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/*
* Copyright (C) 2015 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#pragma once
#include <errno.h>
#include <string.h>
#include <sys/cdefs.h>
#include <sys/mman.h>
#include <sys/user.h>
#include <async_safe/log.h>
#include "platform/bionic/macros.h"
template <typename T>
union WriteProtectedContents {
T value;
char padding[PAGE_SIZE];
WriteProtectedContents() = default;
BIONIC_DISALLOW_COPY_AND_ASSIGN(WriteProtectedContents);
} __attribute__((aligned(PAGE_SIZE)));
// Write protected wrapper class that aligns its contents to a page boundary,
// and sets the memory protection to be non-writable, except when being modified
// explicitly.
template <typename T>
class WriteProtected {
public:
static_assert(sizeof(T) < PAGE_SIZE,
"WriteProtected only supports contents up to PAGE_SIZE");
static_assert(__is_pod(T), "WriteProtected only supports POD contents");
WriteProtected() = default;
BIONIC_DISALLOW_COPY_AND_ASSIGN(WriteProtected);
void initialize() {
// Not strictly necessary, but this will hopefully segfault if we initialize
// multiple times by accident.
memset(&contents, 0, sizeof(contents));
set_protection(PROT_READ);
}
const T* operator->() {
return &contents.value;
}
const T& operator*() {
return contents.value;
}
template <typename Mutator>
void mutate(Mutator mutator) {
set_protection(PROT_READ | PROT_WRITE);
mutator(&contents.value);
set_protection(PROT_READ);
}
private:
WriteProtectedContents<T> contents;
void set_protection(int prot) {
auto addr = &contents;
#if __has_feature(hwaddress_sanitizer)
// The mprotect system call does not currently untag pointers, so do it
// ourselves.
addr = untag_address(addr);
#endif
if (mprotect(reinterpret_cast<void*>(addr), PAGE_SIZE, prot) == -1) {
async_safe_fatal("WriteProtected mprotect %x failed: %s", prot, strerror(errno));
}
}
};
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