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* Copyright (C) 2008 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
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* See the License for the specific language governing permissions and
* limitations under the License.
#include <stddef.h>
#include <sys/types.h>
#include <map>
#include <mutex>
#include <string>
#include "android-base/thread_annotations.h"
namespace art {
#if defined(__LP64__) && (defined(__aarch64__) || defined(__mips__) || defined(__APPLE__))
#if defined(__LP64__) && !defined(__x86_64__)
#error "Unrecognized 64-bit architecture."
#ifdef __linux__
static constexpr bool kMadviseZeroes = true;
static constexpr bool kMadviseZeroes = false;
// We cannot ever perform MemMap::ReplaceWith on non-linux hosts since the syscall is not
// present.
// Used to keep track of mmap segments.
// On 64b systems not supporting MAP_32BIT, the implementation of MemMap will do a linear scan
// for free pages. For security, the start of this scan should be randomized. This requires a
// dynamic initializer.
// For this to work, it is paramount that there are no other static initializers that access MemMap.
// Otherwise, calls might see uninitialized values.
class MemMap {
static constexpr bool kCanReplaceMapping = HAVE_MREMAP_SYSCALL;
// Replace the data in this memmmap with the data in the memmap pointed to by source. The caller
// relinquishes ownership of the source mmap.
// For the call to be successful:
// * The range [dest->Begin, dest->Begin() + source->Size()] must not overlap with
// [source->Begin(), source->End()].
// * Neither source nor dest may be 'reused' mappings (they must own all the pages associated
// with them.
// * kCanReplaceMapping must be true.
// * Neither source nor dest may use manual redzones.
// * Both source and dest must have the same offset from the nearest page boundary.
// * mremap must succeed when called on the mappings.
// If this call succeeds it will return true and:
// * Deallocate *source
// * Sets *source to nullptr
// * The protection of this will remain the same.
// * The size of this will be the size of the source
// * The data in this will be the data from source.
// If this call fails it will return false and make no changes to *source or this. The ownership
// of the source mmap is returned to the caller.
bool ReplaceWith(/*in-out*/MemMap** source, /*out*/std::string* error);
// Request an anonymous region of length 'byte_count' and a requested base address.
// Use null as the requested base address if you don't care.
// "reuse" allows re-mapping an address range from an existing mapping.
// The word "anonymous" in this context means "not backed by a file". The supplied
// 'name' will be used -- on systems that support it -- to give the mapping
// a name.
// On success, returns returns a MemMap instance. On failure, returns null.
static MemMap* MapAnonymous(const char* name,
uint8_t* addr,
size_t byte_count,
int prot,
bool low_4gb,
bool reuse,
std::string* error_msg,
bool use_ashmem = true);
// Create placeholder for a region allocated by direct call to mmap.
// This is useful when we do not have control over the code calling mmap,
// but when we still want to keep track of it in the list.
// The region is not considered to be owned and will not be unmmaped.
static MemMap* MapDummy(const char* name, uint8_t* addr, size_t byte_count);
// Map part of a file, taking care of non-page aligned offsets. The
// "start" offset is absolute, not relative.
// On success, returns returns a MemMap instance. On failure, returns null.
static MemMap* MapFile(size_t byte_count,
int prot,
int flags,
int fd,
off_t start,
bool low_4gb,
const char* filename,
std::string* error_msg) {
return MapFileAtAddress(nullptr,
// Map part of a file, taking care of non-page aligned offsets. The "start" offset is absolute,
// not relative. This version allows requesting a specific address for the base of the mapping.
// "reuse" allows us to create a view into an existing mapping where we do not take ownership of
// the memory. If error_msg is null then we do not print /proc/maps to the log if
// MapFileAtAddress fails. This helps improve performance of the fail case since reading and
// printing /proc/maps takes several milliseconds in the worst case.
