Shmem.h
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: sw=2 ts=8 et :
*/
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
#ifndef mozilla_ipc_Shmem_h
#define mozilla_ipc_Shmem_h
#include "mozilla/Attributes.h"
#include "base/basictypes.h"
#include "base/process.h"
#include "nscore.h"
#include "nsDebug.h"
#include "ipc/IPCMessageUtils.h"
#include "mozilla/ipc/SharedMemory.h"
/**
* |Shmem| is one agent in the IPDL shared memory scheme. The way it
works is essentially
*
* (1) C++ code calls, say, |parentActor->AllocShmem(size)|
* (2) IPDL-generated code creates a |mozilla::ipc::SharedMemory|
* wrapping the bare OS shmem primitives. The code then adds the new
* SharedMemory to the set of shmem segments being managed by IPDL.
*
* (3) IPDL-generated code "shares" the new SharedMemory to the child
* process, and then sends a special asynchronous IPC message to the
* child notifying it of the creation of the segment. (What this
* means is OS specific.)
*
* (4a) The child receives the special IPC message, and using the
* |SharedMemory{SysV,Basic}::Handle| it was passed, creates a
* |mozilla::ipc::SharedMemory| in the child
* process.
*
* (4b) After sending the "shmem-created" IPC message, IPDL-generated
* code in the parent returns a |mozilla::ipc::Shmem| back to the C++
* caller of |parentActor->AllocShmem()|. The |Shmem| is a "weak
* reference" to the underlying |SharedMemory|, which is managed by
* IPDL-generated code. C++ consumers of |Shmem| can't get at the
* underlying |SharedMemory|.
*
* If parent code wants to give access rights to the Shmem to the
* child, it does so by sending its |Shmem| to the child, in an IPDL
* message. The parent's |Shmem| then "dies", i.e. becomes
* inaccessible. This process could be compared to passing a
* "shmem-access baton" between parent and child.
*/
namespace mozilla {
namespace layers {
class ShadowLayerForwarder;
}
namespace ipc {
class Shmem MOZ_FINAL
{
friend struct IPC::ParamTraits<mozilla::ipc::Shmem>;
#ifdef DEBUG
// For ShadowLayerForwarder::CheckSurfaceDescriptor
friend class mozilla::layers::ShadowLayerForwarder;
#endif
public:
typedef int32_t id_t;
// Low-level wrapper around platform shmem primitives.
typedef mozilla::ipc::SharedMemory SharedMemory;
typedef SharedMemory::SharedMemoryType SharedMemoryType;
struct IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead {};
Shmem() :
mSegment(0),
mData(0),
mSize(0),
mId(0)
{
}
Shmem(const Shmem& aOther) :
mSegment(aOther.mSegment),
mData(aOther.mData),
mSize(aOther.mSize),
mId(aOther.mId)
{
}
#if !defined(DEBUG)
Shmem(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
SharedMemory* aSegment, id_t aId) :
mSegment(aSegment),
mData(aSegment->memory()),
mSize(0),
mId(aId)
{
mSize = static_cast<size_t>(*PtrToSize(mSegment));
}
#else
Shmem(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
SharedMemory* aSegment, id_t aId);
#endif
~Shmem()
{
// Shmem only holds a "weak ref" to the actual segment, which is
// owned by IPDL. So there's nothing interesting to be done here
forget(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead());
}
Shmem& operator=(const Shmem& aRhs)
{
mSegment = aRhs.mSegment;
mData = aRhs.mData;
mSize = aRhs.mSize;
mId = aRhs.mId;
return *this;
}
bool operator==(const Shmem& aRhs) const
{
// need to compare IDs because of AdoptShmem(); two Shmems might
// refer to the same segment but with different IDs for different
// protocol trees. (NB: it's possible for this method to
// spuriously return true if AdoptShmem() gives the same ID for
// two protocol trees, but I don't think that can cause any
// problems since the Shmems really would be indistinguishable.)
