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Summary: The following are risks associated with pointer-to-pointer reinterpret_cast: * Can produce the "wrong result" (crash or memory corruption). IIRC, in theory this can happen for any up-cast or down-cast for a non-standard-layout type, though in practice would only happen for multiple inheritance cases (where the base class pointer might be "inside" the derived object). We don't use multiple inheritance a lot, but we do. * Can mask useful compiler errors upon code change, including converting between unrelated pointer types that you are expecting to be related, and converting between pointer and scalar types unintentionally. I can only think of some obscure cases where static_cast could be troublesome when it compiles as a replacement: * Going through `void*` could plausibly cause unnecessary or broken pointer arithmetic. Suppose we have `struct Derived: public Base1, public Base2`. If we have `Derived*` -> `void*` -> `Base2*` -> `Derived*` through reinterpret casts, this could plausibly work (though technical UB) assuming the `Base2*` is not dereferenced. Changing to static cast could introduce breaking pointer arithmetic. * Unnecessary (but safe) pointer arithmetic could arise in a case like `Derived*` -> `Base2*` -> `Derived*` where before the Base2 pointer might not have been dereferenced. This could potentially affect performance. With some light scripting, I tried replacing pointer-to-pointer reinterpret_casts with static_cast and kept the cases that still compile. Most occurrences of reinterpret_cast have successfully been changed (except for java/ and third-party/). 294 changed, 257 remain. A couple of related interventions included here: * Previously Cache::Handle was not actually derived from in the implementations and just used as a `void*` stand-in with reinterpret_cast. Now there is a relationship to allow static_cast. In theory, this could introduce pointer arithmetic (as described above) but is unlikely without multiple inheritance AND non-empty Cache::Handle. * Remove some unnecessary casts to void* as this is allowed to be implicit (for better or worse). Most of the remaining reinterpret_casts are for converting to/from raw bytes of objects. We could consider better idioms for these patterns in follow-up work. I wish there were a way to implement a template variant of static_cast that would only compile if no pointer arithmetic is generated, but best I can tell, this is not possible. AFAIK the best you could do is a dynamic check that the void* conversion after the static cast is unchanged. Pull Request resolved: https://github.com/facebook/rocksdb/pull/12308 Test Plan: existing tests, CI Reviewed By: ltamasi Differential Revision: D53204947 Pulled By: pdillinger fbshipit-source-id: 9de23e618263b0d5b9820f4e15966876888a16e2
53 lines
1.5 KiB
C++
53 lines
1.5 KiB
C++
// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
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// This source code is licensed under both the GPLv2 (found in the
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// COPYING file in the root directory) and Apache 2.0 License
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// (found in the LICENSE.Apache file in the root directory).
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//
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// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file. See the AUTHORS file for names of contributors.
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#include "db/malloc_stats.h"
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#include <cstring>
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#include <memory>
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#include "port/jemalloc_helper.h"
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namespace ROCKSDB_NAMESPACE {
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#ifdef ROCKSDB_JEMALLOC
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struct MallocStatus {
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char* cur;
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char* end;
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};
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static void GetJemallocStatus(void* mstat_arg, const char* status) {
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MallocStatus* mstat = static_cast<MallocStatus*>(mstat_arg);
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size_t status_len = status ? strlen(status) : 0;
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size_t buf_size = (size_t)(mstat->end - mstat->cur);
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if (!status_len || status_len > buf_size) {
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return;
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}
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snprintf(mstat->cur, buf_size, "%s", status);
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mstat->cur += status_len;
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}
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void DumpMallocStats(std::string* stats) {
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if (!HasJemalloc()) {
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return;
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}
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MallocStatus mstat;
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const unsigned int kMallocStatusLen = 1000000;
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std::unique_ptr<char[]> buf{new char[kMallocStatusLen + 1]};
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mstat.cur = buf.get();
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mstat.end = buf.get() + kMallocStatusLen;
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malloc_stats_print(GetJemallocStatus, &mstat, "");
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stats->append(buf.get());
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}
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#else
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void DumpMallocStats(std::string*) {}
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#endif // ROCKSDB_JEMALLOC
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} // namespace ROCKSDB_NAMESPACE
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