Implement base Unix page allocator

BepInEx.IL2CPP/Hook/Allocator/UnixPageAllocator.cs

@@ -1,6 +1,8 @@
 using System;
 using System.Collections.Generic;
+using System.ComponentModel;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using MonoMod.Utils;
 
 namespace BepInEx.IL2CPP.Allocator
@@ -59,12 +61,79 @@ namespace BepInEx.IL2CPP.Allocator
 
 		protected override IntPtr AllocateChunk(IntPtr hint)
 		{
-			throw new NotImplementedException();
+			/* From https://github.com/kubo/funchook/blob/master/src/funchook_unix.c#L251-L254:
+			 * Loop three times just to avoid rare cases such as
+			 * unused memory region is used between 'get_free_address()'
+			 * and 'mmap()'.
+			*/
+			const int retryCount = 3;
+
+			for (var attempt = 0; attempt < retryCount; attempt++)
+			{
+				var freeAdrresses = GetFreeAddresses(hint);
+				// Try to use addr[1] (allocated after original method) first, then try before
+				for (int i = freeAdrresses.Length - 1; i >= 0; i--)
+				{
+					var addr = freeAdrresses[i];
+					if (addr == IntPtr.Zero)
+						continue;
+					var result = Unix.mmap(freeAdrresses[i], (UIntPtr)PAGE_SIZE, Unix.Protection.PROT_READ | Unix.Protection.PROT_WRITE, Unix.MapFlags.MAP_PRIVATE | Unix.MapFlags.MAP_ANONYMOUS, -1, 0);
+					if (result == freeAdrresses[i])
+						return result;
+					if (result == Unix.MAP_FAILED)
+						throw new Win32Exception(Marshal.GetLastWin32Error()); // Yes, this should work on unix too
+					Unix.munmap(result, (UIntPtr)PAGE_SIZE);
+				}
+			}
+
+			throw new PageAllocatorException("Failed to allocate memory in unused regions");
 		}
 
 		protected interface IMemoryMapper : IDisposable
 		{
 			bool FindNextFree(out IntPtr start, out IntPtr end);
 		}
+
+		private static class Unix
+		{
+			public static readonly IntPtr MAP_FAILED = new IntPtr(-1);
+
+			[DynDllImport("mmap")]
+			public static mmapDelegate mmap;
+
+			[DynDllImport("munmap")]
+			public static munmapDelegate munmap;
+
+			public delegate IntPtr mmapDelegate(IntPtr addr, UIntPtr length, Protection prot, MapFlags flags, int fd, int offset);
+
+			public delegate int munmapDelegate(IntPtr addr, UIntPtr length);
+
+			[Flags]
+			public enum MapFlags
+			{
+				MAP_PRIVATE = 0x02,
+				MAP_ANONYMOUS = 0x20
+			}
+
+			[Flags]
+			public enum Protection
+			{
+				PROT_READ = 0x1,
+				PROT_WRITE = 0x2
+			}
+
+			static Unix()
+			{
+				typeof(Unix).ResolveDynDllImports(new Dictionary<string, List<DynDllMapping>>
+				{
+					["libc"] = new List<DynDllMapping>
+					{
+						"libc.so.6",               // Ubuntu glibc
+						"libc",                    // Linux glibc,
+						"/usr/lib/libSystem.dylib" // OSX POSIX
+					}
+				});
+			}
+		}
 	}
 }