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| 1 // Copyright (c) 2015 The Chromium Authors. All rights reserved. |
| 2 // Use of this source code is governed by a BSD-style license that can be |
| 3 // found in the LICENSE file. |
| 4 |
| 5 #ifndef BASE_MEMORY_SHARED_MEMORY_ALLOCATOR_H_ |
| 6 #define BASE_MEMORY_SHARED_MEMORY_ALLOCATOR_H_ |
| 7 |
| 8 #include <stdint.h> |
| 9 |
| 10 #include "base/atomicops.h" |
| 11 #include "base/base_export.h" |
| 12 #include "base/macros.h" |
| 13 |
| 14 namespace base { |
| 15 |
| 16 // Simple allocator for pieces of a memory block that may be shared across |
| 17 // multiple processes. |
| 18 // |
| 19 // This class provides for thread-secure (i.e. safe against other threads |
| 20 // or processes that may be compromised and thus have malicious intent) |
| 21 // allocation of memory within a designated block and also a mechanism by |
| 22 // which other threads can learn of the allocations with any additional |
| 23 // shared information. |
| 24 // |
| 25 // There is (currently) no way to release an allocated block of data because |
| 26 // doing so would risk invalidating pointers held by other processes and |
| 27 // greatly complicate the allocation algorithm. |
| 28 // |
| 29 // Construction of this object can accept new, clean (i.e. zeroed) memory |
| 30 // or previously initialized memory. In the first case, construction must |
| 31 // be allowed to complete before letting other allocators attach to the same |
| 32 // segment. In other words, don't share the segment until at least one |
| 33 // allocator has been attached to it. |
| 34 // |
| 35 // It should be noted that memory doesn't need to actually have zeros written |
| 36 // throughout; it just needs to read as zero until something diffferent is |
| 37 // written to a location. This is an important distinction as it supports the |
| 38 // use-case of non-pinned memory, such as from a demand-allocated region by |
| 39 // the OS or a memory-mapped file that auto-grows from a starting size of zero. |
| 40 class BASE_EXPORT SharedMemoryAllocator { |
| 41 public: |
| 42 // Internal state information when iterating over memory allocations. |
| 43 struct Iterator { |
| 44 int32_t last; |
| 45 uint32_t niter; |
| 46 }; |
| 47 |
| 48 // Returned information about the internal state of the heap. |
| 49 struct MemoryInfo { |
| 50 int32_t total; |
| 51 int32_t free; |
| 52 }; |
| 53 |
| 54 enum : int32_t { |
| 55 kTypeIdAny = 0 // Match any type-id inside GetAsObject(). |
| 56 }; |
| 57 |
| 58 // The allocator operates on any arbitrary block of memory. Creation and |
| 59 // sharing of that block with another process is the responsibility of the |
| 60 // caller. The allocator needs to know only the block's |base| address, the |
| 61 // total |size| of the block, and any internal |page| size (zero if not |
| 62 // paged) across which allocations should not span. |
| 63 // |
| 64 // SharedMemoryAllocator does NOT take ownership of this memory block. The |
| 65 // caller must manage it and ensure it stays available throughout the lifetime |
| 66 // of this object. |
| 67 // |
| 68 // Memory segments for sharing must have had an allocator attached to them |
| 69 // before actually being shared. If the memory segment was just created, it |
| 70 // should be zeroed. If it was an existing segment, the values here will |
| 71 // be compared to copies stored in the shared segment as a guard against |
| 72 // corruption. |
| 73 SharedMemoryAllocator(void* base, int32_t size, int32_t page_size); |
| 74 ~SharedMemoryAllocator(); |
| 75 |
| 76 // Get an object referenced by an |offset|. For safety reasons, the |type_id| |
| 77 // code and size-of(|T|) are compared to ensure the reference is valid |
| 78 // and cannot return an object outside of the memory segment. A |type_id| of |
| 79 // zero will match any though the size is still checked. NULL is returned |
| 80 // if any problem is detected, such as corrupted storage or incorrect |
