/*-
 * SPDX-License-Identifier: BSD-2-Clause
 *
 * Copyright (c) 1999 Assar Westerlund
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <sys/param.h>
#include <sys/kernel.h>
#include <sys/lock.h>
#include <sys/module.h>
#include <sys/mutex.h>
#include <sys/proc.h>
#include <sys/resourcevar.h>
#include <sys/sx.h>
#include <sys/syscall.h>
#include <sys/syscallsubr.h>
#include <sys/sysent.h>
#include <sys/sysproto.h>
#include <sys/systm.h>
#include <machine/atomic.h>

/*
 * Acts like "nosys" but can be identified in sysent for dynamic call
 * number assignment for a limited number of calls.
 *
 * Place holder for system call slots reserved for loadable modules.
 */
int
lkmnosys(struct thread *td, struct nosys_args *args)
{

        return (kern_nosys(td, 0));
}

int
lkmressys(struct thread *td, struct nosys_args *args)
{

        return (kern_nosys(td, 0));
}

struct sysent nosys_sysent = {
        .sy_call =      (sy_call_t *)nosys,
        .sy_systrace_args_func = NULL,
        .sy_narg =      0,
        .sy_flags =     SYF_CAPENABLED,
        .sy_auevent =   AUE_NULL,
        .sy_entry =     0, /* DTRACE_IDNONE */
        .sy_return =    0,
        .sy_thrcnt =    SY_THR_STATIC,
};

static void
syscall_thread_drain(struct sysent *se)
{
        uint32_t cnt, oldcnt;

        do {
                oldcnt = se->sy_thrcnt;
                KASSERT((oldcnt & SY_THR_STATIC) == 0,
                    ("drain on static syscall"));
                cnt = oldcnt | SY_THR_DRAINING;
        } while (atomic_cmpset_acq_32(&se->sy_thrcnt, oldcnt, cnt) == 0);
        while (atomic_cmpset_32(&se->sy_thrcnt, SY_THR_DRAINING,
            SY_THR_ABSENT) == 0)
                pause("scdrn", hz/2);
}

void
syscall_thread_enter(struct thread *td, struct sysent **se)
{
        uint32_t cnt, oldcnt;

        KASSERT(((*se)->sy_thrcnt & SY_THR_STATIC) == 0,
            ("%s: not a static syscall", __func__));

        do {
                oldcnt = (*se)->sy_thrcnt;
                if ((oldcnt & (SY_THR_DRAINING | SY_THR_ABSENT)) != 0) {
                        *se = &nosys_sysent;
                        break;
                }
                cnt = oldcnt + SY_THR_INCR;
        } while (atomic_cmpset_acq_32(&(*se)->sy_thrcnt, oldcnt, cnt) == 0);
}

void
syscall_thread_exit(struct thread *td, struct sysent *se)
{
        uint32_t cnt, oldcnt;

        KASSERT((se->sy_thrcnt & SY_THR_STATIC) == 0,
            ("%s: not a static syscall", __func__));

        do {
                oldcnt = se->sy_thrcnt;
                cnt = oldcnt - SY_THR_INCR;
        } while (atomic_cmpset_rel_32(&se->sy_thrcnt, oldcnt, cnt) == 0);
}

int
kern_syscall_register(struct sysent *sysents, int *offset,
    struct sysent *new_sysent, struct sysent *old_sysent, int flags)
{
        int i;

        if ((flags & ~SY_THR_STATIC) != 0)
                return (EINVAL);

        if (*offset == NO_SYSCALL) {
                for (i = 1; i < SYS_MAXSYSCALL; ++i)
                        if (sysents[i].sy_call == (sy_call_t *)lkmnosys)
                                break;
                if (i == SYS_MAXSYSCALL)
                        return (ENFILE);
                *offset = i;
        } else if (*offset < 0 || *offset >= SYS_MAXSYSCALL) {
                return (EINVAL);
        } else if (sysents[*offset].sy_call != (sy_call_t *)lkmnosys &&
            sysents[*offset].sy_call != (sy_call_t *)lkmressys) {
                KASSERT(sysents[*offset].sy_call != NULL,
                    ("undefined syscall %d", *offset));
                return (EEXIST);
        }

