binder: Use variant for backend specific Parcel fields

This frees up some space to add RPC binder specific bookkeeping to
Parcel. `variant` has a size overhead of one pointer, but since there is
at least one pointer worth of fields in each case, it works out as an
overall win. We could probably do it cheaper by using a union and hiding
a bit field somewhere to indicate the active case, but that isn't
necessary yet.

Bug: 185909244
Test: TH
Change-Id: Id53ea0cbfe51b3246e6f9f1f3d9df7c8c193808e

3 files changed, 372 insertions, 225 deletions

diff --git a/libs/binder/Parcel.cpp b/libs/binder/Parcel.cpp
index 58b0b35323..038ce381e2 100644
--- a/libs/binder/Parcel.cpp
+++ b/libs/binder/Parcel.cpp
@@ -151,6 +151,10 @@ static void release_object(const sp<ProcessState>& proc, const flat_binder_objec
     ALOGE("Invalid object type 0x%08x", obj.hdr.type);
 }
 
+Parcel::RpcFields::RpcFields(const sp<RpcSession>& session) : mSession(session) {
+    LOG_ALWAYS_FATAL_IF(mSession == nullptr);
+}
+
 status_t Parcel::finishFlattenBinder(const sp<IBinder>& binder)
 {
     internal::Stability::tryMarkCompilationUnit(binder.get());
@@ -182,13 +186,14 @@ status_t Parcel::flattenBinder(const sp<IBinder>& binder) {
     if (binder) local = binder->localBinder();
     if (local) local->setParceled();
 
-    if (isForRpc()) {
+    if (const auto* rpcFields = maybeRpcFields()) {
         if (binder) {
             status_t status = writeInt32(1); // non-null
             if (status != OK) return status;
             uint64_t address;
             // TODO(b/167966510): need to undo this if the Parcel is not sent
-            status = mSession->state()->onBinderLeaving(mSession, binder, &address);
+            status = rpcFields->mSession->state()->onBinderLeaving(rpcFields->mSession, binder,
+                                                                   &address);
             if (status != OK) return status;
             status = writeUint64(address);
             if (status != OK) return status;
@@ -259,9 +264,7 @@ status_t Parcel::flattenBinder(const sp<IBinder>& binder) {
 
 status_t Parcel::unflattenBinder(sp<IBinder>* out) const
 {
-    if (isForRpc()) {
-        LOG_ALWAYS_FATAL_IF(mSession == nullptr, "RpcSession required to read from remote parcel");
-
+    if (const auto* rpcFields = maybeRpcFields()) {
         int32_t isPresent;
         status_t status = readInt32(&isPresent);
         if (status != OK) return status;
@@ -271,10 +274,14 @@ status_t Parcel::unflattenBinder(sp<IBinder>* out) const
         if (isPresent & 1) {
             uint64_t addr;
             if (status_t status = readUint64(&addr); status != OK) return status;
-            if (status_t status = mSession->state()->onBinderEntering(mSession, addr, &binder);
+            if (status_t status =
+                        rpcFields->mSession->state()->onBinderEntering(rpcFields->mSession, addr,
+                                                                       &binder);
                 status != OK)
                 return status;
-            if (status_t status = mSession->state()->flushExcessBinderRefs(mSession, addr, binder);
+            if (status_t status =
+                        rpcFields->mSession->state()->flushExcessBinderRefs(rpcFields->mSession,
+                                                                            addr, binder);
                 status != OK)
                 return status;
         }
@@ -378,8 +385,10 @@ void Parcel::setDataPosition(size_t pos) const
     }
 
     mDataPos = pos;
-    mNextObjectHint = 0;
-    mObjectsSorted = false;
+    if (const auto* kernelFields = maybeKernelFields()) {
+        kernelFields->mNextObjectHint = 0;
+        kernelFields->mObjectsSorted = false;
+    }
 }
 
 status_t Parcel::setDataCapacity(size_t size)
@@ -406,25 +415,27 @@ status_t Parcel::setData(const uint8_t* buffer, size_t len)
     if (err == NO_ERROR) {
         memcpy(const_cast<uint8_t*>(data()), buffer, len);
         mDataSize = len;
-        mFdsKnown = false;
+        if (auto* kernelFields = maybeKernelFields()) {
+            kernelFields->mFdsKnown = false;
+        }
     }
     return err;
 }
 
-status_t Parcel::appendFrom(const Parcel *parcel, size_t offset, size_t len)
-{
-    if (mSession != parcel->mSession) {
+status_t Parcel::appendFrom(const Parcel* parcel, size_t offset, size_t len) {
+    if (isForRpc() != parcel->isForRpc()) {
         ALOGE("Cannot append Parcel from one context to another. They may be different formats, "
               "and objects are specific to a context.");
         return BAD_TYPE;
     }
+    if (isForRpc() && maybeRpcFields()->mSession != parcel->maybeRpcFields()->mSession) {
+        ALOGE("Cannot append Parcels from different sessions");
+        return BAD_TYPE;
+    }
 
     status_t err;
-    const uint8_t *data = parcel->mData;
-    const binder_size_t *objects = parcel->mObjects;
-    size_t size = parcel->mObjectsSize;
+    const uint8_t* data = parcel->mData;
     int startPos = mDataPos;
-    int firstIndex = -1, lastIndex = -2;
 
     if (len == 0) {
         return NO_ERROR;
@@ -443,18 +454,6 @@ status_t Parcel::appendFrom(const Parcel *parcel, size_t offset, size_t len)
         return BAD_VALUE;
     }
 
-    // Count objects in range
-    for (int i = 0; i < (int) size; i++) {
-        size_t off = objects[i];
-        if ((off >= offset) && (off + sizeof(flat_binder_object) <= offset + len)) {
-            if (firstIndex == -1) {
-                firstIndex = i;
-            }
-            lastIndex = i;
-        }
-    }
-    int numObjects = lastIndex - firstIndex + 1;
-
     if ((mDataSize+len) > mDataCapacity) {
         // grow data
         err = growData(len);
@@ -470,43 +469,63 @@ status_t Parcel::appendFrom(const Parcel *parcel, size_t offset, size_t len)
 
     err = NO_ERROR;
 
