Shmem.cpp

/* -*- 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/. */

#include "Shmem.h"

#include "ProtocolUtils.h"
#include "SharedMemoryBasic.h"
#include "SharedMemorySysV.h"

#include "mozilla/unused.h"


namespace mozilla {
namespace ipc {

class ShmemCreated : public IPC::Message
{
private:
  typedef Shmem::id_t id_t;

public:
  ShmemCreated(int32_t routingId,
               const id_t& aIPDLId,
               const size_t& aSize,
               const SharedMemoryBasic::Handle& aHandle) :
    IPC::Message(routingId, SHMEM_CREATED_MESSAGE_TYPE, PRIORITY_NORMAL)
  {
    IPC::WriteParam(this, aIPDLId);
    IPC::WriteParam(this, aSize);
    IPC::WriteParam(this, int32_t(SharedMemory::TYPE_BASIC)),
    IPC::WriteParam(this, aHandle);
  }

  // Instead of a single Read() function, we have ReadInfo() and
  // ReadHandle().  The reason is that the handle type is specific to
  // the shmem type.  These functions should only be called in the
  // order ReadInfo(); ReadHandle();, and only once each.

  static bool
  ReadInfo(const Message* msg, void** iter,
           id_t* aIPDLId,
           size_t* aSize,
           SharedMemory::SharedMemoryType* aType)
  {
    if (!IPC::ReadParam(msg, iter, aIPDLId) ||
        !IPC::ReadParam(msg, iter, aSize) ||
        !IPC::ReadParam(msg, iter, reinterpret_cast<int32_t*>(aType)))
      return false;
    return true;
  }

  static bool
  ReadHandle(const Message* msg, void** iter,
             SharedMemoryBasic::Handle* aHandle)
  {
    if (!IPC::ReadParam(msg, iter, aHandle))
      return false;
    msg->EndRead(*iter);
    return true;
  }

#ifdef MOZ_HAVE_SHAREDMEMORYSYSV
  ShmemCreated(int32_t routingId,
               const id_t& aIPDLId,
               const size_t& aSize,
               const SharedMemorySysV::Handle& aHandle) :
    IPC::Message(routingId, SHMEM_CREATED_MESSAGE_TYPE, PRIORITY_NORMAL)
  {
    IPC::WriteParam(this, aIPDLId);
    IPC::WriteParam(this, aSize);
    IPC::WriteParam(this, int32_t(SharedMemory::TYPE_SYSV)),
    IPC::WriteParam(this, aHandle);
  }

  static bool
  ReadHandle(const Message* msg, void** iter,
             SharedMemorySysV::Handle* aHandle)
  {
    if (!IPC::ReadParam(msg, iter, aHandle))
      return false;
    msg->EndRead(*iter);
    return true;
  }
#endif

  void Log(const std::string& aPrefix,
           FILE* aOutf) const
  {
    fputs("(special ShmemCreated msg)", aOutf);
  }
};

class ShmemDestroyed : public IPC::Message
{
private:
  typedef Shmem::id_t id_t;

public:
  ShmemDestroyed(int32_t routingId,
                 const id_t& aIPDLId) :
    IPC::Message(routingId, SHMEM_DESTROYED_MESSAGE_TYPE, PRIORITY_NORMAL)
  {
    IPC::WriteParam(this, aIPDLId);
  }
};


#ifdef MOZ_HAVE_SHAREDMEMORYSYSV
static already_AddRefed<Shmem::SharedMemory>
CreateSegment(size_t aNBytes, SharedMemorySysV::Handle aHandle)
{
  nsRefPtr<SharedMemory> segment;

  if (SharedMemorySysV::IsHandleValid(aHandle)) {
    segment = new SharedMemorySysV(aHandle);
  }
  else {
    segment = new SharedMemorySysV();

    if (!segment->Create(aNBytes))
      return nullptr;
  }
  if (!segment->Map(aNBytes))
    return nullptr;

  return segment.forget();
}
#endif

static already_AddRefed<Shmem::SharedMemory>
CreateSegment(size_t aNBytes, SharedMemoryBasic::Handle aHandle)
{
  nsRefPtr<SharedMemory> segment;

  if (SharedMemoryBasic::IsHandleValid(aHandle)) {
    segment = new SharedMemoryBasic(aHandle);
  }
  else {
    segment = new SharedMemoryBasic();

    if (!segment->Create(aNBytes))
      return nullptr;
  }
  if (!segment->Map(aNBytes))
    return nullptr;

  return segment.forget();
}

static void
DestroySegment(SharedMemory* aSegment)
{
  // the SharedMemory dtor closes and unmaps the actual OS shmem segment
  if (aSegment)
    aSegment->Release();
}