        KASSERT(sysents[*offset].sy_thrcnt == SY_THR_ABSENT,
            ("dynamic syscall is not protected"));
        *old_sysent = sysents[*offset];
        new_sysent->sy_thrcnt = SY_THR_ABSENT;
        sysents[*offset] = *new_sysent;
        atomic_store_rel_32(&sysents[*offset].sy_thrcnt, flags);
        return (0);
}

int
kern_syscall_deregister(struct sysent *sysents, int offset,
    const struct sysent *old_sysent)
{
        struct sysent *se;

        if (offset == 0) {
                /*
                 * Syscall #0 is reserved and is not dynamically registered.
                 * Treat deregistration as a no-op to simplify module unload
                 * paths.
                 */
                return (0);
        }

        se = &sysents[offset];
        if ((se->sy_thrcnt & SY_THR_STATIC) != 0)
                return (EINVAL);
        syscall_thread_drain(se);
        sysents[offset] = *old_sysent;
        return (0);
}

int
syscall_module_handler(struct module *mod, int what, void *arg)
{

        return (kern_syscall_module_handler(sysent, mod, what, arg));
}

int
kern_syscall_module_handler(struct sysent *sysents, struct module *mod,
    int what, void *arg)
{
        struct syscall_module_data *data = arg;
        modspecific_t ms;
        int error;

        bzero(&ms, sizeof(ms));
        switch (what) {
        case MOD_LOAD:
                error = kern_syscall_register(sysents, data->offset,
                    data->new_sysent, &data->old_sysent, data->flags);
                if (error) {
                        /* Leave a mark so we know to safely unload below. */
                        data->offset = NULL;
                        return (error);
                }
                ms.intval = *data->offset;
                MOD_XLOCK;
                module_setspecific(mod, &ms);
                MOD_XUNLOCK;
                if (data->chainevh)
                        error = data->chainevh(mod, what, data->chainarg);
                return (error);
        case MOD_UNLOAD:
                /*
                 * MOD_LOAD failed, so just return without calling the
                 * chained handler since we didn't pass along the MOD_LOAD
                 * event.
                 */
                if (data->offset == NULL)
                        return (0);
                if (data->chainevh) {
                        error = data->chainevh(mod, what, data->chainarg);
                        if (error)
                                return error;
                }
                error = kern_syscall_deregister(sysents, *data->offset,
                    &data->old_sysent);
                return (error);
        default:
                if (data->chainevh)
                        return (data->chainevh(mod, what, data->chainarg));
                return (EOPNOTSUPP);
        }

        /* NOTREACHED */
}

int
syscall_helper_register(struct syscall_helper_data *sd, int flags)
{

        return (kern_syscall_helper_register(sysent, sd, flags));
}

int
kern_syscall_helper_register(struct sysent *sysents,
    struct syscall_helper_data *sd, int flags)
{
        struct syscall_helper_data *sd1;
        int error;

        for (sd1 = sd; sd1->syscall_no != NO_SYSCALL; sd1++) {
                error = kern_syscall_register(sysents, &sd1->syscall_no,
                    &sd1->new_sysent, &sd1->old_sysent, flags);
                if (error != 0) {
                        kern_syscall_helper_unregister(sysents, sd);
                        return (error);
                }
                sd1->registered = 1;
        }
        return (0);
}

int
syscall_helper_unregister(struct syscall_helper_data *sd)
{

        return (kern_syscall_helper_unregister(sysent, sd));
}

int
kern_syscall_helper_unregister(struct sysent *sysents,
    struct syscall_helper_data *sd)
{
        struct syscall_helper_data *sd1;

        for (sd1 = sd; sd1->registered != 0; sd1++) {
                kern_syscall_deregister(sysents, sd1->syscall_no,
                    &sd1->old_sysent);
                sd1->registered = 0;
        }
        return (0);
}