-    if (numObjects > 0) {
-        const sp<ProcessState> proc(ProcessState::self());
-        // grow objects
-        if (mObjectsCapacity < mObjectsSize + numObjects) {
-            if ((size_t) numObjects > SIZE_MAX - mObjectsSize) return NO_MEMORY; // overflow
-            if (mObjectsSize + numObjects > SIZE_MAX / 3) return NO_MEMORY; // overflow
-            size_t newSize = ((mObjectsSize + numObjects)*3)/2;
-            if (newSize > SIZE_MAX / sizeof(binder_size_t)) return NO_MEMORY; // overflow
-            binder_size_t *objects =
-                (binder_size_t*)realloc(mObjects, newSize*sizeof(binder_size_t));
-            if (objects == (binder_size_t*)nullptr) {
-                return NO_MEMORY;
+    if (auto* kernelFields = maybeKernelFields()) {
+        auto* otherKernelFields = parcel->maybeKernelFields();
+        LOG_ALWAYS_FATAL_IF(otherKernelFields == nullptr);
+
+        const binder_size_t* objects = otherKernelFields->mObjects;
+        size_t size = otherKernelFields->mObjectsSize;
+        // Count objects in range
+        int firstIndex = -1, lastIndex = -2;
+        for (int i = 0; i < (int)size; i++) {
+            size_t off = objects[i];
+            if ((off >= offset) && (off + sizeof(flat_binder_object) <= offset + len)) {
+                if (firstIndex == -1) {
+                    firstIndex = i;
+                }
+                lastIndex = i;
             }
-            mObjects = objects;
-            mObjectsCapacity = newSize;
         }
+        int numObjects = lastIndex - firstIndex + 1;
+        if (numObjects > 0) {
+            const sp<ProcessState> proc(ProcessState::self());
+            // grow objects
+            if (kernelFields->mObjectsCapacity < kernelFields->mObjectsSize + numObjects) {
+                if ((size_t)numObjects > SIZE_MAX - kernelFields->mObjectsSize)
+                    return NO_MEMORY; // overflow
+                if (kernelFields->mObjectsSize + numObjects > SIZE_MAX / 3)
+                    return NO_MEMORY; // overflow
+                size_t newSize = ((kernelFields->mObjectsSize + numObjects) * 3) / 2;
+                if (newSize > SIZE_MAX / sizeof(binder_size_t)) return NO_MEMORY; // overflow
+                binder_size_t* objects = (binder_size_t*)realloc(kernelFields->mObjects,
+                                                                 newSize * sizeof(binder_size_t));
+                if (objects == (binder_size_t*)nullptr) {
+                    return NO_MEMORY;
+                }
+                kernelFields->mObjects = objects;
+                kernelFields->mObjectsCapacity = newSize;
+            }
+
+            // append and acquire objects
+            int idx = kernelFields->mObjectsSize;
+            for (int i = firstIndex; i <= lastIndex; i++) {
+                size_t off = objects[i] - offset + startPos;
+                kernelFields->mObjects[idx++] = off;
+                kernelFields->mObjectsSize++;
+
+                flat_binder_object* flat = reinterpret_cast<flat_binder_object*>(mData + off);
+                acquire_object(proc, *flat, this);
 
-        // append and acquire objects
-        int idx = mObjectsSize;
-        for (int i = firstIndex; i <= lastIndex; i++) {
-            size_t off = objects[i] - offset + startPos;
-            mObjects[idx++] = off;
-            mObjectsSize++;
-
-            flat_binder_object* flat
-                = reinterpret_cast<flat_binder_object*>(mData + off);
-            acquire_object(proc, *flat, this);
-
-            if (flat->hdr.type == BINDER_TYPE_FD) {
-                // If this is a file descriptor, we need to dup it so the
-                // new Parcel now owns its own fd, and can declare that we
-                // officially know we have fds.
-                flat->handle = fcntl(flat->handle, F_DUPFD_CLOEXEC, 0);
-                flat->cookie = 1;
-                mHasFds = mFdsKnown = true;
-                if (!mAllowFds) {
-                    err = FDS_NOT_ALLOWED;
+                if (flat->hdr.type == BINDER_TYPE_FD) {
+                    // If this is a file descriptor, we need to dup it so the
+                    // new Parcel now owns its own fd, and can declare that we
+                    // officially know we have fds.
+                    flat->handle = fcntl(flat->handle, F_DUPFD_CLOEXEC, 0);
+                    flat->cookie = 1;
+                    kernelFields->mHasFds = kernelFields->mFdsKnown = true;
+                    if (!mAllowFds) {
+                        err = FDS_NOT_ALLOWED;
+                    }
                 }
             }
         }
@@ -563,18 +582,27 @@ void Parcel::restoreAllowFds(bool lastValue)
 
 bool Parcel::hasFileDescriptors() const
 {
-    if (!mFdsKnown) {
+    if (const auto* rpcFields = maybeRpcFields()) {
+        return false;
+    }
+    auto* kernelFields = maybeKernelFields();
+    if (!kernelFields->mFdsKnown) {
         scanForFds();
     }
-    return mHasFds;
+    return kernelFields->mHasFds;
 }
 
 std::vector<sp<IBinder>> Parcel::debugReadAllStrongBinders() const {
     std::vector<sp<IBinder>> ret;
 
+    const auto* kernelFields = maybeKernelFields();
+    if (kernelFields == nullptr) {
+        return ret;
+    }
+
     size_t initPosition = dataPosition();
-    for (size_t i = 0; i < mObjectsSize; i++) {
-        binder_size_t offset = mObjects[i];
+    for (size_t i = 0; i < kernelFields->mObjectsSize; i++) {
+        binder_size_t offset = kernelFields->mObjects[i];
         const flat_binder_object* flat =
                 reinterpret_cast<const flat_binder_object*>(mData + offset);
         if (flat->hdr.type != BINDER_TYPE_BINDER) continue;
@@ -592,9 +620,14 @@ std::vector<sp<IBinder>> Parcel::debugReadAllStrongBinders() const {
 std::vector<int> Parcel::debugReadAllFileDescriptors() const {
     std::vector<int> ret;
 