#if defined(DEBUG)

static const char sMagic[] =
    "This little piggy went to market.\n"
    "This little piggy stayed at home.\n"
    "This little piggy has roast beef,\n"
    "This little piggy had none.\n"
    "And this little piggy cried \"Wee! Wee! Wee!\" all the way home";


struct Header {
  // Don't use size_t or bool here because their size depends on the
  // architecture.
  uint32_t mSize;
  uint32_t mUnsafe;
  char mMagic[sizeof(sMagic)];
};

static void
GetSections(Shmem::SharedMemory* aSegment,
            Header** aHeader,
            char** aFrontSentinel,
            char** aData,
            char** aBackSentinel)
{
  NS_ABORT_IF_FALSE(aSegment && aFrontSentinel && aData && aBackSentinel,
                    "NULL param(s)");

  *aFrontSentinel = reinterpret_cast<char*>(aSegment->memory());
  NS_ABORT_IF_FALSE(*aFrontSentinel, "NULL memory()");

  *aHeader = reinterpret_cast<Header*>(*aFrontSentinel);

  size_t pageSize = Shmem::SharedMemory::SystemPageSize();
  *aData = *aFrontSentinel + pageSize;

  *aBackSentinel = *aFrontSentinel + aSegment->Size() - pageSize;
}

static Header*
GetHeader(Shmem::SharedMemory* aSegment)
{
  Header* header;
  char* dontcare;
  GetSections(aSegment, &header, &dontcare, &dontcare, &dontcare);
  return header;
}

static void
Protect(SharedMemory* aSegment)
{
  NS_ABORT_IF_FALSE(aSegment, "NULL segment");
  aSegment->Protect(reinterpret_cast<char*>(aSegment->memory()),
                    aSegment->Size(),
                    RightsNone);
}

static void
Unprotect(SharedMemory* aSegment)
{
  NS_ABORT_IF_FALSE(aSegment, "NULL segment");
  aSegment->Protect(reinterpret_cast<char*>(aSegment->memory()),
                    aSegment->Size(),
                    RightsRead | RightsWrite);
}

//
// In debug builds, we specially allocate shmem segments.  The layout
// is as follows
//
//   Page 0: "front sentinel"
//     size of mapping
//     magic bytes
//   Page 1 through n-1:
//     user data
//   Page n: "back sentinel"
//     [nothing]
//
// The mapping can be in one of the following states, wrt to the
// current process.
//
//   State "unmapped": all pages are mapped with no access rights.
//
//   State "mapping": all pages are mapped with read/write access.
//
//   State "mapped": the front and back sentinels are mapped with no
//     access rights, and all the other pages are mapped with
//     read/write access.
//
// When a SharedMemory segment is first allocated, it starts out in
// the "mapping" state for the process that allocates the segment, and
// in the "unmapped" state for the other process.  The allocating
// process will then create a Shmem, which takes the segment into the
// "mapped" state, where it can be accessed by clients.
//
// When a Shmem is sent to another process in an IPDL message, the
// segment transitions into the "unmapped" state for the sending
// process, and into the "mapping" state for the receiving process.
// The receiving process will then create a Shmem from the underlying
// segment, and take the segment into the "mapped" state.
//
// In the "mapping" state, we use the front sentinel to verify the
// integrity of the shmem segment.  If valid, it has a size_t
// containing the number of bytes the user allocated followed by the
// magic bytes above.
//
// In the "mapped" state, the front and back sentinels have no access
// rights.  They act as guards against buffer overflows and underflows
// in client code; if clients touch a sentinel, they die with SIGSEGV.
//
// The "unmapped" state is used to enforce single-owner semantics of
// the shmem segment.  If a process other than the current owner tries
// to touch the segment, it dies with SIGSEGV.
//