// On success, returns returns a MemMap instance. On failure, returns null.
static MemMap* MapFileAtAddress(uint8_t* addr,
size_t byte_count,
int prot,
int flags,
int fd,
off_t start,
bool low_4gb,
bool reuse,
const char* filename,
std::string* error_msg);
// Releases the memory mapping.
~MemMap() REQUIRES(!MemMap::mem_maps_lock_);
const std::string& GetName() const {
return name_;
bool Sync();
bool Protect(int prot);
void MadviseDontNeedAndZero();
int GetProtect() const {
return prot_;
uint8_t* Begin() const {
return begin_;
size_t Size() const {
return size_;
// Resize the mem-map by unmapping pages at the end. Currently only supports shrinking.
void SetSize(size_t new_size);
uint8_t* End() const {
return Begin() + Size();
void* BaseBegin() const {
return base_begin_;
size_t BaseSize() const {
return base_size_;
void* BaseEnd() const {
return reinterpret_cast<uint8_t*>(BaseBegin()) + BaseSize();
bool HasAddress(const void* addr) const {
return Begin() <= addr && addr < End();
// Unmap the pages at end and remap them to create another memory map.
MemMap* RemapAtEnd(uint8_t* new_end,
const char* tail_name,
int tail_prot,
std::string* error_msg,
bool use_ashmem = true);
static bool CheckNoGaps(MemMap* begin_map, MemMap* end_map)
static void DumpMaps(std::ostream& os, bool terse = false)
// Init and Shutdown are NOT thread safe.
// Both may be called multiple times and MemMap objects may be created any
// time after the first call to Init and before the first call to Shutodwn.
static void Init() REQUIRES(!MemMap::mem_maps_lock_);
static void Shutdown() REQUIRES(!MemMap::mem_maps_lock_);
// If the map is PROT_READ, try to read each page of the map to check it is in fact readable (not
// faulting). This is used to diagnose a bug b/19894268 where mprotect doesn't seem to be working
// intermittently.
void TryReadable();
// Align the map by unmapping the unaligned parts at the lower and the higher ends.
void AlignBy(size_t size);
// For annotation reasons.
static std::mutex* GetMemMapsLock() RETURN_CAPABILITY(mem_maps_lock_) {
return nullptr;
MemMap(const std::string& name,
uint8_t* begin,
size_t size,
void* base_begin,
size_t base_size,
int prot,
bool reuse,
size_t redzone_size = 0) REQUIRES(!MemMap::mem_maps_lock_);
static void DumpMapsLocked(std::ostream& os, bool terse)
static bool HasMemMap(MemMap* map)
static MemMap* GetLargestMemMapAt(void* address)
static bool ContainedWithinExistingMap(uint8_t* ptr, size_t size, std::string* error_msg)
// Internal version of mmap that supports low 4gb emulation.
static void* MapInternal(void* addr,
size_t length,
int prot,
int flags,
int fd,
off_t offset,
bool low_4gb)
static void* MapInternalArtLow4GBAllocator(size_t length,
int prot,
int flags,
int fd,
off_t offset)
const std::string name_;
uint8_t* begin_; // Start of data. May be changed by AlignBy.
size_t size_; // Length of data.
void* base_begin_; // Page-aligned base address. May be changed by AlignBy.
size_t base_size_; // Length of mapping. May be changed by RemapAtEnd (ie Zygote).
int prot_; // Protection of the map.
// When reuse_ is true, this is just a view of an existing mapping
// and we do not take ownership and are not responsible for
// unmapping.
const bool reuse_;
// When already_unmapped_ is true the destructor will not call munmap.
bool already_unmapped_;
const size_t redzone_size_;
static uintptr_t next_mem_pos_; // Next memory location to check for low_4g extent.
static std::mutex* mem_maps_lock_;
friend class MemMapTest; // To allow access to base_begin_ and base_size_.
std::ostream& operator<<(std::ostream& os, const MemMap& mem_map);
// Zero and release pages if possible, no requirements on alignments.
void ZeroAndReleasePages(void* address, size_t length);
} // namespace art