom_debug();
+                if (asprintf(&delegate_gid_fn, "g" GID_FMT ".delegate", delegate->start_gid) < 0)
+                        return log_oom_debug();
 
                 if (existing.n_ancestor_userns > 0) {
                         _cleanup_(sd_json_variant_unrefp) sd_json_variant *delegate_def = NULL, *ancestor_array = NULL;
@@ -876,10 +878,18 @@ int userns_registry_remove(int dir_fd, UserNamespaceInfo *info) {
                                 return log_debug_errno(r, "Failed to format delegation JSON object: %m");
 
                         r = write_string_file_at(dir_fd, delegate_uid_fn, delegate_buf, WRITE_STRING_FILE_CREATE|WRITE_STRING_FILE_ATOMIC);
-                        if (r < 0)
+                        if (r < 0) {
                                 RET_GATHER(ret, log_debug_errno(r, "Failed to write restored delegation data to '%s' in registry: %m", delegate_uid_fn));
+                                continue;
+                        }
+
+                        /* The atomic write above replaced the UID file with a new inode, so the
+                         * hardlink to the GID file is now broken. Re-create it to keep the two in
+                         * sync. */
+                        r = linkat_replace(dir_fd, delegate_uid_fn, dir_fd, delegate_gid_fn);
+                        if (r < 0)
+                                RET_GATHER(ret, log_debug_errno(r, "Failed to re-link '%s' to '%s' in registry: %m", delegate_uid_fn, delegate_gid_fn));
 
-                        /* GID link already points to the UID file, no need to update it */
                         continue;
                 }
 
@@ -891,10 +901,6 @@ int userns_registry_remove(int dir_fd, UserNamespaceInfo *info) {
                 if (r < 0)
                         RET_GATHER(ret, log_debug_errno(r, "Failed to remove %s: %m", delegate_uid_fn));
 
-                _cleanup_free_ char *delegate_gid_fn = NULL;
-                if (asprintf(&delegate_gid_fn, "g" GID_FMT ".delegate", delegate->start_gid) < 0)
-                        return log_oom_debug();
-
                 r = RET_NERRNO(unlinkat(dir_fd, delegate_gid_fn, 0));
                 if (r < 0)
                         RET_GATHER(ret, log_debug_errno(r, "Failed to remove %s: %m", delegate_gid_fn));