| 81 // parameters. Callers MUST check that the returned value is not-null EVERY |
| 82 // TIME before accessing it or risk crashing! Once dereferenced, the pointer |
| 83 // is safe to reuse forever. |
| 84 // |
| 85 // NOTE: Though this method will guarantee that an object of the specified |
| 86 // type can be accessed without going outside the bounds of the memory |
| 87 // segment, it makes not guarantees of the validity of the data within the |
| 88 // object itself. If it is expected that the contents of the segment could |
| 89 // be compromised with malicious intent, the object must be hardened as well. |
| 90 template <typename T> |
| 91 T* GetAsObject(int32_t offset, int32_t type_id) { |
| 92 return static_cast<T*>(GetBlockData(offset, type_id, sizeof(T), false)); |
| 93 } |
| 94 |
| 95 // Reserve space in the memory segment of the desired |size| and |type_id|. |
| 96 // A return value of zero indicates the allocation failed, otherwise the |
| 97 // returned offset can be used by any process to get a real pointer via |
| 98 // the GetAsObject() call. |
| 99 int32_t Allocate(int32_t size, int32_t type_id); |
| 100 |
| 101 // Allocated objects can be added to an internal list that can then be |
| 102 // iterated over by other processes. If an allocated object can be found |
| 103 // another way, such as by having its offset within a different object |
| 104 // that will be made iterable, then this call is not necessary. This always |
| 105 // succeeds unless corruption is detected; check IsCorrupted() to find out. |
| 106 void MakeIterable(int32_t offset); |
| 107 |
| 108 // Get the information about the amount of free space in the allocator. The |
| 109 // amount of free space should be treated as approximate due to extras from |
| 110 // alignment and metadata. Concurrent allocations from other threads will |
| 111 // also make the true amount less than what is reported. It will never |
| 112 // return _less_ than could actually be allocated. |
| 113 void GetMemoryInfo(MemoryInfo* meminfo); |
| 114 |
| 115 // Iterating uses a |state| structure (initialized by CreateIterator) and |
| 116 // returns both the offset reference to the object as well as the |type_id| |
| 117 // of that object. A zero return value indicates there are currently no more |
| 118 // objects to be found but future attempts can be made without having to |
| 119 // reset the iterator to "first". |
| 120 void CreateIterator(Iterator* state); |
| 121 int32_t GetNextIterable(Iterator* state, int32_t* type_id); |
| 122 |
| 123 // If there is some indication that the shared memory has become corrupted, |
| 124 // calling this will attempt to prevent further damage by indicating to |
| 125 // all processes that something is not as expected. |
| 126 void SetCorrupted(); |
| 127 |
| 128 // This can be called to determine if corruption has been detected in the |
| 129 // shared segment, possibly my a malicious actor. Once detected, future |
| 130 // allocations will fail and iteration may not locate all objects. |
| 131 bool IsCorrupted(); |
| 132 |
| 133 // Flag set if an allocation has failed because memory was full. |
| 134 bool IsFull(); |
| 135 |
| 136 private: |
| 137 struct SharedMetadata; |
| 138 struct BlockHeader; |
| 139 |
| 140 BlockHeader* GetBlock(int32_t offset, int32_t type_id, int32_t size, |
| 141 bool special); |
| 142 void* GetBlockData(int32_t offset, int32_t type_id, int32_t size, |
| 143 bool special); |
| 144 |
| 145 SharedMetadata* shared_meta_; // Pointer to start of memory segment. |
| 146 char* mem_base_; // Same. (char because sizeof guaranteed 1) |
| 147 int32_t mem_size_; // Size of entire memory segment. |
| 148 int32_t mem_page_; // Page size allocations shouldn't cross. |
| 149 subtle::Atomic32 corrupted_; // TODO(bcwhite): Use std::atomic<char> when ok. |
| 150 |
| 151 DISALLOW_COPY_AND_ASSIGN(SharedMemoryAllocator); |
| 152 }; |
| 153 |
| 154 } // namespace base |
| 155 |
| 156 #endif // BASE_MEMORY_SHARED_MEMORY_ALLOCATOR_H_ |
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