Shmem::Shmem(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
             SharedMemory* aSegment, id_t aId) :
    mSegment(aSegment),
    mData(nullptr),
    mSize(0)
{
  NS_ABORT_IF_FALSE(mSegment, "NULL segment");
  NS_ABORT_IF_FALSE(aId != 0, "invalid ID");

  Unprotect(mSegment);

  Header* header;
  char* frontSentinel;
  char* data;
  char* backSentinel;
  GetSections(aSegment, &header, &frontSentinel, &data, &backSentinel);

  // do a quick validity check to avoid weird-looking crashes in libc
  char check = *frontSentinel;
  (void)check;

  NS_ABORT_IF_FALSE(!strncmp(header->mMagic, sMagic, sizeof(sMagic)),
                      "invalid segment");
  mSize = static_cast<size_t>(header->mSize);

  size_t pageSize = SharedMemory::SystemPageSize();
  // transition into the "mapped" state by protecting the front and
  // back sentinels (which guard against buffer under/overflows)
  mSegment->Protect(frontSentinel, pageSize, RightsNone);
  mSegment->Protect(backSentinel, pageSize, RightsNone);

  // don't set these until we know they're valid
  mData = data;
  mId = aId;
}

void
Shmem::AssertInvariants() const
{
  NS_ABORT_IF_FALSE(mSegment, "NULL segment");
  NS_ABORT_IF_FALSE(mData, "NULL data pointer");
  NS_ABORT_IF_FALSE(mSize > 0, "invalid size");
  // if the segment isn't owned by the current process, these will
  // trigger SIGSEGV
  char checkMappingFront = *reinterpret_cast<char*>(mData);
  char checkMappingBack = *(reinterpret_cast<char*>(mData) + mSize - 1);

  // avoid "unused" warnings for these variables:
  unused << checkMappingFront;
  unused << checkMappingBack;
}

void
Shmem::RevokeRights(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead)
{
  AssertInvariants();

  size_t pageSize = SharedMemory::SystemPageSize();
  Header* header = GetHeader(mSegment);

  // Open this up for reading temporarily
  mSegment->Protect(reinterpret_cast<char*>(header), pageSize, RightsRead);

  if (!header->mUnsafe) {
    Protect(mSegment);
  } else {
    mSegment->Protect(reinterpret_cast<char*>(header), pageSize, RightsNone);
  }
}

// static
already_AddRefed<Shmem::SharedMemory>
Shmem::Alloc(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
             size_t aNBytes,
             SharedMemoryType aType,
             bool aUnsafe,
             bool aProtect)
{
  NS_ASSERTION(aNBytes <= UINT32_MAX, "Will truncate shmem segment size!");
  NS_ABORT_IF_FALSE(!aProtect || !aUnsafe, "protect => !unsafe");

  size_t pageSize = SharedMemory::SystemPageSize();
  nsRefPtr<SharedMemory> segment;
  // |2*pageSize| is for the front and back sentinel
  size_t segmentSize = SharedMemory::PageAlignedSize(aNBytes + 2*pageSize);

  if (aType == SharedMemory::TYPE_BASIC)
    segment = CreateSegment(segmentSize, SharedMemoryBasic::NULLHandle());
#ifdef MOZ_HAVE_SHAREDMEMORYSYSV
  else if (aType == SharedMemory::TYPE_SYSV)
    segment = CreateSegment(segmentSize, SharedMemorySysV::NULLHandle());
#endif
  else {
    NS_ERROR("unknown shmem type");
    return nullptr;
  }

  if (!segment)
    return nullptr;

  Header* header;
  char *frontSentinel;
  char *data;
  char *backSentinel;
  GetSections(segment, &header, &frontSentinel, &data, &backSentinel);

