[Bug 294912] Linuxulator: munmap is missing EINVAL for unaligned addresses

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=294912

            Bug ID: 294912
           Summary: Linuxulator: munmap is missing EINVAL for unaligned
                    addresses
           Product: Base System
           Version: Unspecified
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: iwtcex@gmail.com

It might be acceptable for the native FreeBSD code to truncate
the passed address to the nearest page boundary
(I personally don't agree), but the actual Linux behavior is different
and some Unity engine games inadvertently rely on it not to crash.

https://store.steampowered.com/app/3605120/Trails is a good example.

The reason is too stupid to be believed, but Mono apparently has
a wrapper around mmap (mono_valloc_aligned) that is used to align
addresses to the page boundaries. (Yes, the addresses from mmap.
Yes, I'm aware they are already aligned. No, I have no idea what's
wrong with those people.) To do that mono_valloc_aligned always mmaps
one additional page, "aligns" the returned address and unmaps
the excess memory. That kind of works for Mono because it doesn't seem
to actually to invoke mono_valloc_aligned with sizes that are not
divisible by 4096. The Unity game in question somehow does and
by doing so unmaps the memory that it's actually using.

See
https://github.com/mono/mono/blob/0f53e9e151d92944cacab3e24ac359410c606df6/mono/utils/mono-mmap.c#L767

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