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 |
index 0000000000000000000000000000000000000000..4eab83cd52e8a9236b8184cce7cc0e6018013102 |
--- /dev/null |
+++ b/base/memory/shared_memory_allocator.cc |
@@ -0,0 +1,460 @@ |
+// 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 <algorithm> |
+ |
+#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. |
mdempsky
2015/11/10 19:47:55
I notice that 1) there's no checks to ensure the s
bcwhite
2015/11/10 21:17:34
There are size_t checks where it's used (which is
mdempsky
2015/11/10 22:45:52
Style guide says "Do not use C-style casts."
|
+ |
+namespace { |
+ |
+// Required range of memory segment sizes. It has to fit in a signed 32-bit |
+// number and should be a power of 2 in order to accomodate almost any page |
+// size. |
+const int32_t kSegmentMinSize = 1 << 10; // 1 KiB |
+const int32_t kSegmentMaxSize = 1 << 30; // 1 GiB |
+ |
+// All allocations and data-structures must be aligned to this byte boundary. |
+// Alignment as large as the physical bus between CPU and RAM is _required_ |
+// for some architectures, is simply more efficient on other CPUs, and |
+// generally a Good Idea(tm) for all platforms as it reduces/eliminates the |
+// chance that a type will span cache lines. Alignment mustn't be less |
+// than 8 to ensure proper alignment for all types. The rest is a balance |
+// between reducing spans across multiple cache lines and wasted space spent |
+// padding out allocations. An alignment of 16 would ensure that the block |
+// header structure always sits in a single cache line. An average of about |
+// 1/2 this value will be wasted with every allocation. |
+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; |
+ |
+// TODO(bcwhite): When acceptable, consider moving flags to std::atomic<char> |
+// types rather than combined bitfield. |
+ |
+// Flags stored in the flags_ field of the SharedMetaData structure below. |
+enum : int32_t { |
+ kFlagCorrupt = 1 << 0, |
+ kFlagFull = 1 << 1 |
+}; |
+ |
+bool CheckFlag(base::subtle::Atomic32* flags, int flag) { |
+ base::subtle::Atomic32 loaded_flags = base::subtle::Acquire_Load(flags); |
+ return (loaded_flags & flag) != 0; |
+} |
+ |
+void SetFlag(base::subtle::Atomic32* flags, int flag) { |
+ for (;;) { |
+ base::subtle::Atomic32 loaded_flags = base::subtle::Acquire_Load(flags); |
+ base::subtle::Atomic32 new_flags = |
+ (loaded_flags & ~flag) | flag; |
+ if (base::subtle::Release_CompareAndSwap( |
+ flags, loaded_flags, new_flags) == loaded_flags) { |
+ break; |
+ } |
+ } |
+} |
+ |
+} // 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. |
+ uint32_t type_id; // 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/ref to first free space in the segment. |
+ subtle::Atomic32 flags; // Bitfield of information flags. |
+ 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. |
mdempsky
2015/11/10 19:47:55
FWIW, this is valid C++:
class Foo {
struct
bcwhite
2015/11/10 21:17:34
That would require the constant to be declared in
mdempsky
2015/11/10 22:45:52
Declared, yes. Is that problematic somehow?
bcwhite
2015/11/11 13:25:14
Nope. Just making sure I understood.
|
+#define REF_QUEUE offsetof(SharedMetadata, queue) |
+#define REF_NULL 0 // the equivalest NULL value for a reference |
mdempsky
2015/11/10 19:47:55
typo: equivalent
bcwhite
2015/11/10 21:17:34
Done.