+    const auto* kernelFields = maybeKernelFields();
+    if (kernelFields == nullptr) {
+        return ret;
+    }
+
     size_t initPosition = dataPosition();
-    for (size_t i = 0; i < mObjectsSize; i++) {
-        binder_size_t offset = mObjects[i];
+    for (size_t i = 0; i < kernelFields->mObjectsSize; i++) {
+        binder_size_t offset = kernelFields->mObjects[i];
         const flat_binder_object* flat =
                 reinterpret_cast<const flat_binder_object*>(mData + offset);
         if (flat->hdr.type != BINDER_TYPE_FD) continue;
@@ -611,6 +644,10 @@ std::vector<int> Parcel::debugReadAllFileDescriptors() const {
 }
 
 status_t Parcel::hasFileDescriptorsInRange(size_t offset, size_t len, bool* result) const {
+    const auto* kernelFields = maybeKernelFields();
+    if (kernelFields == nullptr) {
+        return BAD_TYPE;
+    }
     if (len > INT32_MAX || offset > INT32_MAX) {
         // Don't accept size_t values which may have come from an inadvertent conversion from a
         // negative int.
@@ -621,12 +658,15 @@ status_t Parcel::hasFileDescriptorsInRange(size_t offset, size_t len, bool* resu
         return BAD_VALUE;
     }
     *result = false;
-    for (size_t i = 0; i < mObjectsSize; i++) {
-        size_t pos = mObjects[i];
+    for (size_t i = 0; i < kernelFields->mObjectsSize; i++) {
+        size_t pos = kernelFields->mObjects[i];
         if (pos < offset) continue;
         if (pos + sizeof(flat_binder_object) > offset + len) {
-          if (mObjectsSorted) break;
-          else continue;
+            if (kernelFields->mObjectsSorted) {
+                break;
+            } else {
+                continue;
+            }
         }
         const flat_binder_object* flat = reinterpret_cast<const flat_binder_object*>(mData + pos);
         if (flat->hdr.type == BINDER_TYPE_FD) {
@@ -654,20 +694,24 @@ void Parcel::markForRpc(const sp<RpcSession>& session) {
     LOG_ALWAYS_FATAL_IF(mData != nullptr && mOwner == nullptr,
                         "format must be set before data is written OR on IPC data");
 
-    LOG_ALWAYS_FATAL_IF(session == nullptr, "markForRpc requires session");
-    mSession = session;
+    mVariantFields.emplace<RpcFields>(session);
 }
 
 bool Parcel::isForRpc() const {
-    return mSession != nullptr;
+    return std::holds_alternative<RpcFields>(mVariantFields);
 }
 
 void Parcel::updateWorkSourceRequestHeaderPosition() const {
+    auto* kernelFields = maybeKernelFields();
+    if (kernelFields == nullptr) {
+        return;
+    }
+
     // Only update the request headers once. We only want to point
     // to the first headers read/written.
-    if (!mRequestHeaderPresent) {
-        mWorkSourceRequestHeaderPosition = dataPosition();
-        mRequestHeaderPresent = true;
+    if (!kernelFields->mRequestHeaderPresent) {
+        kernelFields->mWorkSourceRequestHeaderPosition = dataPosition();
+        kernelFields->mRequestHeaderPresent = true;
     }
 }
 
@@ -686,7 +730,7 @@ status_t Parcel::writeInterfaceToken(const String16& interface)
 }
 
 status_t Parcel::writeInterfaceToken(const char16_t* str, size_t len) {
-    if (CC_LIKELY(!isForRpc())) {
+    if (auto* kernelFields = maybeKernelFields()) {
         const IPCThreadState* threadState = IPCThreadState::self();
         writeInt32(threadState->getStrictModePolicy() | STRICT_MODE_PENALTY_GATHER);
         updateWorkSourceRequestHeaderPosition();
@@ -701,12 +745,16 @@ status_t Parcel::writeInterfaceToken(const char16_t* str, size_t len) {
 
 bool Parcel::replaceCallingWorkSourceUid(uid_t uid)
 {
-    if (!mRequestHeaderPresent) {
+    auto* kernelFields = maybeKernelFields();
+    if (kernelFields == nullptr) {
+        return false;
+    }
+    if (!kernelFields->mRequestHeaderPresent) {
         return false;
     }
 
     const size_t initialPosition = dataPosition();
-    setDataPosition(mWorkSourceRequestHeaderPosition);
+    setDataPosition(kernelFields->mWorkSourceRequestHeaderPosition);
     status_t err = writeInt32(uid);
     setDataPosition(initialPosition);
     return err == NO_ERROR;
@@ -714,12 +762,16 @@ bool Parcel::replaceCallingWorkSourceUid(uid_t uid)
 
 uid_t Parcel::readCallingWorkSourceUid() const
 {
-    if (!mRequestHeaderPresent) {
+    auto* kernelFields = maybeKernelFields();
+    if (kernelFields == nullptr) {
+        return false;
+    }
+    if (!kernelFields->mRequestHeaderPresent) {
         return IPCThreadState::kUnsetWorkSource;
     }
 
     const size_t initialPosition = dataPosition();
-    setDataPosition(mWorkSourceRequestHeaderPosition);
+    setDataPosition(kernelFields->mWorkSourceRequestHeaderPosition);
     uid_t uid = readInt32();
     setDataPosition(initialPosition);
     return uid;
@@ -740,7 +792,7 @@ bool Parcel::enforceInterface(const char16_t* interface,
                               size_t len,
                               IPCThreadState* threadState) const
 {
-    if (CC_LIKELY(!isForRpc())) {
+    if (auto* kernelFields = maybeKernelFields()) {
         // StrictModePolicy.
         int32_t strictPolicy = readInt32();
         if (threadState == nullptr) {
@@ -795,7 +847,10 @@ binder::Status Parcel::enforceNoDataAvail() const {
 
 size_t Parcel::objectsCount() const
 {
-    return mObjectsSize;
+    if (const auto* kernelFields = maybeKernelFields()) {
+        return kernelFields->mObjectsSize;
+    }
+    return 0;
 }
 
 status_t Parcel::errorCheck() const
@@ -1401,8 +1456,11 @@ status_t Parcel::write(const FlattenableHelperInterface& val)
 
 status_t Parcel::writeObject(const flat_binder_object& val, bool nullMetaData)
 {
+    auto* kernelFields = maybeKernelFields();
+    LOG_ALWAYS_FATAL_IF(kernelFields == nullptr, "Can't write flat_binder_object to RPC Parcel");
+
     const bool enoughData = (mDataPos+sizeof(val)) <= mDataCapacity;
-    const bool enoughObjects = mObjectsSize < mObjectsCapacity;
+    const bool enoughObjects = kernelFields->mObjectsSize < kernelFields->mObjectsCapacity;
     if (enoughData && enoughObjects) {
 restart_write:
         *reinterpret_cast<flat_binder_object*>(mData+mDataPos) = val;
@@ -1413,14 +1471,14 @@ restart_write:
                 // fail before modifying our object index
                 return FDS_NOT_ALLOWED;
             }
-            mHasFds = mFdsKnown = true;
+            kernelFields->mHasFds = kernelFields->mFdsKnown = true;
         }
 
