Mmap native images at default base address if env variable COMPlus_UseDefaultBaseAddr=0x1 is setup

Author: gbalykov

src/pal/src/include/pal/init.h

@@ -41,6 +41,8 @@ extern Volatile<INT> init_count;
 
 extern SIZE_T g_defaultStackSize;
 
+extern BOOL g_useDefaultBaseAddr;
+
 /*++
 MACRO:
   PALIsInitialized

src/pal/src/init/pal.cpp

@@ -104,6 +104,9 @@ static pthread_mutex_t init_critsec_mutex = PTHREAD_MUTEX_INITIALIZER;
 // The default minimum stack size
 SIZE_T g_defaultStackSize = 0;
 
+// The default value of parameter, whether to mmap images at default base address or not
+BOOL g_useDefaultBaseAddr = FALSE;
+
 /* critical section to protect access to init_count. This is allocated on the
    very first PAL_Initialize call, and is freed afterward. */
 static PCRITICAL_SECTION init_critsec = NULL;
@@ -382,6 +385,20 @@ Initialize(
         }
 #endif // ENSURE_PRIMARY_STACK_SIZE
 
+        char* useDefaultBaseAddr = getenv("COMPlus_UseDefaultBaseAddr");
+        if (useDefaultBaseAddr != NULL)
+        {
+            errno = 0;
+            // Like all numeric values specific by the COMPlus_xxx variables, it is a
+            // hexadecimal string without any prefix.
+            long int flag = strtol(useDefaultBaseAddr, NULL, 16);
+
+            if (errno == 0)
+            {
+                g_useDefaultBaseAddr = (BOOL) flag;
+            }
+        }
+
         // Initialize the TLS lookaside cache
         if (FALSE == TLSInitialize())
         {

src/pal/src/map/map.cpp

@@ -2350,9 +2350,14 @@ void * MAPMapPEFile(HANDLE hFile)
 
     if (loadedBase == NULL)
     {
+        void *usedBaseAddr = NULL;
+        if (g_useDefaultBaseAddr)
+        {
+            usedBaseAddr = (void*) preferredBase;
+        }
         // MAC64 requires we pass MAP_SHARED (or MAP_PRIVATE) flags - otherwise, the call is failed.
         // Refer to mmap documentation at http://www.manpagez.com/man/2/mmap/ for details.
-        loadedBase = mmap(NULL, virtualSize, PROT_NONE, MAP_ANON|MAP_PRIVATE, -1, 0);
+        loadedBase = mmap(usedBaseAddr, virtualSize, PROT_NONE, MAP_ANON|MAP_PRIVATE, -1, 0);
     }
 
     if (MAP_FAILED == loadedBase)