Index: base/memory/shared_memory_allocator.cc |
diff --git a/base/memory/shared_memory_allocator.cc b/base/memory/shared_memory_allocator.cc |
new file mode 100644 |
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--- /dev/null |
+++ b/base/memory/shared_memory_allocator.cc |
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+// Copyright (c) 2015 The Chromium Authors. All rights reserved. |
+// Use of this source code is governed by a BSD-style license that can be |
+// found in the LICENSE file. |
+ |
+#include "base/memory/shared_memory_allocator.h" |
+ |
+#include <assert.h> |
+ |
+#include "base/atomicops.h" |
+#include "base/logging.h" |
+ |
+// All integer constants in this file are signed because Atomic32 is signed |
+// and keeping all others consistent with this avoids a lot of unnecessary |
+// casting to avoid signed/unsigned operations just to avoid compiler errors. |
+// This means an occasonal cast of a constant from sizeof() to "int" but |
+// is far simpler than the alternative. |
+ |
+namespace { |
+ |
+// All allocations and data-structures must be aligned to this byte boundary. |
+// It shouldn't be less than 8 so that 64-bit values can be read in a single |
+// RAM bus access. 16 can be used so that the block header would always fall |
+// within a single cache line. |
+const int32_t kAllocAlignment = 8; |
+ |
+// A constant (random) value placed in the shared metadata to identify |
+// an already initialized memory segment. |
+const int32_t kGlobalCookie = 0x408305DC; |
+ |
+// The current version of the metadata. If updates are made that change |
+// the metadata, the version number can be queried to operate in a backward- |
+// compatible manner until the memory segment is completely re-initalized. |
+const int32_t kGlobalVersion = 1; |
+ |
+// Constant values placed in the block headers to indicate its state. |
+const int32_t kBlockCookieFree = 0; |
+const int32_t kBlockCookieQueue = 1; |
+const int32_t kBlockCookieWasted = -1; |
+const int32_t kBlockCookieAllocated = 0xC8799269; |
+ |
+} // namespace |
+ |
+namespace base { |
+ |
+// The block-header is placed at the top of every allocation within the |
+// segment to describe the data that follows it. |
+struct SharedMemoryAllocator::BlockHeader { |
+ int32_t size; // Number of bytes in this block, including header. |
+ int32_t cookie; // Constant value indicating completed allocation. |
+ int32_t type; // A number provided by caller indicating data type. |
+ subtle::Atomic32 next; // Pointer to the next block when iterating |
+}; |
+ |
+// The shared metadata exists once at the top of the memory segment to |
+// describe the state of the allocator to all processes. |
+struct SharedMemoryAllocator::SharedMetadata { |
+ int32_t cookie; // Some value that indicates complete initialization. |
+ int32_t size; // Total size of memory segment. |
+ int32_t page_size; // Paging size within memory segment. |
+ int32_t version; // Version code so upgrades don't break. |
+ subtle::Atomic32 freeptr; // Offset to first free space in the segment. |
+ char corrupted; // Flag indicating that corruption has been detected. |
+ char full; // Flag indicating alloc failed because segment is full. |
+ char flags[2]; // Future flags. (exact padding to int boundary) |
+ int32_t reserved; // Padding to ensure size is multiple of alignment. |
+ |
+ // The "iterable" queue is an M&S Queue as described here, append-only: |
+ // https://www.research.ibm.com/people/m/michael/podc-1996.pdf |
+ subtle::Atomic32 tailptr; // Last block available for iteration. |
+ BlockHeader queue; // Empty block for linked-list head/tail. (must be last) |
+}; |
+ |
+// The "queue" block header is used to detect "last node" so that zero/null |
+// can be used to indicate that it hasn't been added at all. It is part of |
+// the SharedMetadata structure which itself is always located at offset zero. |
+// This can't be a constant because SharedMetadata is a private definition. |
+#define OFFSET_QUEUE offsetof(SharedMetadata, queue) |
+#define OFFSET_NULL 0 // the equivalest NULL value for an offset |