         // Need to write meta-data?
         if (nullMetaData || val.binder != 0) {
-            mObjects[mObjectsSize] = mDataPos;
+            kernelFields->mObjects[kernelFields->mObjectsSize] = mDataPos;
             acquire_object(ProcessState::self(), val, this);
-            mObjectsSize++;
+            kernelFields->mObjectsSize++;
         }
 
         return finishWrite(sizeof(flat_binder_object));
@@ -1431,14 +1489,15 @@ restart_write:
         if (err != NO_ERROR) return err;
     }
     if (!enoughObjects) {
-        if (mObjectsSize > SIZE_MAX - 2) return NO_MEMORY; // overflow
-        if ((mObjectsSize + 2) > SIZE_MAX / 3) return NO_MEMORY; // overflow
-        size_t newSize = ((mObjectsSize+2)*3)/2;
+        if (kernelFields->mObjectsSize > SIZE_MAX - 2) return NO_MEMORY;       // overflow
+        if ((kernelFields->mObjectsSize + 2) > SIZE_MAX / 3) return NO_MEMORY; // overflow
+        size_t newSize = ((kernelFields->mObjectsSize + 2) * 3) / 2;
         if (newSize > SIZE_MAX / sizeof(binder_size_t)) return NO_MEMORY; // overflow
-        binder_size_t* objects = (binder_size_t*)realloc(mObjects, newSize*sizeof(binder_size_t));
+        binder_size_t* objects =
+                (binder_size_t*)realloc(kernelFields->mObjects, newSize * sizeof(binder_size_t));
         if (objects == nullptr) return NO_MEMORY;
-        mObjects = objects;
-        mObjectsCapacity = newSize;
+        kernelFields->mObjects = objects;
+        kernelFields->mObjectsCapacity = newSize;
     }
 
     goto restart_write;
@@ -1452,54 +1511,64 @@ status_t Parcel::writeNoException()
 
 status_t Parcel::validateReadData(size_t upperBound) const
 {
+    const auto* kernelFields = maybeKernelFields();
+    if (kernelFields == nullptr) {
+        // Can't validate RPC Parcel reads because the location of binder
+        // objects is unknown.
+        return OK;
+    }
+
     // Don't allow non-object reads on object data
-    if (mObjectsSorted || mObjectsSize <= 1) {
-data_sorted:
+    if (kernelFields->mObjectsSorted || kernelFields->mObjectsSize <= 1) {
+    data_sorted:
         // Expect to check only against the next object
-        if (mNextObjectHint < mObjectsSize && upperBound > mObjects[mNextObjectHint]) {
+        if (kernelFields->mNextObjectHint < kernelFields->mObjectsSize &&
+            upperBound > kernelFields->mObjects[kernelFields->mNextObjectHint]) {
             // For some reason the current read position is greater than the next object
             // hint. Iterate until we find the right object
-            size_t nextObject = mNextObjectHint;
+            size_t nextObject = kernelFields->mNextObjectHint;
             do {
-                if (mDataPos < mObjects[nextObject] + sizeof(flat_binder_object)) {
+                if (mDataPos < kernelFields->mObjects[nextObject] + sizeof(flat_binder_object)) {
                     // Requested info overlaps with an object
                     ALOGE("Attempt to read from protected data in Parcel %p", this);
                     return PERMISSION_DENIED;
                 }
                 nextObject++;
-            } while (nextObject < mObjectsSize && upperBound > mObjects[nextObject]);
-            mNextObjectHint = nextObject;
+            } while (nextObject < kernelFields->mObjectsSize &&
+                     upperBound > kernelFields->mObjects[nextObject]);
+            kernelFields->mNextObjectHint = nextObject;
         }
         return NO_ERROR;
     }
     // Quickly determine if mObjects is sorted.
-    binder_size_t* currObj = mObjects + mObjectsSize - 1;
+    binder_size_t* currObj = kernelFields->mObjects + kernelFields->mObjectsSize - 1;
     binder_size_t* prevObj = currObj;
-    while (currObj > mObjects) {
+    while (currObj > kernelFields->mObjects) {
         prevObj--;
         if(*prevObj > *currObj) {
             goto data_unsorted;
         }
         currObj--;
     }
-    mObjectsSorted = true;
+    kernelFields->mObjectsSorted = true;
     goto data_sorted;
 
 data_unsorted:
     // Insertion Sort mObjects
     // Great for mostly sorted lists. If randomly sorted or reverse ordered mObjects become common,
     // switch to std::sort(mObjects, mObjects + mObjectsSize);
-    for (binder_size_t* iter0 = mObjects + 1; iter0 < mObjects + mObjectsSize; iter0++) {
+    for (binder_size_t* iter0 = kernelFields->mObjects + 1;
+         iter0 < kernelFields->mObjects + kernelFields->mObjectsSize; iter0++) {
         binder_size_t temp = *iter0;
         binder_size_t* iter1 = iter0 - 1;
-        while (iter1 >= mObjects && *iter1 > temp) {
+        while (iter1 >= kernelFields->mObjects && *iter1 > temp) {
             *(iter1 + 1) = *iter1;
             iter1--;
         }
         *(iter1 + 1) = temp;
     }
-    mNextObjectHint = 0;
-    mObjectsSorted = true;
+    kernelFields->mNextObjectHint = 0;
+    kernelFields->mObjectsSorted = true;
     goto data_sorted;
 }
 
@@ -1513,7 +1582,8 @@ status_t Parcel::read(void* outData, size_t len) const
 
     if ((mDataPos+pad_size(len)) >= mDataPos && (mDataPos+pad_size(len)) <= mDataSize
             && len <= pad_size(len)) {
-        if (mObjectsSize > 0) {
+        const auto* kernelFields = maybeKernelFields();
+        if (kernelFields != nullptr && kernelFields->mObjectsSize > 0) {
             status_t err = validateReadData(mDataPos + pad_size(len));
             if(err != NO_ERROR) {
                 // Still increment the data position by the expected length
@@ -1540,7 +1610,8 @@ const void* Parcel::readInplace(size_t len) const
 
     if ((mDataPos+pad_size(len)) >= mDataPos && (mDataPos+pad_size(len)) <= mDataSize
             && len <= pad_size(len)) {
-        if (mObjectsSize > 0) {
+        const auto* kernelFields = maybeKernelFields();
+        if (kernelFields != nullptr && kernelFields->mObjectsSize > 0) {
             status_t err = validateReadData(mDataPos + pad_size(len));
             if(err != NO_ERROR) {
                 // Still increment the data position by the expected length
@@ -1587,7 +1658,8 @@ status_t Parcel::readAligned(T *pArg) const {
     static_assert(std::is_trivially_copyable_v<T>);
 
