Linux shared memory only allow read access

I have a parent process that allocates shared memory and writes to it. It also starts child processes that only read the shared memory. However I don't have any control over the insights of those child processes. They are written by other programmers. These child processes are not supposed to write on the shared memory. So I wondered if I can allow them read permissions, but not write access.

However with shmget you can only specify general permission without being able to distinguish between read and write access.

I also thought about switching to shm_open & mmap as it seems that you can open the shared memory with O_RDONLY which forces the use of PROT_READ (read-only access) in the mmap call. Is it somehow possible to create two file descriptors with shm_open in the parent process: one with O_RDONLY and one with O_RDWR and then passing the O_RDONLY one to the child processes which can then map it into their process space? Of course, the children shouldn't have the permission to open the shared memory on their own by using shm_open because that would enable them to open it with O_RDWR.

Or did I get the concept wrong? Is it what I want even possible?