|
+ |
+SharedMemoryAllocator::SharedMemoryAllocator(void* base, |
+ size_t size, |
+ size_t page_size) |
+ : mem_base_(static_cast<char*>(base)), |
+ mem_size_((int32_t)size), |
+ mem_page_((int32_t)(page_size ? page_size : size)), |
+ corrupted_(0) { |
+ 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"); |
+ |
+ CHECK(base && reinterpret_cast<uintptr_t>(base) % kAllocAlignment == 0); |
+ CHECK(size >= kSegmentMinSize && size <= kSegmentMaxSize && |
+ size % kAllocAlignment == 0); |
+ CHECK(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 || |
+ subtle::NoBarrier_Load(&shared_meta()->flags) != 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_id != 0 || |
+ first_block->next != 0) { |
+ // ...or something malicious has been playing with the metadata. |
+ NOTREACHED(); |
+ SetCorrupt(); |
+ } |
+ |
+ // This is still safe to do even if corruption has been detected. |
+ shared_meta()->cookie = kGlobalCookie; |
+ shared_meta()->size = mem_size_; |
+ shared_meta()->page_size = mem_page_; |
+ 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, REF_QUEUE); |
+ subtle::NoBarrier_Store(&shared_meta()->tailptr, REF_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 != mem_size_ || |
+ shared_meta()->page_size != mem_page_) { |
+ NOTREACHED(); |
+ SetCorrupt(); |
+ } |
+ } |
+} |
+ |
+SharedMemoryAllocator::~SharedMemoryAllocator() {} |
+ |
+size_t SharedMemoryAllocator::GetAllocSize(Reference ref) { |
+ BlockHeader* block = GetBlock(ref, 0, 0, false, false); |
+ if (!block) |
+ return 0; |
+ int32_t size = block->size; |
+ // Header was verified by GetBlock() but a malicious actor could change |
+ // the value between there and here. Check it again. |
+ if (size <= (int)sizeof(BlockHeader) || ref + size >= mem_size_) |
+ return 0; |
+ return (size_t)size - sizeof(BlockHeader); |
+} |
+ |
+int32_t SharedMemoryAllocator::Allocate(size_t usize, uint32_t type_id) { |
+ // Round up the requested size, plus header, to the next allocation alignment. |
+ int32_t size = (int)usize + sizeof(BlockHeader); |
+ size = (size + (kAllocAlignment - 1)) & ~(kAllocAlignment - 1); |
+ if (usize > (size_t)std::numeric_limits<int32_t>::max() || |
+ size <= (int)sizeof(BlockHeader) || size > mem_page_) { |
+ NOTREACHED(); |
+ return REF_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 (IsCorrupt()) |
+ return REF_NULL; |
+ |
+ // Get the current start of unallocated memory. Other threads may |
+ // update this at any time and cause us to retry these operations. |
+ int32_t freeptr = subtle::NoBarrier_Load(&shared_meta()->freeptr); |
+ if (freeptr + size > mem_size_) { |
+ SetFlag(&shared_meta()->flags, kFlagFull); |
+ return REF_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, 0, false, true); |
+ if (!block) { |
+ SetCorrupt(); |
+ return REF_NULL; |
+ } |
+ |
+ // An allocation cannot cross page boundaries. If it would, create a |
+ // "wasted" block and begin again at the top of the next page. This |
+ // area could just be left empty but we fill in the block header just |
+ // for completeness sake. |
+ int32_t page_free = mem_page_ - freeptr % mem_page_; |
+ if (size > page_free) { |
+ if (page_free <= sizeof(BlockHeader)) { |
+ SetCorrupt(); |
+ return REF_NULL; |
+ } |
+ int32_t new_freeptr = freeptr + page_free; |
+ if (subtle::NoBarrier_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_) { |
+ SetCorrupt(); |
+ return REF_NULL; |
+ } |
+ |
+ if (subtle::NoBarrier_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_id != 0 || |
+ subtle::NoBarrier_Load(&block->next) != 0) { |
+ SetCorrupt(); |
+ return REF_NULL; |
+ } |
+ |
+ block->size = size; |
+ block->cookie = kBlockCookieAllocated; |
+ block->type_id = type_id; |
+ return freeptr; |
+ } |
+} |
+ |
+void SharedMemoryAllocator::GetMemoryInfo(MemoryInfo* meminfo) { |
+ int32_t remaining = |
+ mem_size_ - subtle::NoBarrier_Load(&shared_meta()->freeptr); |
+ meminfo->total = mem_size_; |
+ meminfo->free = IsCorrupt() ? 0 : remaining - sizeof(BlockHeader); |
+} |
+ |
+void SharedMemoryAllocator::MakeIterable(Reference ref) { |
+ if (IsCorrupt()) |
+ return; |
+ BlockHeader* block = GetBlock(ref, 0, 0, false, false); |
+ if (!block) // invalid reference |
+ return; |
+ if (subtle::Acquire_Load(&block->next) != 0) // previously set iterable |
+ return; |