     if ((mDataPos+sizeof(T)) <= mDataSize) {
-        if (mObjectsSize > 0) {
+        const auto* kernelFields = maybeKernelFields();
+        if (kernelFields != nullptr && kernelFields->mObjectsSize > 0) {
             status_t err = validateReadData(mDataPos + sizeof(T));
             if(err != NO_ERROR) {
                 // Still increment the data position by the expected length
@@ -2132,6 +2204,11 @@ status_t Parcel::read(FlattenableHelperInterface& val) const
 }
 const flat_binder_object* Parcel::readObject(bool nullMetaData) const
 {
+    const auto* kernelFields = maybeKernelFields();
+    if (kernelFields == nullptr) {
+        return nullptr;
+    }
+
     const size_t DPOS = mDataPos;
     if ((DPOS+sizeof(flat_binder_object)) <= mDataSize) {
         const flat_binder_object* obj
@@ -2146,9 +2223,9 @@ const flat_binder_object* Parcel::readObject(bool nullMetaData) const
         }
 
         // Ensure that this object is valid...
-        binder_size_t* const OBJS = mObjects;
-        const size_t N = mObjectsSize;
-        size_t opos = mNextObjectHint;
+        binder_size_t* const OBJS = kernelFields->mObjects;
+        const size_t N = kernelFields->mObjectsSize;
+        size_t opos = kernelFields->mNextObjectHint;
 
         if (N > 0) {
             ALOGV("Parcel %p looking for obj at %zu, hint=%zu",
@@ -2167,7 +2244,7 @@ const flat_binder_object* Parcel::readObject(bool nullMetaData) const
                 // Found it!
                 ALOGV("Parcel %p found obj %zu at index %zu with forward search",
                      this, DPOS, opos);
-                mNextObjectHint = opos+1;
+                kernelFields->mNextObjectHint = opos + 1;
                 ALOGV("readObject Setting data pos of %p to %zu", this, mDataPos);
                 return obj;
             }
@@ -2180,7 +2257,7 @@ const flat_binder_object* Parcel::readObject(bool nullMetaData) const
                 // Found it!
                 ALOGV("Parcel %p found obj %zu at index %zu with backward search",
                      this, DPOS, opos);
-                mNextObjectHint = opos+1;
+                kernelFields->mNextObjectHint = opos + 1;
                 ALOGV("readObject Setting data pos of %p to %zu", this, mDataPos);
                 return obj;
             }
@@ -2191,16 +2268,19 @@ const flat_binder_object* Parcel::readObject(bool nullMetaData) const
     return nullptr;
 }
 
-void Parcel::closeFileDescriptors()
-{
-    size_t i = mObjectsSize;
+void Parcel::closeFileDescriptors() {
+    auto* kernelFields = maybeKernelFields();
+    if (kernelFields == nullptr) {
+        return;
+    }
+    size_t i = kernelFields->mObjectsSize;
     if (i > 0) {
         //ALOGI("Closing file descriptors for %zu objects...", i);
     }
     while (i > 0) {
         i--;
-        const flat_binder_object* flat
-            = reinterpret_cast<flat_binder_object*>(mData+mObjects[i]);
+        const flat_binder_object* flat =
+                reinterpret_cast<flat_binder_object*>(mData + kernelFields->mObjects[i]);
         if (flat->hdr.type == BINDER_TYPE_FD) {
             //ALOGI("Closing fd: %ld", flat->handle);
             close(flat->handle);
@@ -2220,35 +2300,43 @@ size_t Parcel::ipcDataSize() const
 
 uintptr_t Parcel::ipcObjects() const
 {
-    return reinterpret_cast<uintptr_t>(mObjects);
+    if (const auto* kernelFields = maybeKernelFields()) {
+        return reinterpret_cast<uintptr_t>(kernelFields->mObjects);
+    }
+    return 0;
 }
 
 size_t Parcel::ipcObjectsCount() const
 {
-    return mObjectsSize;
+    if (const auto* kernelFields = maybeKernelFields()) {
+        return kernelFields->mObjectsSize;
+    }
+    return 0;
 }
 
-void Parcel::ipcSetDataReference(const uint8_t* data, size_t dataSize,
-    const binder_size_t* objects, size_t objectsCount, release_func relFunc)
-{
+void Parcel::ipcSetDataReference(const uint8_t* data, size_t dataSize, const binder_size_t* objects,
+                                 size_t objectsCount, release_func relFunc) {
     // this code uses 'mOwner == nullptr' to understand whether it owns memory
     LOG_ALWAYS_FATAL_IF(relFunc == nullptr, "must provide cleanup function");
 
     freeData();
 
+    auto* kernelFields = maybeKernelFields();
+    LOG_ALWAYS_FATAL_IF(kernelFields == nullptr); // guaranteed by freeData.
+
     mData = const_cast<uint8_t*>(data);
     mDataSize = mDataCapacity = dataSize;
-    mObjects = const_cast<binder_size_t*>(objects);
-    mObjectsSize = mObjectsCapacity = objectsCount;
+    kernelFields->mObjects = const_cast<binder_size_t*>(objects);
+    kernelFields->mObjectsSize = kernelFields->mObjectsCapacity = objectsCount;
     mOwner = relFunc;
 
     binder_size_t minOffset = 0;
-    for (size_t i = 0; i < mObjectsSize; i++) {
-        binder_size_t offset = mObjects[i];
+    for (size_t i = 0; i < kernelFields->mObjectsSize; i++) {
+        binder_size_t offset = kernelFields->mObjects[i];
         if (offset < minOffset) {
             ALOGE("%s: bad object offset %" PRIu64 " < %" PRIu64 "\n",
                   __func__, (uint64_t)offset, (uint64_t)minOffset);
-            mObjectsSize = 0;
+            kernelFields->mObjectsSize = 0;
             break;
         }
         const flat_binder_object* flat
@@ -2266,7 +2354,7 @@ void Parcel::ipcSetDataReference(const uint8_t* data, size_t dataSize,
 
             // WARNING: callers of ipcSetDataReference need to make sure they
             // don't rely on mObjectsSize in their release_func.
-            mObjectsSize = 0;
+            kernelFields->mObjectsSize = 0;
             break;
         }
         minOffset = offset + sizeof(flat_binder_object);
@@ -2274,6 +2362,21 @@ void Parcel::ipcSetDataReference(const uint8_t* data, size_t dataSize,
     scanForFds();
 }
 