  // initialize the segment with Shmem-internal information

  // NB: this can't be a static assert because technically pageSize
  // isn't known at compile time, event though in practice it's always
  // going to be 4KiB
  NS_ABORT_IF_FALSE(sizeof(Header) <= pageSize,
                    "Shmem::Header has gotten too big");
  memcpy(header->mMagic, sMagic, sizeof(sMagic));
  header->mSize = static_cast<uint32_t>(aNBytes);
  header->mUnsafe = aUnsafe;

  if (aProtect)
    Protect(segment);

  return segment.forget();
}

// static
already_AddRefed<Shmem::SharedMemory>
Shmem::OpenExisting(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
                    const IPC::Message& aDescriptor,
                    id_t* aId,
                    bool aProtect)
{
  if (SHMEM_CREATED_MESSAGE_TYPE != aDescriptor.type()) {
    NS_ERROR("expected 'shmem created' message");
    return nullptr;
  }

  void* iter = nullptr;
  SharedMemory::SharedMemoryType type;
  size_t size;
  if (!ShmemCreated::ReadInfo(&aDescriptor, &iter, aId, &size, &type))
    return nullptr;

  nsRefPtr<SharedMemory> segment;
  size_t pageSize = SharedMemory::SystemPageSize();
  // |2*pageSize| is for the front and back sentinels
  size_t segmentSize = SharedMemory::PageAlignedSize(size + 2*pageSize);

  if (SharedMemory::TYPE_BASIC == type) {
    SharedMemoryBasic::Handle handle;
    if (!ShmemCreated::ReadHandle(&aDescriptor, &iter, &handle))
      return nullptr;

    if (!SharedMemoryBasic::IsHandleValid(handle)) {
      NS_ERROR("trying to open invalid handle");
      return nullptr;
    }
    segment = CreateSegment(segmentSize, handle);
  }
#ifdef MOZ_HAVE_SHAREDMEMORYSYSV
  else if (SharedMemory::TYPE_SYSV == type) {
    SharedMemorySysV::Handle handle;
    if (!ShmemCreated::ReadHandle(&aDescriptor, &iter, &handle))
      return nullptr;

    if (!SharedMemorySysV::IsHandleValid(handle)) {
      NS_ERROR("trying to open invalid handle");
      return nullptr;
    }
    segment = CreateSegment(segmentSize, handle);
  }
#endif
  else {
    NS_ERROR("unknown shmem type");
    return nullptr;
  }

  if (!segment)
    return nullptr;

  Header* header = GetHeader(segment);

  if (size != header->mSize) {
    NS_ERROR("Wrong size for this Shmem!");
    return nullptr;
  }

  // The caller of this function may not know whether the segment is
  // unsafe or not
  if (!header->mUnsafe && aProtect)
    Protect(segment);

  return segment.forget();
}

// static
void
Shmem::Dealloc(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
               SharedMemory* aSegment)
{
  if (!aSegment)
    return;

  size_t pageSize = SharedMemory::SystemPageSize();
  Header* header;
  char *frontSentinel;
  char *data;
  char *backSentinel;
  GetSections(aSegment, &header, &frontSentinel, &data, &backSentinel);

  aSegment->Protect(frontSentinel, pageSize, RightsWrite | RightsRead);
  memset(header->mMagic, 0, sizeof(sMagic));
  header->mSize = 0;
  header->mUnsafe = false;          // make it "safe" so as to catch errors

  DestroySegment(aSegment);
}


#else  // !defined(DEBUG)

// static
already_AddRefed<Shmem::SharedMemory>
Shmem::Alloc(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
             size_t aNBytes,
             SharedMemoryType aType,
             bool /*unused*/,
             bool /*unused*/)
{
  nsRefPtr<SharedMemory> segment;

  if (aType == SharedMemory::TYPE_BASIC)
    segment = CreateSegment(SharedMemory::PageAlignedSize(aNBytes + sizeof(uint32_t)),
                            SharedMemoryBasic::NULLHandle());
#ifdef MOZ_HAVE_SHAREDMEMORYSYSV
  else if (aType == SharedMemory::TYPE_SYSV)
    segment = CreateSegment(SharedMemory::PageAlignedSize(aNBytes + sizeof(uint32_t)),
                            SharedMemorySysV::NULLHandle());
#endif
  else {
    return nullptr;
  }

  if (!segment)
    return nullptr;

