Does recovery write to backup_label ?

Just saw this in a PG 11.6 cluster starting a recovery:

2020-02-07 10:45:40 EST FATAL:  42501: could not open file
"backup_label": Permission denied
2020-02-07 10:45:40 EST LOCATION:  fsync_fname_ext, fd.c:3531

The label file was written with mode 0400 by a script that got
the contents from pg_stop_backup(boolean,boolean).

But during recovery, it is being poked at by fsync_fname_ext
which wants to open it O_RDWR.

I had assumed the label file would be treated as readonly
during recovery.

If the file needs to have 0600 permissions, should there be
a note in the nonexclusive-mode backup docs to say so?

Regards,
-Chap