+ subtle::Release_Store(&block->next, REF_QUEUE); // will be tail block |
+ |
+ // Try to add this block to the tail of the queue. May take multiple tries. |
+ int32_t tail; |
+ for (;;) { |
+ // Acquire the current tail-pointer released by previous call to this |
+ // method and validate it. |
+ tail = subtle::Acquire_Load(&shared_meta()->tailptr); |
+ block = GetBlock(tail, 0, 0, true, false); |
+ if (!block) { |
+ SetCorrupt(); |
+ return; |
+ } |
+ |
+ // Try to insert the block at the tail of the queue. The tail node always |
+ // has an existing value of REF_QUEUE; if that is not the value returned, |
+ // another thread has acted in the meantime. |
+ int32_t next = subtle::Release_CompareAndSwap(&block->next, REF_QUEUE, ref); |
+ if (next == REF_QUEUE) { |
+ // Update the tail pointer to the new offset. If the "else" clause did |
+ // not exist, then this could be a simple Release_Store to set the new |
+ // value but because it does, it's possible that other threads could add |
+ // one or more nodes at the tail before reaching this point. We don't |
+ // have to check the return value because it either operates correctly |
+ // or the exact same operation has already been done (by the "else" |
+ // clause). |
+ subtle::Release_CompareAndSwap(&shared_meta()->tailptr, tail, ref); |
+ return; |
+ } else { |
+ // In the unlikely case that a thread crashed or was killed between the |
+ // update of "next" and the update of "tailptr", it is necessary to |
+ // perform the operation that would have been done. There's no explicit |
+ // check for crash/kill which means that this operation may also happen |
+ // even when the other thread is in perfect working order which is what |
+ // necessitates the CompareAndSwap above. |
+ subtle::Release_CompareAndSwap(&shared_meta()->tailptr, tail, next); |
+ } |
+ } |
+} |
+ |
+void SharedMemoryAllocator::CreateIterator(Iterator* state) { |
+ state->last = REF_QUEUE; |
+ state->niter = 0; |
+} |
+ |
+int32_t SharedMemoryAllocator::GetNextIterable(Iterator* state, |
+ uint32_t* type_id) { |
+ const BlockHeader* block = GetBlock(state->last, 0, 0, true, false); |
+ if (!block) // invalid iterator state |
+ return REF_NULL; |
+ |
+ // The compiler and CPU can freely reorder all memory accesses on which |
+ // there are no dependencies. It could, for example, move the load of |
+ // "freeptr" above this point because there are no explicit dependencies |
+ // between it and "next". If it did, however, then another block could |
+ // be queued after that but before the following load meaning there is |
+ // one more queued block than the future "detect loop by having more |
+ // blocks that could fit before freeptr" will allow. |
+ // |
+ // By "acquiring" the "next" value here, it's synchronized to the enqueue |
+ // of the node which in turn is synchronized to the allocation (which sets |
+ // freeptr). Thus, the scenario above cannot happen. |
+ int32_t next = subtle::Acquire_Load(&block->next); |
+ block = GetBlock(next, 0, 0, false, false); |
+ if (!block) // no next allocation in queue |
+ return REF_NULL; |
+ |
+ // 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. We do this |
+ // simply by making sure we don't iterate more than the absolute maximum |
+ // number of allocations that could have been made. Callers are likely |
+ // to loop multiple times before it is detected but at least it stops. |
+ int32_t freeptr = std::min(subtle::Acquire_Load(&shared_meta()->freeptr), |
+ mem_size_); |
+ if (state->niter > freeptr / (sizeof(BlockHeader) + kAllocAlignment)) { |
+ SetCorrupt(); |
+ return REF_NULL; |
+ } |
+ |
+ state->last = next; |
+ state->niter++; |
+ *type_id = block->type_id; |
+ |
+ return next; |
+} |
+ |
+// The "corrupted" state is held both locally and globally (shared). The |
+// shared flag can't be trusted since a malicious actor could overwrite it. |
+// The local version is immune to foreign actors. Thus, if seen shared, |
+// copy it locally and, once known, always restore it globally. |
+void SharedMemoryAllocator::SetCorrupt() { |
+ LOG(ERROR) << "Corruption detected in shared-memory segment."; |
+ subtle::NoBarrier_Store(&corrupted_, 1); |
mdempsky
2015/11/10 19:47:55
(I take individual SharedMemoryAllocator objects m
bcwhite
2015/11/10 21:17:34
Yes.
