lib/libc/i386/gen/makecontext.c

   1 /*
   2  * Copyright (c) 2001 Daniel M. Eischen <deischen@freebsd.org>
   3  * All rights reserved.
   4  * Copyright (c) 2007 Matthew Dillon <dillon@backplane.com>
   5  * All rights reserved.
   6  *
   7  * Redistribution and use in source and binary forms, with or without
   8  * modification, are permitted provided that the following conditions
   9  * are met:
  10  * 1. Redistributions of source code must retain the above copyright
  11  *    notice, this list of conditions and the following disclaimer.
  12  * 2. Neither the name of the author nor the names of its contributors
  13  *    may be used to endorse or promote products derived from this software
  14  *    without specific prior written permission.
  15  *
  16  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
  17  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  18  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  19  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
  20  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  21  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  22  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  23  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  24  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  25  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  26  * SUCH DAMAGE.
  27  *
  28  * $FreeBSD: src/lib/libc/i386/gen/makecontext.c,v 1.5 2004/12/05 21:22:08 deischen Exp $
  29  * $DragonFly: src/lib/libc/i386/gen/makecontext.c,v 1.1 2007/01/16 07:16:23 dillon Exp $
  30  */
  31
  32 #include <sys/cdefs.h>
  33 #include <sys/param.h>
  34 #include <sys/signal.h>
  35 #include <sys/ucontext.h>
  36
  37 #include <errno.h>
  38 #include <stdarg.h>
  39 #include <stdlib.h>
  40 #include <unistd.h>
  41
  42 /* Prototypes */
  43 extern void _ctx_start(ucontext_t *, int argc, ...);
  44
  45 __weak_reference(_makecontext, makecontext);
  46
  47 /*
  48  * _ctx_done - terminate a context
  49  *
  50  * The function specified by makecontext() is called by _ctx_start,
  51  * returns, and then _ctx_start calls _ctx_done to terminate the context.
  52  */
  53 void
  54 _ctx_done (ucontext_t *ucp)
  55 {
  56         if (ucp->uc_link == NULL) {
  57                 exit(0);
  58         } else {
  59                 /*
  60                  * Since this context has finished, don't allow it
  61                  * to be restarted without being reinitialized (via
  62                  * setcontext or swapcontext).
  63                  */
  64                 ucp->uc_mcontext.mc_len = 0;
  65
  66                 /* Set context to next one in link */
  67                 /* XXX - what to do for error, abort? */
  68                 setcontext((const ucontext_t *)ucp->uc_link);
  69                 abort();        /* should never get here */
  70         }
  71 }
  72
  73 /*
  74  * makecontext() associates a stack with a user thread context and sets
  75  * up to call the start function when switched to.  The start function
  76  * returns to _ctx_start which then calls _ctx_done to terminate the
  77  * context.
  78  */
  79 void
  80 _makecontext(ucontext_t *ucp, void (*start)(void), int argc, ...)
  81 {
  82         va_list         ap;
  83         char            *stack_top;
  84         intptr_t        *argp;
  85         int             i;
  86
  87         if (ucp == NULL)
  88                 return;
  89
  90         /*
  91          * Invalidate a context which did not have a stack associated with
  92          * it or for which the stack was too small.  The stack check is
  93          * kinda silly, though, since we have no control over the stack
  94          * usage of the code being set up to run.
  95          */
  96         if ((ucp->uc_stack.ss_sp == NULL) ||
  97             (ucp->uc_stack.ss_size < MINSIGSTKSZ)) {
  98                 ucp->uc_mcontext.mc_len = 0;
  99         }
 100         if (argc < 0 || argc > NCARGS)
 101                 ucp->uc_mcontext.mc_len = 0;
 102
 103         if (ucp->uc_mcontext.mc_len == sizeof(mcontext_t)) {
 104                 /*
 105                  * Arrange the stack as follows:
 106                  *
 107                  *      _ctx_start      - dummy return frame for stack trace
 108                  *      start_ptr       - user start routine  <<<< ESP PTR
 109                  *      arg1            - first argument, aligned(16)
 110                  *      ...
 111                  *      argn
 112                  *      ucp             - this context, %ebp points here
 113                  *
 114                  * When the context is started, control will return to
 115                  * the context start wrapper _ctx_start which will pop the
 116                  * user start routine from the top of the stack.  After that,
 117                  * the top of the stack will be setup with all arguments
 118                  * necessary for calling the start routine.  When the
 119                  * start routine returns, the context wrapper then sets
 120                  * the stack pointer to %ebp which was setup to point to
 121                  * the base of the stack (and where ucp is stored).  It
 122                  * will then call _ctx_done() to swap in the next context
 123                  * (uc_link != 0) or exit the program (uc_link == 0).
 124                  */
 125                 stack_top = (char *)(ucp->uc_stack.ss_sp +
 126                             ucp->uc_stack.ss_size - sizeof(intptr_t));
 127
 128                 /*
 129                  * Adjust top of stack to allow for 3 pointers (return
 130                  * address, _ctx_start, and ucp) and argc arguments.
 131                  * We allow the arguments to be pointers also.  The first
 132                  * argument to the user function must be properly aligned.
 133                  */
 134                 stack_top = stack_top - (sizeof(intptr_t) * (1 + argc));
 135                 stack_top = (char *)((unsigned)stack_top & ~15);
 136                 stack_top = stack_top - (2 * sizeof(intptr_t));
 137                 argp = (intptr_t *)stack_top;
 138
 139                 /*
 140                  * Setup the top of the stack with the user start routine
 141                  * followed by all of its aguments and the pointer to the
 142                  * ucontext.  We need to leave a spare spot at the top of
 143                  * the stack because setcontext will move eip to the top
 144                  * of the stack before returning.
 145                  */
 146                 *argp = (intptr_t)_ctx_start;  /* overwritten with same value */
 147                 argp++;
 148                 *argp = (intptr_t)start;
 149                 argp++;
 150
 151                 /* Add all the arguments: */
 152                 va_start(ap, argc);
 153                 for (i = 0; i < argc; i++) {
 154                         *argp = va_arg(ap, intptr_t);
 155                         argp++;
 156                 }
 157                 va_end(ap);
 158
 159                 /* The ucontext is placed at the bottom of the stack. */
 160                 *argp = (intptr_t)ucp;
 161
 162                 /*
 163                  * Set the machine context to point to the top of the
 164                  * stack and the program counter to the context start
 165                  * wrapper.  Note that setcontext() pushes the return
 166                  * address onto the top of the stack, so allow for this
 167                  * by adjusting the stack downward 1 slot.  Also set
 168                  * %esi to point to the base of the stack where ucp
 169                  * is stored.
 170                  */
 171                 ucp->uc_mcontext.mc_esi = (int)argp;
 172                 ucp->uc_mcontext.mc_ebp = 0;
 173                 ucp->uc_mcontext.mc_esp = (int)stack_top + sizeof(caddr_t);
 174                 ucp->uc_mcontext.mc_eip = (int)_ctx_start;
 175         }
 176 }
 177