+void Parcel::rpcSetDataReference(const sp<RpcSession>& session, const uint8_t* data,
+                                 size_t dataSize, release_func relFunc) {
+    // this code uses 'mOwner == nullptr' to understand whether it owns memory
+    LOG_ALWAYS_FATAL_IF(relFunc == nullptr, "must provide cleanup function");
+
+    LOG_ALWAYS_FATAL_IF(session == nullptr);
+
+    freeData();
+    markForRpc(session);
+
+    mData = const_cast<uint8_t*>(data);
+    mDataSize = mDataCapacity = dataSize;
+    mOwner = relFunc;
+}
+
 void Parcel::print(TextOutput& to, uint32_t /*flags*/) const
 {
     to << "Parcel(";
@@ -2284,14 +2387,16 @@ void Parcel::print(TextOutput& to, uint32_t /*flags*/) const
     } else if (dataSize() > 0) {
         const uint8_t* DATA = data();
         to << indent << HexDump(DATA, dataSize()) << dedent;
-        const binder_size_t* OBJS = mObjects;
-        const size_t N = objectsCount();
-        for (size_t i=0; i<N; i++) {
-            const flat_binder_object* flat
-                = reinterpret_cast<const flat_binder_object*>(DATA+OBJS[i]);
-            to << endl << "Object #" << i << " @ " << (void*)OBJS[i] << ": "
-                << TypeCode(flat->hdr.type & 0x7f7f7f00)
-                << " = " << flat->binder;
+        if (const auto* kernelFields = maybeKernelFields()) {
+            const binder_size_t* OBJS = kernelFields->mObjects;
+            const size_t N = objectsCount();
+            for (size_t i = 0; i < N; i++) {
+                const flat_binder_object* flat =
+                        reinterpret_cast<const flat_binder_object*>(DATA + OBJS[i]);
+                to << endl
+                   << "Object #" << i << " @ " << (void*)OBJS[i] << ": "
+                   << TypeCode(flat->hdr.type & 0x7f7f7f00) << " = " << flat->binder;
+            }
         }
     } else {
         to << "NULL";
@@ -2302,34 +2407,42 @@ void Parcel::print(TextOutput& to, uint32_t /*flags*/) const
 
 void Parcel::releaseObjects()
 {
-    size_t i = mObjectsSize;
+    auto* kernelFields = maybeKernelFields();
+    if (kernelFields == nullptr) {
+        return;
+    }
+
+    size_t i = kernelFields->mObjectsSize;
     if (i == 0) {
         return;
     }
     sp<ProcessState> proc(ProcessState::self());
     uint8_t* const data = mData;
-    binder_size_t* const objects = mObjects;
+    binder_size_t* const objects = kernelFields->mObjects;
     while (i > 0) {
         i--;
-        const flat_binder_object* flat
-            = reinterpret_cast<flat_binder_object*>(data+objects[i]);
+        const flat_binder_object* flat = reinterpret_cast<flat_binder_object*>(data + objects[i]);
         release_object(proc, *flat, this);
     }
 }
 
 void Parcel::acquireObjects()
 {
-    size_t i = mObjectsSize;
+    auto* kernelFields = maybeKernelFields();
+    if (kernelFields == nullptr) {
+        return;
+    }
+
+    size_t i = kernelFields->mObjectsSize;
     if (i == 0) {
         return;
     }
     const sp<ProcessState> proc(ProcessState::self());
     uint8_t* const data = mData;
-    binder_size_t* const objects = mObjects;
+    binder_size_t* const objects = kernelFields->mObjects;
     while (i > 0) {
         i--;
-        const flat_binder_object* flat
-            = reinterpret_cast<flat_binder_object*>(data+objects[i]);
+        const flat_binder_object* flat = reinterpret_cast<flat_binder_object*>(data + objects[i]);
         acquire_object(proc, *flat, this);
     }
 }
@@ -2345,7 +2458,9 @@ void Parcel::freeDataNoInit()
     if (mOwner) {
         LOG_ALLOC("Parcel %p: freeing other owner data", this);
         //ALOGI("Freeing data ref of %p (pid=%d)", this, getpid());
-        mOwner(this, mData, mDataSize, mObjects, mObjectsSize);
+        auto* kernelFields = maybeKernelFields();
+        mOwner(this, mData, mDataSize, kernelFields ? kernelFields->mObjects : nullptr,
+               kernelFields ? kernelFields->mObjectsSize : 0);
     } else {
         LOG_ALLOC("Parcel %p: freeing allocated data", this);
         releaseObjects();
@@ -2358,7 +2473,8 @@ void Parcel::freeDataNoInit()
             }
             free(mData);
         }
-        if (mObjects) free(mObjects);
+        auto* kernelFields = maybeKernelFields();
+        if (kernelFields && kernelFields->mObjects) free(kernelFields->mObjects);
     }
 }
 
@@ -2433,13 +2549,15 @@ status_t Parcel::restartWrite(size_t desired)
     ALOGV("restartWrite Setting data size of %p to %zu", this, mDataSize);
     ALOGV("restartWrite Setting data pos of %p to %zu", this, mDataPos);
 
-    free(mObjects);
-    mObjects = nullptr;
-    mObjectsSize = mObjectsCapacity = 0;
-    mNextObjectHint = 0;
-    mObjectsSorted = false;
-    mHasFds = false;
-    mFdsKnown = true;
+    if (auto* kernelFields = maybeKernelFields()) {
+        free(kernelFields->mObjects);
+        kernelFields->mObjects = nullptr;
+        kernelFields->mObjectsSize = kernelFields->mObjectsCapacity = 0;
+        kernelFields->mNextObjectHint = 0;
+        kernelFields->mObjectsSorted = false;
+        kernelFields->mHasFds = false;
+        kernelFields->mFdsKnown = true;
+    }
     mAllowFds = true;
 
     return NO_ERROR;
@@ -2453,16 +2571,17 @@ status_t Parcel::continueWrite(size_t desired)
         return BAD_VALUE;
     }
 
+    auto* kernelFields = maybeKernelFields();
+
     // If shrinking, first adjust for any objects that appear
     // after the new data size.
-    size_t objectsSize = mObjectsSize;
-    if (desired < mDataSize) {
+    size_t objectsSize = kernelFields ? kernelFields->mObjectsSize : 0;
+    if (kernelFields && desired < mDataSize) {
         if (desired == 0) {
             objectsSize = 0;
         } else {
             while (objectsSize > 0) {
-                if (mObjects[objectsSize-1] < desired)
-                    break;
+                if (kernelFields->mObjects[objectsSize - 1] < desired) break;
                 objectsSize--;
             }
         }
@@ -2495,20 +2614,21 @@ status_t Parcel::continueWrite(size_t desired)
 