|
+ SetFlag(&shared_meta()->flags, kFlagCorrupt); |
+} |
+ |
+bool SharedMemoryAllocator::IsCorrupt() { |
+ if (subtle::NoBarrier_Load(&corrupted_) || |
+ CheckFlag(&shared_meta()->flags, kFlagCorrupt)) { |
+ SetCorrupt(); // Make sure all indicators are set. |
+ return true; |
+ } |
+ return false; |
+} |
+ |
+bool SharedMemoryAllocator::IsFull() { |
+ return CheckFlag(&shared_meta()->flags, kFlagFull); |
+} |
+ |
+// Dereference a block |ref| and ensure that it's valid for the desired |
+// |type_id| 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( |
+ Reference ref, |
+ uint32_t type_id, |
+ int32_t size, |
+ bool queue_ok, |
+ bool free_ok) { |
+ // Validation of parameters. |
+ if (ref % kAllocAlignment != 0) |
+ return nullptr; |
+ if (ref < (int)(queue_ok ? REF_QUEUE : sizeof(SharedMetadata))) |
+ return nullptr; |
+ size += sizeof(BlockHeader); |
+ if (ref + size > mem_size_) |
+ return nullptr; |
+ |
+ // Validation of referenced block-header. |
+ if (!free_ok) { |
+ int32_t freeptr = subtle::NoBarrier_Load(&shared_meta()->freeptr); |
+ if (ref + size > freeptr) |
+ return nullptr; |
+ const BlockHeader* block = |
+ reinterpret_cast<BlockHeader*>(mem_base_ + ref); |
+ if (block->size < size) |
mdempsky
2015/11/10 19:47:55
Like JF and I mentioned, for C++ correctness, bloc
Alexander Potapenko
2015/11/10 20:47:10
By "volatile" did you mean "atomic"?
(this is basi
bcwhite
2015/11/10 21:17:35
You mean "volatile T* GetAsObject(...) {...}"? I
mdempsky
2015/11/10 22:45:52
JF said they need to both volatile and atomic (and
mdempsky
2015/11/10 22:45:52
Yes.
|
+ return nullptr; |
+ if (ref != REF_QUEUE && block->cookie != kBlockCookieAllocated) |
+ return nullptr; |
+ if (type_id != 0 && block->type_id != type_id) |
+ return nullptr; |
+ } |
+ |
+ // Return pointer to block data. |
+ return reinterpret_cast<BlockHeader*>(mem_base_ + ref); |
+} |
+ |
+void* SharedMemoryAllocator::GetBlockData(Reference ref, |
+ uint32_t type_id, |
+ int32_t size) { |
+ DCHECK(size > 0); |
+ BlockHeader* block = GetBlock(ref, type_id, size, false, false); |
+ if (!block) |
+ return nullptr; |
+ return reinterpret_cast<char*>(block) + sizeof(BlockHeader); |
+} |
+ |
+} // namespace base |