+ |
+SharedMemoryAllocator::SharedMemoryAllocator(void* base, int32_t size, |
+ int32_t page_size) |
+ : shared_meta_(static_cast<SharedMetadata*>(base)), |
+ mem_base_(static_cast<char*>(base)), |
+ mem_size_(size), |
+ mem_page_(page_size ? page_size : size), |
+ last_seen_(0), |
+ corrupted_(false) { |
+ static_assert(sizeof(BlockHeader) % kAllocAlignment == 0, |
+ "BlockHeader is not a multiple of kAllocAlignment"); |
+ static_assert(sizeof(SharedMetadata) % kAllocAlignment == 0, |
+ "SharedMetadata is not a multiple of kAllocAlignment"); |
+ |
+ DCHECK(base && reinterpret_cast<uintptr_t>(base) % kAllocAlignment == 0); |
+ DCHECK(size >= 1 << 10 && size <= 1 << 20 && // 1 KiB <= size <= 1 MiB |
+ size % kAllocAlignment == 0); |
+ DCHECK(page_size >= 0 && (page_size == 0 || size % page_size == 0)); |
+ |
+ if (shared_meta_->cookie != kGlobalCookie) { |
+ // This block is only executed when a completely new memory segment is |
+ // being initialized. It's unshared and single-threaded... |
+ const BlockHeader* first_block = reinterpret_cast<BlockHeader*>( |
+ mem_base_ + sizeof(SharedMetadata)); |
+ if (shared_meta_->cookie != 0 || |
+ shared_meta_->size != 0 || |
+ shared_meta_->version != 0 || |
+ subtle::NoBarrier_Load(&shared_meta_->freeptr) != 0 || |
+ shared_meta_->corrupted != 0 || |
+ shared_meta_->full != 0 || |
+ shared_meta_->tailptr != 0 || |
+ shared_meta_->queue.cookie != 0 || |
+ subtle::NoBarrier_Load(&shared_meta_->queue.next) != 0 || |
+ first_block->size != 0 || |
+ first_block->cookie != 0 || |
+ first_block->type != 0 || |
+ first_block->next != 0) { |
+ // ...or something malicious has been playing with the metadata. |
+ NOTREACHED(); |
+ SetCorrupted(); |
+ } |
+ |
+ // This is still safe to do even if corruption has been detected. |
+ shared_meta_->cookie = kGlobalCookie; |
+ shared_meta_->size = size; |
+ shared_meta_->page_size = page_size; |
+ shared_meta_->version = kGlobalVersion; |
+ subtle::NoBarrier_Store(&shared_meta_->freeptr, sizeof(SharedMetadata)); |
+ |
+ // Set up the queue of iterable allocations. |
+ shared_meta_->queue.size = sizeof(BlockHeader); |
+ shared_meta_->queue.cookie = kBlockCookieQueue; |
+ subtle::NoBarrier_Store(&shared_meta_->queue.next, OFFSET_QUEUE); |
+ subtle::NoBarrier_Store(&shared_meta_->tailptr, OFFSET_QUEUE); |
+ } else { |
+ // The allocator is attaching to a previously initialized segment of |
+ // memory. Make sure the embedded data matches what has been passed. |
+ if (shared_meta_->size != size || |
+ shared_meta_->page_size != page_size) { |
+ NOTREACHED(); |
+ SetCorrupted(); |
+ } |
+ } |
+} |
+ |
+SharedMemoryAllocator::~SharedMemoryAllocator() { |
+} |
+ |
+int32_t SharedMemoryAllocator::Allocate(int32_t size, int32_t type) { |
+ if (size < 0) { |
+ NOTREACHED(); |
+ return OFFSET_NULL; |
+ } |
+ |
+ // Round up the requested size, plus header, to the next allocation alignment. |
+ size += sizeof(BlockHeader); |
+ size = (size + (kAllocAlignment - 1)) & ~(kAllocAlignment - 1); |
+ if (size > mem_page_) |
+ return OFFSET_NULL; |
+ |
+ // Allocation is lockless so we do all our caculation and then, if saving |
+ // indicates a change has occurred since we started, scrap everything and |
+ // start over. |
+ for (;;) { |
+ if (IsCorrupted()) |
+ return OFFSET_NULL; |
+ |
+ int32_t freeptr = subtle::Acquire_Load(&shared_meta_->freeptr); |
+ if (freeptr + size > mem_size_) { |
+ shared_meta_->full = true; |
+ return OFFSET_NULL; |
+ } |
+ |
+ // Get pointer to the "free" block. It doesn't even have a header; pass |
+ // -sizeof(header) so accouting for that will yield an expected size of |
+ // zero which is what will be stored at that location. If something |
+ // has been allocated since the load of freeptr above, it is still safe |
+ // as nothing will be written to that location until after the CAS below. |
+ BlockHeader* block = GetBlock(freeptr, 0, -(int)sizeof(BlockHeader), true); |
+ if (!block) { |
+ SetCorrupted(); |
+ return OFFSET_NULL; |
+ } |
+ |
+ // An allocation cannot cross page boundaries. If it would, create a |
