i386/gen/makecontext.c

   1 /*
   2  * Copyright (c) 2007, 2009 Apple Inc. All rights reserved.
   3  *
   4  * @APPLE_LICENSE_HEADER_START@
   5  *
   6  * This file contains Original Code and/or Modifications of Original Code
   7  * as defined in and that are subject to the Apple Public Source License
   8  * Version 2.0 (the 'License'). You may not use this file except in
   9  * compliance with the License. Please obtain a copy of the License at
  10  * http://www.opensource.apple.com/apsl/ and read it before using this
  11  * file.
  12  *
  13  * The Original Code and all software distributed under the License are
  14  * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
  15  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
  16  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
  17  * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
  18  * Please see the License for the specific language governing rights and
  19  * limitations under the License.
  20  *
  21  * @APPLE_LICENSE_HEADER_END@
  22  */
  23
  24 /*
  25  * Copyright (c) 2001 Daniel M. Eischen <deischen@freebsd.org>
  26  * All rights reserved.
  27  *
  28  * Redistribution and use in source and binary forms, with or without
  29  * modification, are permitted provided that the following conditions
  30  * are met:
  31  * 1. Redistributions of source code must retain the above copyright
  32  *    notice, this list of conditions and the following disclaimer.
  33  * 2. Neither the name of the author nor the names of its contributors
  34  *    may be used to endorse or promote products derived from this software
  35  *    without specific prior written permission.
  36  *
  37  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  38  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  39  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  40  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  41  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  42  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  43  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  44  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  45  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  46  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  47  * SUCH DAMAGE.
  48  */
  49
  50 #if defined(__i386__)
  51
  52 #include <sys/cdefs.h>
  53
  54 #include <sys/param.h>
  55 #include <sys/signal.h>
  56 #include <sys/ucontext.h>
  57
  58 #include <errno.h>
  59 #include <stdarg.h>
  60 #include <stdlib.h>
  61 #include <ucontext.h>
  62 #include <unistd.h>
  63
  64 /* Prototypes */
  65 extern void _ctx_start(ucontext_t *, int argc, ...);
  66
  67 void
  68 _ctx_done (ucontext_t *ucp)
  69 {
  70         if (ucp->uc_link == NULL)
  71                 exit(0);
  72         else {
  73                 /*
  74                  * Since this context has finished, don't allow it
  75                  * to be restarted without being reinitialized (via
  76                  * setcontext or swapcontext).
  77                  */
  78                 ucp->uc_mcsize = 0;
  79
  80                 /* Set context to next one in link */
  81                 /* XXX - what to do for error, abort? */
  82                 setcontext((const ucontext_t *)ucp->uc_link);
  83                 LIBC_ABORT("setcontext failed");        /* should never get here */
  84         }
  85 }
  86
  87 void
  88 makecontext(ucontext_t *ucp, void (*start)(), int argc, ...)
  89 {
  90         va_list         ap;
  91         char            *stack_top;
  92         intptr_t        *argp;
  93         int             i;
  94
  95         if (ucp == NULL)
  96                 return;
  97         else if ((ucp->uc_stack.ss_sp == NULL) ||
  98             (ucp->uc_stack.ss_size < MINSIGSTKSZ)) {
  99                 /*
 100                  * This should really return -1 with errno set to ENOMEM
 101                  * or something, but the spec says that makecontext is
 102                  * a void function.   At least make sure that the context
 103                  * isn't valid so it can't be used without an error.
 104                  */
 105                 ucp->uc_mcsize = 0;
 106         }
 107         /* XXX - Do we want to sanity check argc? */
 108         else if ((argc < 0) || (argc > NCARGS)) {
 109                 ucp->uc_mcsize = 0;
 110         }
 111         /* Make sure the context is valid. */
 112         else {
 113                 /*
 114                  * Arrange the stack as follows:
 115                  *
 116                  *      _ctx_start()    - context start wrapper
 117                  *      start()         - user start routine
 118                  *      arg1            - first argument, aligned(16)
 119                  *      ...
 120                  *      argn
 121                  *      ucp             - this context, %ebp points here
 122                  *
 123                  * When the context is started, control will return to
 124                  * the context start wrapper which will pop the user
 125                  * start routine from the top of the stack.  After that,
 126                  * the top of the stack will be setup with all arguments
 127                  * necessary for calling the start routine.  When the
 128                  * start routine returns, the context wrapper then sets
 129                  * the stack pointer to %ebp which was setup to point to
 130                  * the base of the stack (and where ucp is stored).  It
 131                  * will then call _ctx_done() to swap in the next context
 132                  * (uc_link != 0) or exit the program (uc_link == 0).
 133                  */
 134                 mcontext_t mc;
 135
 136                 stack_top = (char *)(ucp->uc_stack.ss_sp +
 137                     ucp->uc_stack.ss_size - sizeof(intptr_t));
 138
 139                 /*
 140                  * Adjust top of stack to allow for 3 pointers (return
 141                  * address, _ctx_start, and ucp) and argc arguments.
 142                  * We allow the arguments to be pointers also.  The first
 143                  * argument to the user function must be properly aligned.
 144                  */
 145                 stack_top = stack_top - (sizeof(intptr_t) * (1 + argc));
 146                 stack_top = (char *)((unsigned)stack_top & ~15);
 147                 stack_top = stack_top - (2 * sizeof(intptr_t));
 148                 argp = (intptr_t *)stack_top;
 149
 150                 /*
 151                  * Setup the top of the stack with the user start routine
 152                  * followed by all of its aguments and the pointer to the
 153                  * ucontext.  We need to leave a spare spot at the top of
 154                  * the stack because setcontext will move eip to the top
 155                  * of the stack before returning.
 156                  */
 157                 *argp = (intptr_t)_ctx_start;  /* overwritten with same value */
 158                 argp++;
 159                 *argp = (intptr_t)start;
 160                 argp++;
 161
 162                 /* Add all the arguments: */
 163                 va_start(ap, argc);
 164                 for (i = 0; i < argc; i++) {
 165                         *argp = va_arg(ap, intptr_t);
 166                         argp++;
 167                 }
 168                 va_end(ap);
 169
 170                 /* The ucontext is placed at the bottom of the stack. */
 171                 *argp = (intptr_t)ucp;
 172
 173                 /*
 174                  * Set the machine context to point to the top of the
 175                  * stack and the program counter to the context start
 176                  * wrapper.  Note that setcontext() pushes the return
 177                  * address onto the top of the stack, so allow for this
 178                  * by adjusting the stack downward 1 slot.  Also set
 179                  * %esi to point to the base of the stack where ucp
 180                  * is stored.
 181                  */
 182                 mc = ucp->uc_mcontext;
 183
 184                 mc->__ss.__esi = (int)argp;
 185                 mc->__ss.__ebp = 0;
 186                 mc->__ss.__esp = (int)stack_top + sizeof(caddr_t);
 187                 mc->__ss.__eip = (int)_ctx_start;
 188         }
 189 }
 190
 191 #endif /* __i386__ */