serenity/Kernel/Syscalls/futex.cpp

/*
 * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/Singleton.h>
#include <Kernel/Debug.h>
#include <Kernel/Memory/MemoryManager.h>
#include <Kernel/Process.h>

namespace Kernel {

void Process::clear_futex_queues_on_exec()
{
    SpinlockLocker lock(m_futex_lock);
    for (auto& it : m_futex_queues) {
        bool did_wake_all;
        it.value->wake_all(did_wake_all);
        VERIFY(did_wake_all); // No one should be left behind...
    }
    m_futex_queues.clear();
}

ErrorOr<FlatPtr> Process::sys$futex(Userspace<Syscall::SC_futex_params const*> user_params)
{
    VERIFY_PROCESS_BIG_LOCK_ACQUIRED(this);
    auto params = TRY(copy_typed_from_user(user_params));

    Thread::BlockTimeout timeout;
    u32 cmd = params.futex_op & FUTEX_CMD_MASK;

    bool use_realtime_clock = (params.futex_op & FUTEX_CLOCK_REALTIME) != 0;
    if (use_realtime_clock && cmd != FUTEX_WAIT && cmd != FUTEX_WAIT_BITSET) {
        return ENOSYS;
    }

    switch (cmd) {
    case FUTEX_WAIT:
    case FUTEX_WAIT_BITSET:
    case FUTEX_REQUEUE:
    case FUTEX_CMP_REQUEUE: {
        if (params.timeout) {
            auto timeout_time = TRY(copy_time_from_user(params.timeout));
            bool is_absolute = cmd != FUTEX_WAIT;
            clockid_t clock_id = use_realtime_clock ? CLOCK_REALTIME_COARSE : CLOCK_MONOTONIC_COARSE;
            timeout = Thread::BlockTimeout(is_absolute, &timeout_time, nullptr, clock_id);
        }
        if (cmd == FUTEX_WAIT_BITSET && params.val3 == FUTEX_BITSET_MATCH_ANY)
            cmd = FUTEX_WAIT;
        break;
    case FUTEX_WAKE_BITSET:
        if (params.val3 == FUTEX_BITSET_MATCH_ANY)
            cmd = FUTEX_WAKE;
        break;
    }
    }

    auto find_futex_queue = [&](FlatPtr user_address, bool create_if_not_found, bool* did_create = nullptr) -> RefPtr<FutexQueue> {
        VERIFY(!create_if_not_found || did_create != nullptr);
        auto* queues = &m_futex_queues;
        auto it = m_futex_queues.find(user_address);
        if (it != m_futex_queues.end())
            return it->value;
        if (create_if_not_found) {
            *did_create = true;
            auto futex_queue = adopt_ref(*new FutexQueue);
            auto result = queues->set(user_address, futex_queue);
            VERIFY(result == AK::HashSetResult::InsertedNewEntry);
            return futex_queue;
        }
        return {};
    };

    auto remove_futex_queue = [&](FlatPtr user_address) {
        if (auto it = m_futex_queues.find(user_address); it != m_futex_queues.end()) {
            if (it->value->try_remove()) {
                m_futex_queues.remove(it);
            }
        }
    };

    auto do_wake = [&](FlatPtr user_address, u32 count, Optional<u32> bitmask) -> int {
        if (count == 0)
            return 0;
        SpinlockLocker locker(m_futex_lock);
        auto futex_queue = find_futex_queue(user_address, false);
        if (!futex_queue)
            return 0;
        bool is_empty;
        u32 woke_count = futex_queue->wake_n(count, bitmask, is_empty);
        if (is_empty) {
            // If there are no more waiters, we want to get rid of the futex!
            remove_futex_queue(user_address);
        }
        return (int)woke_count;
    };

    auto user_address = FlatPtr(params.userspace_address);
    auto user_address2 = FlatPtr(params.userspace_address2);

    auto do_wait = [&](u32 bitset) -> ErrorOr<FlatPtr> {
        bool did_create;
        RefPtr<FutexQueue> futex_queue;
        do {
            auto user_value = user_atomic_load_relaxed(params.userspace_address);
            if (!user_value.has_value())
                return EFAULT;
            if (user_value.value() != params.val) {
                dbgln_if(FUTEX_DEBUG, "futex wait: EAGAIN. user value: {:p} @ {:p} != val: {}", user_value.value(), params.userspace_address, params.val);
                return EAGAIN;
            }
            atomic_thread_fence(AK::MemoryOrder::memory_order_acquire);

            SpinlockLocker locker(m_futex_lock);
            did_create = false;
            futex_queue = find_futex_queue(user_address, true, &did_create);
            VERIFY(futex_queue);
            // We need to try again if we didn't create this queue and the existing queue
            // was removed before we were able to queue an imminent wait.
        } while (!did_create && !futex_queue->queue_imminent_wait());

        // We must not hold the lock before blocking. But we have a reference
        // to the FutexQueue so that we can keep it alive.

