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memory backend file unnecessarily requires write permission while it is only mapped privately
Description of problem:
One day I wanted to boot the machine with physical memory initialized with a file, in a copy-on-write style. That is why I tried out `-mem-path` and `-object memory-backend-file`. Actually `-mem-path` already works if not considering that qemu dislikes the backing file being readonly and requires it to be writeable even when only private mappings are used here.

I sadly found out that when using memory-backend-file, and when `share=off`, if `readonly=on`, then file is `open`ed with `O_RDONLY` and mmap prot is `PROT_READ`; if `readonly=off`, then the file is `open`ed with `O_RDWR` and mmap prot is `PROT_READ|PROT_WRITE`. I want `O_RDONLY` and `PROT_READ|PROT_WRITE` but I cannot find it anywhere.

In my opinion, expected behavior should be that if `share=off`, the file can already be opened with `O_RDONLY` no matter what prot the mmap is. That is how linux `MAP_PRIVATE` works - basically copy on write. When I only need copy on write for the content of file, why do I require write permission for it?

Now I cannot find a setup that opens the file with `fd=open(*, O_RDONLY)` and mmap it with `mmap(*, *, PROT_READ|PROT_WRITE, MAP_PRIVATE|*, fd, *)`.

Tell me if I misunderstood linux (for example certain file behave differently if one open with O_RDONLY and this behavior is necessary) or qemu or other posix systems where copy-on-write does not work like this.