             // Little hack to only acquire references on objects
             // we will be keeping.
-            size_t oldObjectsSize = mObjectsSize;
-            mObjectsSize = objectsSize;
+            size_t oldObjectsSize = kernelFields->mObjectsSize;
+            kernelFields->mObjectsSize = objectsSize;
             acquireObjects();
-            mObjectsSize = oldObjectsSize;
+            kernelFields->mObjectsSize = oldObjectsSize;
         }
 
         if (mData) {
             memcpy(data, mData, mDataSize < desired ? mDataSize : desired);
         }
-        if (objects && mObjects) {
-            memcpy(objects, mObjects, objectsSize*sizeof(binder_size_t));
+        if (objects && kernelFields && kernelFields->mObjects) {
+            memcpy(objects, kernelFields->mObjects, objectsSize * sizeof(binder_size_t));
         }
         //ALOGI("Freeing data ref of %p (pid=%d)", this, getpid());
-        mOwner(this, mData, mDataSize, mObjects, mObjectsSize);
+        mOwner(this, mData, mDataSize, kernelFields ? kernelFields->mObjects : nullptr,
+               kernelFields ? kernelFields->mObjectsSize : 0);
         mOwner = nullptr;
 
         LOG_ALLOC("Parcel %p: taking ownership of %zu capacity", this, desired);
@@ -2516,43 +2636,46 @@ status_t Parcel::continueWrite(size_t desired)
         gParcelGlobalAllocCount++;
 
         mData = data;
-        mObjects = objects;
         mDataSize = (mDataSize < desired) ? mDataSize : desired;
         ALOGV("continueWrite Setting data size of %p to %zu", this, mDataSize);
         mDataCapacity = desired;
-        mObjectsSize = mObjectsCapacity = objectsSize;
-        mNextObjectHint = 0;
-        mObjectsSorted = false;
+        if (kernelFields) {
+            kernelFields->mObjects = objects;
+            kernelFields->mObjectsSize = kernelFields->mObjectsCapacity = objectsSize;
+            kernelFields->mNextObjectHint = 0;
+            kernelFields->mObjectsSorted = false;
+        }
 
     } else if (mData) {
-        if (objectsSize < mObjectsSize) {
+        if (kernelFields && objectsSize < kernelFields->mObjectsSize) {
             // Need to release refs on any objects we are dropping.
             const sp<ProcessState> proc(ProcessState::self());
-            for (size_t i=objectsSize; i<mObjectsSize; i++) {
-                const flat_binder_object* flat
-                    = reinterpret_cast<flat_binder_object*>(mData+mObjects[i]);
+            for (size_t i = objectsSize; i < kernelFields->mObjectsSize; i++) {
+                const flat_binder_object* flat =
+                        reinterpret_cast<flat_binder_object*>(mData + kernelFields->mObjects[i]);
                 if (flat->hdr.type == BINDER_TYPE_FD) {
                     // will need to rescan because we may have lopped off the only FDs
-                    mFdsKnown = false;
+                    kernelFields->mFdsKnown = false;
                 }
                 release_object(proc, *flat, this);
             }
 
             if (objectsSize == 0) {
-                free(mObjects);
-                mObjects = nullptr;
-                mObjectsCapacity = 0;
+                free(kernelFields->mObjects);
+                kernelFields->mObjects = nullptr;
+                kernelFields->mObjectsCapacity = 0;
             } else {
                 binder_size_t* objects =
-                    (binder_size_t*)realloc(mObjects, objectsSize*sizeof(binder_size_t));
+                        (binder_size_t*)realloc(kernelFields->mObjects,
+                                                objectsSize * sizeof(binder_size_t));
                 if (objects) {
-                    mObjects = objects;
-                    mObjectsCapacity = objectsSize;
+                    kernelFields->mObjects = objects;
+                    kernelFields->mObjectsCapacity = objectsSize;
                 }
             }
-            mObjectsSize = objectsSize;
-            mNextObjectHint = 0;
-            mObjectsSorted = false;
+            kernelFields->mObjectsSize = objectsSize;
+            kernelFields->mNextObjectHint = 0;
+            kernelFields->mObjectsSorted = false;
         }
 
         // We own the data, so we can just do a realloc().
@@ -2588,9 +2711,12 @@ status_t Parcel::continueWrite(size_t desired)
             return NO_MEMORY;
         }
 
-        if(!(mDataCapacity == 0 && mObjects == nullptr
-             && mObjectsCapacity == 0)) {
-            ALOGE("continueWrite: %zu/%p/%zu/%zu", mDataCapacity, mObjects, mObjectsCapacity, desired);
+        if (!(mDataCapacity == 0 &&
+              (kernelFields == nullptr ||
+               (kernelFields->mObjects == nullptr && kernelFields->mObjectsCapacity == 0)))) {
+            ALOGE("continueWrite: %zu/%p/%zu/%zu", mDataCapacity,
+                  kernelFields ? kernelFields->mObjects : nullptr,
+                  kernelFields ? kernelFields->mObjectsCapacity : 0, desired);
         }
 
         LOG_ALLOC("Parcel %p: allocating with %zu capacity", this, desired);
@@ -2617,19 +2743,10 @@ void Parcel::initState()
     mDataPos = 0;
     ALOGV("initState Setting data size of %p to %zu", this, mDataSize);
     ALOGV("initState Setting data pos of %p to %zu", this, mDataPos);
-    mSession = nullptr;
-    mObjects = nullptr;
-    mObjectsSize = 0;
-    mObjectsCapacity = 0;
-    mNextObjectHint = 0;
-    mObjectsSorted = false;
-    mHasFds = false;
-    mFdsKnown = true;
+    mVariantFields.emplace<KernelFields>();
     mAllowFds = true;
     mDeallocZero = false;
     mOwner = nullptr;
-    mWorkSourceRequestHeaderPosition = 0;
-    mRequestHeaderPresent = false;
 