+ // "wasted" block and begin again at the top of the next page. |
+ int32_t page_free = mem_page_ - freeptr % mem_page_; |
+ if (size > page_free) { |
+ int32_t new_freeptr = freeptr + page_free; |
+ if (subtle::Release_CompareAndSwap( |
+ &shared_meta_->freeptr, freeptr, new_freeptr) == freeptr) { |
+ block->size = page_free; |
+ block->cookie = kBlockCookieWasted; |
+ } |
+ continue; |
+ } |
+ |
+ // Don't leave a slice at the end of a page too small for anything. This |
+ // can result in an allocation up to two alignment-sizes greater than the |
+ // minimum required by requested-size + header + alignment. |
+ if (page_free - size < (int)(sizeof(BlockHeader) + kAllocAlignment)) |
+ size = page_free; |
+ |
+ int32_t new_freeptr = freeptr + size; |
+ if (new_freeptr > mem_size_) { |
+ SetCorrupted(); |
+ return OFFSET_NULL; |
+ } |
+ |
+ if (subtle::Release_CompareAndSwap( |
+ &shared_meta_->freeptr, freeptr, new_freeptr) != freeptr) { |
+ // Another thread must have completed an allocation while we were working. |
+ // Try again. |
+ continue; |
+ } |
+ |
+ // Given that all memory was zeroed before ever being given to an instance |
+ // of this class and given that we only allocate in a monotomic fashion |
+ // going forward, it must be that the newly allocated block is completely |
+ // full of zeros. If we find anything in the block header that is NOT a |
+ // zero then something must have previously run amuck through memory, |
+ // writing beyond the allocated space and into unallocated space. |
+ if (block->size != 0 || |
+ block->cookie != kBlockCookieFree || |
+ block->type != 0 || |
+ subtle::NoBarrier_Load(&block->next) != 0) { |
+ SetCorrupted(); |
+ return OFFSET_NULL; |
+ } |
+ |
+ block->size = size; |
+ block->cookie = kBlockCookieAllocated; |
+ block->type = type; |
+ return freeptr; |
+ } |
+} |
+ |
+void SharedMemoryAllocator::GetMemoryInfo(MemoryInfo* meminfo) { |
+ int32_t remaining = |
+ mem_size_ - subtle::NoBarrier_Load(&shared_meta_->freeptr); |
+ meminfo->total = mem_size_; |
+ meminfo->free = IsCorrupted() ? 0 : remaining - sizeof(BlockHeader); |
+} |
+ |
+void SharedMemoryAllocator::MakeIterable(int32_t offset) { |
+ if (IsCorrupted()) |
+ return; |
+ BlockHeader* block = GetBlock(offset, 0, 0, false); |
+ if (!block) // invalid offset |
+ return; |
+ if (subtle::NoBarrier_Load(&block->next) != 0) // previously set iterable |
+ return; |
+ subtle::NoBarrier_Store(&block->next, OFFSET_QUEUE); // will be tail block |
+ |
+ // Try to add this block to the tail of the queue. May take multiple tries. |
+ int32_t tail; |
+ for (;;) { |
+ tail = subtle::Acquire_Load(&shared_meta_->tailptr); |
+ block = GetBlock(tail, 0, 0, true); |
+ if (!block) { |
+ SetCorrupted(); |
+ return; |
+ } |
+ int32_t next = subtle::NoBarrier_Load(&block->next); |
+ |
+ // Ensure that the tail pointer didn't change while reading next. Only |
Alexander Potapenko
2015/10/30 21:01:15
If I'm understanding correctly, you don't actually
|
+ // the read of the tail pointer is atomic but we need to read both the |
+ // tail pointer and the next pointer from it in an atomic fashion. The |
+ // way to do this is to read both non-atomically and then verify after |
+ // the second read that the first read is still valid/unchanged. |
+ if (tail == subtle::Release_Load(&shared_meta_->tailptr)) { |
Alexander Potapenko
2015/10/30 20:43:20
The problem with Release_Load() and Acquire_Store(
|
+ // Check if the found block is truely the last in the queue (i.e. it |
+ // points back to the "queue" node). |
+ if (next == OFFSET_QUEUE) { |
+ // Yes. Try to append the passed block after the current tail block. |
+ if (subtle::Release_CompareAndSwap( |
+ &block->next, OFFSET_QUEUE, offset) == OFFSET_QUEUE) { |
+ // Success! The block is enqueued; need to update the tail pointer. |
+ break; |
+ } |
+ } else { |
+ // No. Another thread has stopped between the block-next update |
+ // and the tail-pointer update. Try to update tailptr past the |
+ // found block. That other thread may complete it first or it |
