Force VirtualAlloc allocations to fall below the 2GB mark (#36)

Co-authored-by: ConorBobbleHat <c.github@firstpartners.net>

dll/kernel32.cpp

@@ -1171,6 +1171,10 @@ namespace kernel32 {
 			void *mem = 0;
 			posix_memalign(&mem, 0x1000, dwSize);
 			memset(mem, 0, dwSize);
+
+			// Windows only fences off the lower 2GB of the 32-bit address space for the private use of processes.
+			assert(mem < (void*)0x80000000);
+
 			DEBUG_LOG("-> %p\n", mem);
 			return mem;
 		} else {

main.cpp

@@ -7,6 +7,8 @@
 #include <sys/mman.h>
 #include <sys/syscall.h>
 #include <stdarg.h>
+#include <iostream>
+#include <fstream> 
 
 uint32_t wibo::lastError = 0;
 char *wibo::commandLine;
@@ -233,6 +235,42 @@ int main(int argc, char **argv) {
 	exec.loadPE(f);
 	fclose(f);
 
+	// 32-bit windows only reserves the lowest 2GB of memory for use by a process (https://www.tenouk.com/WinVirtualAddressSpace.html)
+	// Linux, on the other hand, will happily allow nearly the entire 4GB address space to be used.
+	// In order to prevent windows programs from being very confused as to why it's being handed
+	// addresses in "invalid" memory, let's map the upper 2GB of memory to ensure libc can't allocate
+	// anything there.
+	std::ifstream procMap("/proc/self/maps");
+	std::string procLine;
+	unsigned int lastMapEnd = 0;
+
+	const unsigned int FILL_MEMORY_ABOVE = 0x80000000; // 2GB
+
+	while (getline(procMap, procLine)) {
+		std::size_t idx = 0;
+  		unsigned int mapStart = std::stoul(procLine, &idx, 16);
+		unsigned int mapEnd = std::stoul(procLine.substr(idx + 1), nullptr, 16);
+
+		// The empty space we want to map out is now between lastMapEnd and mapStart
+		unsigned int holdingMapStart = lastMapEnd;
+		unsigned int holdingMapEnd = mapStart;
+
+		if ((holdingMapEnd - holdingMapStart) != 0 && holdingMapEnd > FILL_MEMORY_ABOVE) {
+			holdingMapStart = std::max(holdingMapStart, FILL_MEMORY_ABOVE);
+
+			void* holdingMap = mmap((void*) holdingMapStart, holdingMapEnd - holdingMapStart, PROT_READ, MAP_ANONYMOUS|MAP_FIXED|MAP_PRIVATE, -1, 0);
+			
+			if (holdingMap == MAP_FAILED) {
+				perror("Failed to create holding map");
+				return 1;
+			}
+		}
+		
+		lastMapEnd = mapEnd;
+	}
+
+	procMap.close();
+
 	uint16_t tibSegment = (tibDesc.entry_number << 3) | 7;
 	// Invoke the damn thing
 	asm(