     // racing multiple init leads only to multiple identical write
     if (gMaxFds == 0) {
@@ -2645,9 +2762,13 @@ void Parcel::initState()
 }
 
 void Parcel::scanForFds() const {
-    status_t status = hasFileDescriptorsInRange(0, dataSize(), &mHasFds);
+    auto* kernelFields = maybeKernelFields();
+    if (kernelFields == nullptr) {
+        return;
+    }
+    status_t status = hasFileDescriptorsInRange(0, dataSize(), &kernelFields->mHasFds);
     ALOGE_IF(status != NO_ERROR, "Error %d calling hasFileDescriptorsInRange()", status);
-    mFdsKnown = true;
+    kernelFields->mFdsKnown = true;
 }
 
 size_t Parcel::getBlobAshmemSize() const
@@ -2660,10 +2781,15 @@ size_t Parcel::getBlobAshmemSize() const
 
 size_t Parcel::getOpenAshmemSize() const
 {
+    auto* kernelFields = maybeKernelFields();
+    if (kernelFields == nullptr) {
+        return 0;
+    }
+
     size_t openAshmemSize = 0;
-    for (size_t i = 0; i < mObjectsSize; i++) {
+    for (size_t i = 0; i < kernelFields->mObjectsSize; i++) {
         const flat_binder_object* flat =
-                reinterpret_cast<const flat_binder_object*>(mData + mObjects[i]);
+                reinterpret_cast<const flat_binder_object*>(mData + kernelFields->mObjects[i]);
 
         // cookie is compared against zero for historical reasons
         // > obj.cookie = takeOwnership ? 1 : 0;
diff --git a/libs/binder/RpcState.cpp b/libs/binder/RpcState.cpp
index 2a8e9c1d8b..c6460b8330 100644
--- a/libs/binder/RpcState.cpp
+++ b/libs/binder/RpcState.cpp
@@ -601,10 +601,8 @@ status_t RpcState::waitForReply(const sp<RpcSession::RpcConnection>& connection,
     if (rpcReply->status != OK) return rpcReply->status;
 
     data.release();
-    reply->ipcSetDataReference(rpcReply->data, command.bodySize - offsetof(RpcWireReply, data),
-                               nullptr, 0, cleanup_reply_data);
-
-    reply->markForRpc(session);
+    reply->rpcSetDataReference(session, rpcReply->data,
+                               command.bodySize - offsetof(RpcWireReply, data), cleanup_reply_data);
 
     return OK;
 }
@@ -824,11 +822,9 @@ processTransactInternalTailCall:
         // transaction->data is owned by this function. Parcel borrows this data and
         // only holds onto it for the duration of this function call. Parcel will be
         // deleted before the 'transactionData' object.
-        data.ipcSetDataReference(transaction->data,
+        data.rpcSetDataReference(session, transaction->data,
                                  transactionData.size() - offsetof(RpcWireTransaction, data),
-                                 nullptr /*object*/, 0 /*objectCount*/,
                                  do_nothing_to_transact_data);
-        data.markForRpc(session);
 
         if (target) {
             bool origAllowNested = connection->allowNested;
diff --git a/libs/binder/include/binder/Parcel.h b/libs/binder/include/binder/Parcel.h
index 39d0c9068f..0345a5d8e1 100644
--- a/libs/binder/include/binder/Parcel.h
+++ b/libs/binder/include/binder/Parcel.h
@@ -20,6 +20,7 @@
 #include <map> // for legacy reasons
 #include <string>
 #include <type_traits>
+#include <variant>
 #include <vector>
 
 #include <android-base/unique_fd.h>
@@ -605,15 +606,19 @@ private:
     size_t              ipcDataSize() const;
     uintptr_t           ipcObjects() const;
     size_t              ipcObjectsCount() const;
-    void                ipcSetDataReference(const uint8_t* data, size_t dataSize,
-                                            const binder_size_t* objects, size_t objectsCount,
-                                            release_func relFunc);
+    void ipcSetDataReference(const uint8_t* data, size_t dataSize, const binder_size_t* objects,
+                             size_t objectsCount, release_func relFunc);
+    void rpcSetDataReference(const sp<RpcSession>& session, const uint8_t* data, size_t dataSize,
+                             release_func relFunc);
 
     status_t            finishWrite(size_t len);
     void                releaseObjects();
     void                acquireObjects();
     status_t            growData(size_t len);
+    // Clear the Parcel and set the capacity to `desired`.
+    // Doesn't reset the RPC session association.
     status_t            restartWrite(size_t desired);
+    // Set the capacity to `desired`, truncating the Parcel if necessary.
     status_t            continueWrite(size_t desired);
     status_t            writePointer(uintptr_t val);
     status_t            readPointer(uintptr_t *pArg) const;
@@ -1252,19 +1257,40 @@ private:
     uint8_t*            mData;
     size_t              mDataSize;
     size_t              mDataCapacity;
-    mutable size_t      mDataPos;
-    binder_size_t*      mObjects;
-    size_t              mObjectsSize;
-    size_t              mObjectsCapacity;
-    mutable size_t      mNextObjectHint;
-    mutable bool        mObjectsSorted;
+    mutable size_t mDataPos;
 
-    mutable bool        mRequestHeaderPresent;
+    // Fields only needed when parcelling for "kernel Binder".
+    struct KernelFields {
+        binder_size_t* mObjects = nullptr;
+        size_t mObjectsSize = 0;
+        size_t mObjectsCapacity = 0;
+        mutable size_t mNextObjectHint = 0;
 
-    mutable size_t      mWorkSourceRequestHeaderPosition;
+        mutable size_t mWorkSourceRequestHeaderPosition = 0;
+        mutable bool mRequestHeaderPresent = false;
+
+        mutable bool mObjectsSorted = false;
+        mutable bool mFdsKnown = true;
+        mutable bool mHasFds = false;
+    };
+    // Fields only needed when parcelling for RPC Binder.
+    struct RpcFields {
+        RpcFields(const sp<RpcSession>& session);
+
+        // Should always be non-null.
+        const sp<RpcSession> mSession;
+    };
+    std::variant<KernelFields, RpcFields> mVariantFields;
+
+    // Pointer to KernelFields in mVariantFields if present.
+    KernelFields* maybeKernelFields() { return std::get_if<KernelFields>(&mVariantFields); }
+    const KernelFields* maybeKernelFields() const {
+        return std::get_if<KernelFields>(&mVariantFields);
+    }
+    // Pointer to RpcFields in mVariantFields if present.
+    RpcFields* maybeRpcFields() { return std::get_if<RpcFields>(&mVariantFields); }
+    const RpcFields* maybeRpcFields() const { return std::get_if<RpcFields>(&mVariantFields); }
 
-    mutable bool        mFdsKnown;
-    mutable bool        mHasFds;
     bool                mAllowFds;
 
     // if this parcelable is involved in a secure transaction, force the
@@ -1273,7 +1299,6 @@ private:
 
     release_func        mOwner;
 
-    sp<RpcSession> mSession;
     size_t mReserved;
 
     class Blob {