+ // may have crashed. Be fail-safe. |
+ subtle::Release_CompareAndSwap(&shared_meta_->tailptr, tail, next); |
+ } |
+ } |
+ } |
+ |
+ // Block has been enqueued. Now update the tail-pointer past it. This |
+ // could fail if another thread has already completed the operation as |
+ // part of being fail-safe. |
+ subtle::Release_CompareAndSwap(&shared_meta_->tailptr, tail, offset); |
+} |
+ |
+void SharedMemoryAllocator::CreateIterator(Iterator* state) { |
+ state->last = OFFSET_QUEUE; |
+ state->loop_detector = OFFSET_QUEUE; |
+} |
+ |
+int32_t SharedMemoryAllocator::GetNextIterable(Iterator* state, int32_t* type) { |
+ const BlockHeader* block = GetBlock(state->last, 0, 0, true); |
+ if (!block) // invalid iterator state |
+ return OFFSET_NULL; |
+ int32_t next = subtle::NoBarrier_Load(&block->next); |
+ block = GetBlock(next, 0, 0, false); |
+ if (!block) // no next allocation in queue |
+ return OFFSET_NULL; |
+ if (next == state->loop_detector) { |
+ SetCorrupted(); |
+ return OFFSET_NULL; |
+ } |
+ |
+ state->last = next; |
+ *type = block->type; |
+ |
+ // Memory corruption could cause a loop in the list. We need to detect |
+ // that so as to not cause an infinite loop in the caller. This is done |
+ // by having a second pointer that double-increments through the list. |
+ // If it ever comes around to match "last" then we have a loop and need |
+ // to stop iterating. It's possible to not iterate through all items and |
+ // it's possible to loop multiple times before the loop is detected but at |
+ // least it stops. |
+ if (state->loop_detector == OFFSET_QUEUE) |
+ state->loop_detector = next; |
+ block = GetBlock(state->loop_detector, 0, 0, false); |
+ if (block) { |
+ state->loop_detector = subtle::NoBarrier_Load(&block->next); |
+ block = GetBlock(state->loop_detector, 0, 0, false); |
+ if (block) |
+ state->loop_detector = subtle::NoBarrier_Load(&block->next); |
+ } |
+ |
+ return next; |
+} |
+ |
+void SharedMemoryAllocator::SetCorrupted() { |
+ LOG(ERROR) << "Corruption detected in shared-memory segment."; |
+ corrupted_ = true; |
+ shared_meta_->corrupted = true; |
+} |
+ |
+bool SharedMemoryAllocator::IsCorrupted() { |
+ if (corrupted_ || shared_meta_->corrupted) { |
+ SetCorrupted(); // Make sure all indicators are set. |
+ return true; |
+ } |
+ return false; |
+} |
+ |
+bool SharedMemoryAllocator::IsFull() { |
+ return shared_meta_->full != 0; |
+} |
+ |
+// Dereference a block |offset| and ensure that it's valid for the desired |
+// |type| and |size|. |special| indicates that we may try to access block |
+// headers not available to callers but still accessed by this module. By |
+// having internal dereferences go through this same function, the allocator |
+// is hardened against corruption. |
+SharedMemoryAllocator::BlockHeader* SharedMemoryAllocator::GetBlock( |
+ int32_t offset, int32_t type, int32_t size, bool special) { |
+ // Validation of parameters. |
+ if (offset % kAllocAlignment != 0) |
+ return nullptr; |
+ if (offset < (int)(special ? OFFSET_QUEUE : sizeof(SharedMetadata))) |
+ return nullptr; |
+ size += sizeof(BlockHeader); |
+ if (offset + size > mem_size_) |
+ return nullptr; |
+ int32_t freeptr = subtle::NoBarrier_Load(&shared_meta_->freeptr); |
+ if (offset + size > freeptr) |
+ return nullptr; |
+ |
+ // Validation of referenced block-header. |
+ const BlockHeader* block = reinterpret_cast<BlockHeader*>(mem_base_ + offset); |
+ if (block->size < size) |
+ return nullptr; |
+ if (!special && block->cookie != kBlockCookieAllocated) |
+ return nullptr; |
+ if (type != 0 && block->type != type) |
+ return nullptr; |
+ |
+ // Return pointer to block data. |
+ return reinterpret_cast<BlockHeader*>(mem_base_ + offset); |
+} |
+ |
+void* SharedMemoryAllocator::GetBlockData(int32_t offset, int32_t type, |
+ int32_t size, bool special) { |
+ DCHECK(size > 0); |
+ BlockHeader* block = GetBlock(offset, type, size, special); |
+ if (!block) |
+ return nullptr; |
+ return reinterpret_cast<char*>(block) + sizeof(BlockHeader); |
+} |
+ |
+} // namespace base |