/*-
 * SPDX-License-Identifier: BSD-2-Clause
 *
 * Copyright (c) 2003 Marcel Moolenaar
 * 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 ``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 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/types.h>
#include <sys/ucontext.h>
#include <stdarg.h>
#include <stdlib.h>

typedef void (*func_t)(uint64_t, uint64_t, uint64_t, uint64_t, uint64_t,
    uint64_t);

/* Prototypes */
static void makectx_wrapper(ucontext_t *ucp, func_t func, uint64_t *args);

__weak_reference(__makecontext, makecontext);

void
__makecontext(ucontext_t *ucp, void (*start)(void), int argc, ...)
{
        uint64_t *args;
        uint64_t *sp;
        va_list ap;
        int i;

        /* A valid context is required. */
        if ((ucp == NULL) || (ucp->uc_mcontext.mc_len != sizeof(mcontext_t)))
                return;
        else if ((argc < 0) || (argc > 6) || (ucp->uc_stack.ss_sp == NULL) ||
            (ucp->uc_stack.ss_size < MINSIGSTKSZ)) {
                /*
                 * This should really return -1 with errno set to ENOMEM
                 * or something, but the spec says that makecontext is
                 * a void function.   At least make sure that the context
                 * isn't valid so it can't be used without an error.
                 */
                ucp->uc_mcontext.mc_len = 0;
                return;
        }

        /* Align the stack to 16 bytes. */
        sp = (uint64_t *)(ucp->uc_stack.ss_sp + ucp->uc_stack.ss_size);
        sp = (uint64_t *)((uint64_t)sp & ~15UL);

        /* Allocate space for a maximum of 6 arguments on the stack. */
        args = sp - 6;

        /*
         * Account for arguments on stack and do the funky C entry alignment.
         * This means that we need an 8-byte-odd alignment since the ABI expects
         * the return address to be pushed, thus breaking the 16 byte alignment.
         */
        sp -= 7;

        /* Add the arguments: */
        va_start(ap, argc);
        for (i = 0; i < argc; i++)
                args[i] = va_arg(ap, uint64_t);
        va_end(ap);
        for (i = argc; i < 6; i++)
                args[i] = 0;

        ucp->uc_mcontext.mc_rdi = (register_t)ucp;
        ucp->uc_mcontext.mc_rsi = (register_t)start;
        ucp->uc_mcontext.mc_rdx = (register_t)args;
        ucp->uc_mcontext.mc_rbp = 0;
        ucp->uc_mcontext.mc_rbx = (register_t)sp;
        ucp->uc_mcontext.mc_rsp = (register_t)sp;
        ucp->uc_mcontext.mc_rip = (register_t)makectx_wrapper;
}

static void
makectx_wrapper(ucontext_t *ucp, func_t func, uint64_t *args)
{
        (*func)(args[0], args[1], args[2], args[3], args[4], args[5]);
        if (ucp->uc_link == NULL)
                exit(0);
        setcontext((const ucontext_t *)ucp->uc_link);
        /* should never get here */
        abort();
        /* NOTREACHED */
}