return mSegment == aRhs.mSegment && mId == aRhs.mId;
}
// Returns whether this Shmem is writable by you, and thus whether you can
// transfer writability to another actor.
bool
IsWritable() const
{
return mSegment != nullptr;
}
// Returns whether this Shmem is readable by you, and thus whether you can
// transfer readability to another actor.
bool
IsReadable() const
{
return mSegment != nullptr;
}
// Return a pointer to the user-visible data segment.
template<typename T>
T*
get() const
{
AssertInvariants();
AssertAligned<T>();
return reinterpret_cast<T*>(mData);
}
// Return the size of the segment as requested when this shmem
// segment was allocated, in units of T. The underlying mapping may
// actually be larger because of page alignment and private data,
// but this isn't exposed to clients.
template<typename T>
size_t
Size() const
{
AssertInvariants();
AssertAligned<T>();
return mSize / sizeof(T);
}
int GetSysVID() const;
// These shouldn't be used directly, use the IPDL interface instead.
id_t Id(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead) const {
return mId;
}
SharedMemory* Segment(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead) const {
return mSegment;
}
#ifndef DEBUG
void RevokeRights(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead)
{
}
#else
void RevokeRights(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead);
#endif
void forget(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead)
{
mSegment = 0;
mData = 0;
mSize = 0;
mId = 0;
}
static SharedMemory*
Alloc(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
size_t aNBytes,
SharedMemoryType aType,
bool aUnsafe,
bool aProtect=false);
// Prepare this to be shared with |aProcess|. Return an IPC message
// that contains enough information for the other process to map
// this segment in OpenExisting() below. Return a new message if
// successful (owned by the caller), nullptr if not.
IPC::Message*
ShareTo(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
base::ProcessHandle aProcess,
int32_t routingId);
// Stop sharing this with |aProcess|. Return an IPC message that
// contains enough information for the other process to unmap this
// segment. Return a new message if successful (owned by the
// caller), nullptr if not.
IPC::Message*
UnshareFrom(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
base::ProcessHandle aProcess,
int32_t routingId);
// Return a SharedMemory instance in this process using the
// descriptor shared to us by the process that created the
// underlying OS shmem resource. The contents of the descriptor
// depend on the type of SharedMemory that was passed to us.
static SharedMemory*
OpenExisting(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
const IPC::Message& aDescriptor,
id_t* aId,
bool aProtect=false);
static void
Dealloc(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
SharedMemory* aSegment);
private:
template<typename T>
void AssertAligned() const
{
if (0 != (mSize % sizeof(T)))
NS_RUNTIMEABORT("shmem is not T-aligned");
}
#if !defined(DEBUG)
void AssertInvariants() const
{ }
static uint32_t*
PtrToSize(SharedMemory* aSegment)
{
char* endOfSegment =
reinterpret_cast<char*>(aSegment->memory()) + aSegment->Size();
return reinterpret_cast<uint32_t*>(endOfSegment - sizeof(uint32_t));
}
#else
void AssertInvariants() const;
#endif
SharedMemory* mSegment;
void* mData;
size_t mSize;
id_t mId;
};
} // namespace ipc
} // namespace mozilla
namespace IPC {
template<>
struct ParamTraits<mozilla::ipc::Shmem>
{
typedef mozilla::ipc::Shmem paramType;
// NB: Read()/Write() look creepy in that Shmems have a pointer
// member, but IPDL internally uses mId to properly initialize a
// "real" Shmem
static void Write(Message* aMsg, const paramType& aParam)
{
WriteParam(aMsg, aParam.mId);
}
static bool Read(const Message* aMsg, void** aIter, paramType* aResult)
{
paramType::id_t id;
if (!ReadParam(aMsg, aIter, &id))
return false;
aResult->mId = id;
return true;
}
static void Log(const paramType& aParam, std::wstring* aLog)
{
aLog->append(L"(shmem segment)");
}
};
} // namespace IPC
#endif // ifndef mozilla_ipc_Shmem_h
