1 /* 2 * Copyright (C) 2015 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 #ifndef _PRIVATE_WRITEPROTECTED_H 18 #define _PRIVATE_WRITEPROTECTED_H 19 20 #include <errno.h> 21 #include <string.h> 22 #include <sys/cdefs.h> 23 #include <sys/mman.h> 24 #include <sys/user.h> 25 26 #include "private/bionic_macros.h" 27 #include "private/bionic_prctl.h" 28 #include "private/libc_logging.h" 29 30 template <typename T> 31 union WriteProtectedContents { 32 T value; 33 char padding[PAGE_SIZE]; 34 35 WriteProtectedContents() = default; 36 DISALLOW_COPY_AND_ASSIGN(WriteProtectedContents); 37 } __attribute__((aligned(PAGE_SIZE))); 38 39 // Write protected wrapper class that aligns its contents to a page boundary, 40 // and sets the memory protection to be non-writable, except when being modified 41 // explicitly. 42 template <typename T> 43 class WriteProtected { 44 static_assert(sizeof(T) < PAGE_SIZE, 45 "WriteProtected only supports contents up to PAGE_SIZE"); 46 static_assert(__is_pod(T), "WriteProtected only supports POD contents"); 47 48 WriteProtectedContents<T> contents; 49 50 public: 51 WriteProtected() = default; 52 DISALLOW_COPY_AND_ASSIGN(WriteProtected); 53 54 void initialize() { 55 // Not strictly necessary, but this will hopefully segfault if we initialize 56 // multiple times by accident. 57 memset(&contents, 0, sizeof(contents)); 58 59 if (mprotect(&contents, PAGE_SIZE, PROT_READ)) { 60 __libc_fatal("failed to make WriteProtected nonwritable in initialize"); 61 } 62 } 63 64 const T* operator->() { 65 return &contents.value; 66 } 67 68 const T& operator*() { 69 return contents.value; 70 } 71 72 template <typename Mutator> 73 void mutate(Mutator mutator) { 74 if (mprotect(&contents, PAGE_SIZE, PROT_READ | PROT_WRITE) != 0) { 75 __libc_fatal("failed to make WriteProtected writable in mutate: %s", 76 strerror(errno)); 77 } 78 mutator(&contents.value); 79 if (mprotect(&contents, PAGE_SIZE, PROT_READ) != 0) { 80 __libc_fatal("failed to make WriteProtected nonwritable in mutate: %s", 81 strerror(errno)); 82 } 83 } 84 }; 85 86 #endif 87