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 #pragma once 18 19 #include <errno.h> 20 #include <string.h> 21 #include <sys/cdefs.h> 22 #include <sys/mman.h> 23 #include <sys/user.h> 24 25 #include <async_safe/log.h> 26 27 #include "private/bionic_macros.h" 28 29 template <typename T> 30 union WriteProtectedContents { 31 T value; 32 char padding[PAGE_SIZE]; 33 34 WriteProtectedContents() = default; 35 BIONIC_DISALLOW_COPY_AND_ASSIGN(WriteProtectedContents); 36 } __attribute__((aligned(PAGE_SIZE))); 37 38 // Write protected wrapper class that aligns its contents to a page boundary, 39 // and sets the memory protection to be non-writable, except when being modified 40 // explicitly. 41 template <typename T> 42 class WriteProtected { 43 static_assert(sizeof(T) < PAGE_SIZE, 44 "WriteProtected only supports contents up to PAGE_SIZE"); 45 static_assert(__is_pod(T), "WriteProtected only supports POD contents"); 46 47 WriteProtectedContents<T> contents; 48 49 public: 50 WriteProtected() = default; 51 BIONIC_DISALLOW_COPY_AND_ASSIGN(WriteProtected); 52 initialize()53 void initialize() { 54 // Not strictly necessary, but this will hopefully segfault if we initialize 55 // multiple times by accident. 56 memset(&contents, 0, sizeof(contents)); 57 58 if (mprotect(&contents, PAGE_SIZE, PROT_READ)) { 59 async_safe_fatal("failed to make WriteProtected nonwritable in initialize"); 60 } 61 } 62 63 const T* operator->() { 64 return &contents.value; 65 } 66 67 const T& operator*() { 68 return contents.value; 69 } 70 71 template <typename Mutator> mutate(Mutator mutator)72 void mutate(Mutator mutator) { 73 if (mprotect(&contents, PAGE_SIZE, PROT_READ | PROT_WRITE) != 0) { 74 async_safe_fatal("failed to make WriteProtected writable in mutate: %s", 75 strerror(errno)); 76 } 77 mutator(&contents.value); 78 if (mprotect(&contents, PAGE_SIZE, PROT_READ) != 0) { 79 async_safe_fatal("failed to make WriteProtected nonwritable in mutate: %s", 80 strerror(errno)); 81 } 82 } 83 }; 84