Update mmap mapping to be readonly without overwriting existing data

I'm making a custom ELF loader to learn how the dynamic loader works behind the scenes, and one of the program headers often found in them is PT_GNU_RELRO, which tells the loader to make that segment read-only after performing relocations. However, it doesn't look like there's a good way to update existing memory mappings' protections without replacing the entire thing. MAP_UNINITIALIZED seems to be what I'm looking for, but mmap(2) states that it doesn't work on most systems for security reasons. >**MAP_UNINITIALIZED (since Linux 2.6.33)** > >Don't clear anonymous pages. This flag is intended to improve performance on embedded devices. This flag is honored only if the kernel was configured with the CONFIG_MMAP_ALLOW_UNINITIALIZED option. Because of the security implications, that option is normally enabled only on embedded devices (i.e., devices where one has complete control of the contents of user memory). Which is reasonable for loosening permissions, but I'm looking to restrict them. Is there a way, as a user process, to update a mmap mapping to be read-only without replacing existing data at that address?