/*-
 * SPDX-License-Identifier: BSD-2-Clause
 *
 * Copyright (c) 2001 Daniel M. Eischen <deischen@freebsd.org>
 * 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. Neither the name of the author nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * 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/signal.h>
#include <sys/ucontext.h>

#include <errno.h>
#include <stdarg.h>
#include <stdlib.h>
#include <unistd.h>

/* Prototypes */
extern void _ctx_start(ucontext_t *, int argc, ...);


__weak_reference(__makecontext, makecontext);

void
_ctx_done (ucontext_t *ucp)
{
        if (ucp->uc_link == NULL)
                exit(0);
        else {
                /*
                 * Since this context has finished, don't allow it
                 * to be restarted without being reinitialized (via
                 * setcontext or swapcontext).
                 */
                ucp->uc_mcontext.mc_len = 0;

                /* Set context to next one in link */
                /* XXX - what to do for error, abort? */
                setcontext((const ucontext_t *)ucp->uc_link);
                abort();        /* should never get here */
        }
}

void
__makecontext(ucontext_t *ucp, void (*start)(void), int argc, ...)
{
        va_list         ap;
        char            *stack_top;
        intptr_t        *argp;
        int             i;

        if (ucp == NULL)
                return;
        else if ((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;
        }
        /* XXX - Do we want to sanity check argc? */
        else if (argc < 0) {
                ucp->uc_mcontext.mc_len = 0;
        }
        /* Make sure the context is valid. */
        else if (ucp->uc_mcontext.mc_len == sizeof(mcontext_t)) {
                /*
                 * Arrange the stack as follows:
                 *
                 *      _ctx_start()    - context start wrapper
                 *      start()         - user start routine
                 *      arg1            - first argument, aligned(16)
                 *      ...
                 *      argn
                 *      ucp             - this context, %ebp points here
                 *
                 * When the context is started, control will return to
                 * the context start wrapper which will pop the user
                 * start routine from the top of the stack.  After that,
                 * the top of the stack will be setup with all arguments
                 * necessary for calling the start routine.  When the
                 * start routine returns, the context wrapper then sets
                 * the stack pointer to %ebp which was setup to point to
                 * the base of the stack (and where ucp is stored).  It
                 * will then call _ctx_done() to swap in the next context
                 * (uc_link != 0) or exit the program (uc_link == 0).
                 */
                stack_top = (char *)(ucp->uc_stack.ss_sp +
                    ucp->uc_stack.ss_size - sizeof(intptr_t));

                /*
                 * Adjust top of stack to allow for 3 pointers (return
                 * address, _ctx_start, and ucp) and argc arguments.
                 * We allow the arguments to be pointers also.  The first
                 * argument to the user function must be properly aligned.
                 */
                stack_top = stack_top - (sizeof(intptr_t) * (1 + argc));
                stack_top = (char *)((unsigned)stack_top & ~15);
                stack_top = stack_top - (2 * sizeof(intptr_t));
                argp = (intptr_t *)stack_top;

                /*
                 * Setup the top of the stack with the user start routine
                 * followed by all of its arguments and the pointer to the
                 * ucontext.  We need to leave a spare spot at the top of
                 * the stack because setcontext will move eip to the top
                 * of the stack before returning.
                 */
                *argp = (intptr_t)_ctx_start;  /* overwritten with same value */
                argp++;
                *argp = (intptr_t)start;
                argp++;

                /* Add all the arguments: */
                va_start(ap, argc);
                for (i = 0; i < argc; i++) {
                        *argp = va_arg(ap, intptr_t);
                        argp++;
                }
                va_end(ap);

                /* The ucontext is placed at the bottom of the stack. */
                *argp = (intptr_t)ucp;

                /*
                 * Set the machine context to point to the top of the
                 * stack and the program counter to the context start
                 * wrapper.  Note that setcontext() pushes the return
                 * address onto the top of the stack, so allow for this
                 * by adjusting the stack downward 1 slot.  Also set
                 * %esi to point to the base of the stack where ucp
                 * is stored.
                 */
                ucp->uc_mcontext.mc_esi = (int)argp;
                ucp->uc_mcontext.mc_ebp = 0;
                ucp->uc_mcontext.mc_esp = (int)stack_top + sizeof(caddr_t);
                ucp->uc_mcontext.mc_eip = (int)_ctx_start;
        }
}