[Filetests] Use `make_exec` instead of region::protect for writes

[bnjbvr]

Please use let code_page = code_page.make_exec().map_err(|e| e.to_string())?; instead. Maybe switch cranelift-simplejit to that method too.

Originally posted by @bjorn3 in bytecodealliance/cranelift#940

[bnjbvr]

Fixed by bytecodealliance/cranelift#1017, thanks a lot @walac !

[bjorn3]

@walac said he would change simplejit in a follow-up, so it is not yet completely fixed @bnjbvr.

[walac]

A few questions about the implementation in simple-jit:

• memmap crate is used only for POSIX systems (except SELinux). Is there a technical reason not to use memmap in Windows and POSIX? If yes, I understand that we need to keep using code::protect in those systems.
• As MmapMut::make_exec and MmapMut::make_read_only methods return errors, we need either to change the public interface (and the all the calls) of Memory::set_readonly and Memory::set_readable_and_executable or panic in case of errors. Which one would be preferable?

[walac]

^ @bnjbvr

[bnjbvr]

• memmap seems to be only used for SELinux, right? I don't know if we want to use the memmap crate in general; maybe we wouldn't need to rely on region directly, then... Let's ask @sunfishcode; I'm not very clear on this code.
• as a general principle, yielding errors is more user-friendly, since the user can decide what to do with this error in return (panic, or try something different, etc.).

[teapotd]

Hello, I'd like to work on this issue.

[teapotd]

make_exec consumes MmapMut and returns immutable Mmap. Current implementation in simplejit always stores Option<MmapMut> in PtrLen and changes underlying memory to read-execute using region::protect:

wasmtime/cranelift/simplejit/src/memory.rs

Lines 19 to 26 in e2f6c08

	/// A simple struct consisting of a pointer and length.
	struct PtrLen {
	#[cfg(feature = "selinux-fix")]
	map: Option<MmapMut>,
	ptr: *mut u8,
	len: usize,
	}

wasmtime/cranelift/simplejit/src/memory.rs

Lines 177 to 187 in e2f6c08

	#[cfg(feature = "selinux-fix")]
	{
	for &PtrLen { ref map, ptr, len } in &self.allocations[self.executable..] {
	if len != 0 && map.is_some() {
	unsafe {
	region::protect(ptr, len, region::Protection::ReadExecute)
	.expect("unable to make memory readable+executable");
	}
	}
	}
	}

In order to make change to make_exec meaningful we need to store Mmap or MmapMut accordingly. Perhaps we should split PtrLen into PtrLen and PtrLenMut? Also mmap is used only if selinux-fix feature is enabled, so the remaining region::protect's have to stay. I'm not sure if we should make that change, I don't see much benefits.