  *PtrToSize(segment) = static_cast<uint32_t>(aNBytes);

  return segment.forget();
}

// static
already_AddRefed<Shmem::SharedMemory>
Shmem::OpenExisting(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
                    const IPC::Message& aDescriptor,
                    id_t* aId,
                    bool /*unused*/)
{
  if (SHMEM_CREATED_MESSAGE_TYPE != aDescriptor.type()) {
    return nullptr;
  }

  SharedMemory::SharedMemoryType type;
  void* iter = nullptr;
  size_t size;
  if (!ShmemCreated::ReadInfo(&aDescriptor, &iter, aId, &size, &type))
    return nullptr;

  nsRefPtr<SharedMemory> segment;
  size_t segmentSize = SharedMemory::PageAlignedSize(size + sizeof(uint32_t));

  if (SharedMemory::TYPE_BASIC == type) {
    SharedMemoryBasic::Handle handle;
    if (!ShmemCreated::ReadHandle(&aDescriptor, &iter, &handle))
      return nullptr;

    if (!SharedMemoryBasic::IsHandleValid(handle)) {
      return nullptr;
    }

    segment = CreateSegment(segmentSize, handle);
  }
#ifdef MOZ_HAVE_SHAREDMEMORYSYSV
  else if (SharedMemory::TYPE_SYSV == type) {
    SharedMemorySysV::Handle handle;
    if (!ShmemCreated::ReadHandle(&aDescriptor, &iter, &handle))
      return nullptr;

    if (!SharedMemorySysV::IsHandleValid(handle)) {
      return nullptr;
    }
    segment = CreateSegment(segmentSize, handle);
  }
#endif
  else {
    return nullptr;
  }

  if (!segment)
    return nullptr;

  // this is the only validity check done in non-DEBUG builds
  if (size != static_cast<size_t>(*PtrToSize(segment))) {
    return nullptr;
  }

  return segment.forget();
}

// static
void
Shmem::Dealloc(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
               SharedMemory* aSegment)
{
  DestroySegment(aSegment);
}

#endif  // if defined(DEBUG)

int
Shmem::GetSysVID() const
{
#ifdef MOZ_HAVE_SHAREDMEMORYSYSV
  AssertInvariants();

  if (mSegment->Type() != SharedMemory::TYPE_SYSV) {
    NS_ERROR("Can't call GetSysVID() on a non-SysV Shmem!");
    return -1;
  }

  SharedMemorySysV* seg = static_cast<SharedMemorySysV*>(mSegment);
  return seg->GetHandle();
#else
  NS_ERROR("Can't call GetSysVID() with no support for SysV shared memory!");
  return -1;                    // not reached
#endif
}

IPC::Message*
Shmem::ShareTo(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
               base::ProcessHandle aProcess,
               int32_t routingId)
{
  AssertInvariants();

  // kInvalidProcessHandle is used to indicate that it's the same process.
  if (aProcess == kInvalidProcessHandle) {
    aProcess = base::GetCurrentProcessHandle();
  }

  if (SharedMemory::TYPE_BASIC == mSegment->Type()) {
    SharedMemoryBasic* seg = static_cast<SharedMemoryBasic*>(mSegment);
    SharedMemoryBasic::Handle handle;
    if (!seg->ShareToProcess(aProcess, &handle))
      return nullptr;

    return new ShmemCreated(routingId, mId, mSize, handle);
  }
#ifdef MOZ_HAVE_SHAREDMEMORYSYSV
  else if (SharedMemory::TYPE_SYSV == mSegment->Type()) {
    SharedMemorySysV* seg = static_cast<SharedMemorySysV*>(mSegment);
    return new ShmemCreated(routingId, mId, mSize, seg->GetHandle());
  }
#endif
  else {
    NS_ABORT_IF_FALSE(false, "unknown shmem type (here?!)");
    return nullptr;
  }

  return nullptr;
}

IPC::Message*
Shmem::UnshareFrom(IHadBetterBeIPDLCodeCallingThis_OtherwiseIAmADoodyhead,
                   base::ProcessHandle aProcess,
                   int32_t routingId)
{
  AssertInvariants();
  return new ShmemDestroyed(routingId, mId);
}

} // namespace ipc
} // namespace mozilla