        Thread::BlockResult block_result = futex_queue->wait_on(timeout, bitset);

        SpinlockLocker locker(m_futex_lock);
        if (futex_queue->is_empty_and_no_imminent_waits()) {
            // If there are no more waiters, we want to get rid of the futex!
            remove_futex_queue(user_address);
        }
        if (block_result == Thread::BlockResult::InterruptedByTimeout) {
            return ETIMEDOUT;
        }
        return 0;
    };

    auto do_requeue = [&](Optional<u32> val3) -> ErrorOr<FlatPtr> {
        auto user_value = user_atomic_load_relaxed(params.userspace_address);
        if (!user_value.has_value())
            return EFAULT;
        if (val3.has_value() && val3.value() != user_value.value())
            return EAGAIN;
        atomic_thread_fence(AK::MemoryOrder::memory_order_acquire);

        int woken_or_requeued = 0;
        SpinlockLocker locker(m_futex_lock);
        if (auto futex_queue = find_futex_queue(user_address, false)) {
            RefPtr<FutexQueue> target_futex_queue;
            bool is_empty, is_target_empty;
            woken_or_requeued = futex_queue->wake_n_requeue(
                params.val, [&]() -> FutexQueue* {
                    // NOTE: futex_queue's lock is being held while this callback is called
                    // The reason we're doing this in a callback is that we don't want to always
                    // create a target queue, only if we actually have anything to move to it!
                    target_futex_queue = find_futex_queue(user_address2, true);
                    return target_futex_queue.ptr();
                },
                params.val2, is_empty, is_target_empty);
            if (is_empty)
                remove_futex_queue(user_address);
            if (is_target_empty && target_futex_queue)
                remove_futex_queue(user_address2);
        }
        return woken_or_requeued;
    };

    switch (cmd) {
    case FUTEX_WAIT:
        return do_wait(0);

    case FUTEX_WAKE:
        return do_wake(user_address, params.val, {});

    case FUTEX_WAKE_OP: {
        Optional<u32> oldval;
        u32 op_arg = _FUTEX_OP_ARG(params.val3);
        auto op = _FUTEX_OP(params.val3);
        if (op & FUTEX_OP_ARG_SHIFT) {
            op_arg = 1 << op_arg;
            op &= FUTEX_OP_ARG_SHIFT;
        }
        atomic_thread_fence(AK::MemoryOrder::memory_order_release);
        switch (op) {
        case FUTEX_OP_SET:
            oldval = user_atomic_exchange_relaxed(params.userspace_address2, op_arg);
            break;
        case FUTEX_OP_ADD:
            oldval = user_atomic_fetch_add_relaxed(params.userspace_address2, op_arg);
            break;
        case FUTEX_OP_OR:
            oldval = user_atomic_fetch_or_relaxed(params.userspace_address2, op_arg);
            break;
        case FUTEX_OP_ANDN:
            oldval = user_atomic_fetch_and_not_relaxed(params.userspace_address2, op_arg);
            break;
        case FUTEX_OP_XOR:
            oldval = user_atomic_fetch_xor_relaxed(params.userspace_address2, op_arg);
            break;
        default:
            return EINVAL;
        }
        if (!oldval.has_value())
            return EFAULT;
        atomic_thread_fence(AK::MemoryOrder::memory_order_acquire);
        auto result = do_wake(user_address, params.val, {});
        if (params.val2 > 0) {
            bool compare_result;
            switch (_FUTEX_CMP(params.val3)) {
            case FUTEX_OP_CMP_EQ:
                compare_result = (oldval.value() == _FUTEX_CMP_ARG(params.val3));
                break;
            case FUTEX_OP_CMP_NE:
                compare_result = (oldval.value() != _FUTEX_CMP_ARG(params.val3));
                break;
            case FUTEX_OP_CMP_LT:
                compare_result = (oldval.value() < _FUTEX_CMP_ARG(params.val3));
                break;
            case FUTEX_OP_CMP_LE:
                compare_result = (oldval.value() <= _FUTEX_CMP_ARG(params.val3));
                break;
            case FUTEX_OP_CMP_GT:
                compare_result = (oldval.value() > _FUTEX_CMP_ARG(params.val3));
                break;
            case FUTEX_OP_CMP_GE:
                compare_result = (oldval.value() >= _FUTEX_CMP_ARG(params.val3));
                break;
            default:
                return EINVAL;
            }
            if (compare_result)
                result += do_wake(user_address2, params.val2, {});
        }
        return result;
    }

    case FUTEX_REQUEUE:
        return do_requeue({});

    case FUTEX_CMP_REQUEUE:
        return do_requeue(params.val3);

    case FUTEX_WAIT_BITSET:
        VERIFY(params.val3 != FUTEX_BITSET_MATCH_ANY); // we should have turned it into FUTEX_WAIT
        if (params.val3 == 0)
            return EINVAL;
        return do_wait(params.val3);

    case FUTEX_WAKE_BITSET:
        VERIFY(params.val3 != FUTEX_BITSET_MATCH_ANY); // we should have turned it into FUTEX_WAKE
        if (params.val3 == 0)
            return EINVAL;
        return do_wake(user_address, params.val, params.val3);
    }
    return ENOSYS;
}

}