In cgmemmgr, allow writing permissions on the read page for the debugger

The permissions specified in MapViewOfFile specify the maximum permissions
the file can ever have, which means that even permissions bypass by the
debugger is not allowed to write it. Instead, set the maximum permissions
to RWX and VirtualProtect down to the permissions we need. This is the same
behavior as we use on Mac/Linux.

src/cgmemmgr.cpp

@@ -141,13 +141,18 @@ static intptr_t init_shared_map()
 static void *alloc_shared_page(size_t size, size_t *id, bool exec)
 {
     assert(size % jl_page_size == 0);
-    auto file_mode = exec ? PAGE_EXECUTE_READWRITE : PAGE_READWRITE;
+    DWORD file_mode = exec ? PAGE_EXECUTE_READWRITE : PAGE_READWRITE;
     HANDLE hdl = CreateFileMapping(INVALID_HANDLE_VALUE, NULL,
                                    file_mode, 0, size, NULL);
     *id = (size_t)hdl;
-    auto map_mode = FILE_MAP_READ | (exec ? FILE_MAP_EXECUTE : 0);
+    // We set the maximum permissions for this to the maximum for this file, and then
+    // VirtualProtect, such that the debugger can still access these
+    // pages and set breakpoints if it wants to.
+    DWORD map_mode = FILE_MAP_ALL_ACCESS | (exec ? FILE_MAP_EXECUTE : 0);
     void *addr = MapViewOfFile(hdl, map_mode, 0, 0, size);
     assert(addr && "Cannot map RO view");
+    DWORD protect_mode = exec ? PAGE_EXECUTE_READ : PAGE_READONLY;
+    VirtualProtect(addr, size, protect_mode, &file_mode);
     return addr;
 }
 #else // _OS_WINDOWS_

test/misc.jl

@@ -435,3 +435,18 @@ let creds = Base.LibGit2.CachedCredentials()
     securezero!(creds)
     @test LibGit2.get_creds!(creds, "foo", nothing).pass == "\0\0\0"
 end
+
+# Test that we can VirtualProtect jitted code to writable
+if is_windows()
+    @noinline function WeVirtualProtectThisToRWX(x, y)
+        x+y
+    end
+
+    let addr = cfunction(WeVirtualProtectThisToRWX, UInt64, (UInt64, UInt64))
+        addr = addr-(UInt64(addr)%4096)
+        const PAGE_EXECUTE_READWRITE = 0x40
+        oldPerm = Ref{UInt32}()
+        err = ccall(:VirtualProtect,stdcall,Cint,(Ptr{Void}, Csize_t, UInt32, Ptr{UInt32}), addr, 4096, PAGE_EXECUTE_READWRITE, oldPerm)
+        err == 0 && error(Libc.GetLastError())
+    end
+end