usr/src/lib/libc/i386/gen/makectxt.c

   1 /*
   2  * CDDL HEADER START
   3  *
   4  * The contents of this file are subject to the terms of the
   5  * Common Development and Distribution License (the "License").
   6  * You may not use this file except in compliance with the License.
   7  *
   8  * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
   9  * or http://www.opensolaris.org/os/licensing.
  10  * See the License for the specific language governing permissions
  11  * and limitations under the License.
  12  *
  13  * When distributing Covered Code, include this CDDL HEADER in each
  14  * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
  15  * If applicable, add the following below this CDDL HEADER, with the
  16  * fields enclosed by brackets "[]" replaced with your own identifying
  17  * information: Portions Copyright [yyyy] [name of copyright owner]
  18  *
  19  * CDDL HEADER END
  20  */
  21
  22 /*
  23  * Copyright 2008 Sun Microsystems, Inc.  All rights reserved.
  24  * Use is subject to license terms.
  25  */
  26
  27 /*      Copyright (c) 1988 AT&T */
  28 /*        All Rights Reserved   */
  29
  30 #pragma weak _makecontext = makecontext
  31
  32 #include "lint.h"
  33 #include <stdarg.h>
  34 #include <ucontext.h>
  35 #include <sys/stack.h>
  36
  37 /*
  38  * The ucontext_t that the user passes in must have been primed with a
  39  * call to getcontext(2), have the uc_stack member set to reflect the
  40  * stack which this context will use, and have the uc_link member set
  41  * to the context which should be resumed when this context returns.
  42  * When makecontext() returns, the ucontext_t will be set to run the
  43  * given function with the given parameters on the stack specified by
  44  * uc_stack, and which will return to the ucontext_t specified by uc_link.
  45  */
  46
  47 /*
  48  * The original i386 ABI said that the stack pointer need be only 4-byte
  49  * aligned before a function call (STACK_ALIGN == 4).  The ABI supplement
  50  * version 1.0 changed the required alignment to 16-byte for the benefit of
  51  * floating point code compiled using sse2.  The compiler assumes this
  52  * alignment and maintains it for calls it generates.  If the stack is
  53  * initially properly aligned, it will continue to be so aligned.  If it is
  54  * not initially so aligned, it will never become so aligned.
  55  *
  56  * One slightly confusing detail to keep in mind is that the 16-byte
  57  * alignment (%esp & 0xf == 0) is true just *before* the call instruction.
  58  * The call instruction will then push a return value, decrementing %esp by
  59  * 4.  Therefore, if one dumps %esp at the at the very first instruction in
  60  * a function, it will end with a 0xc.  The compiler expects this and
  61  * compensates for it properly.
  62  *
  63  * Note: If you change this value, you need to change it in the following
  64  * files as well:
  65  *
  66  *  - lib/libc/i386/threads/machdep.c
  67  *  - lib/crt/i86/crti.s
  68  *  - lib/crt/i86/crt1.s
  69  */
  70 #undef  STACK_ALIGN
  71 #define STACK_ALIGN     16
  72
  73 static void resumecontext(void);
  74
  75 void
  76 makecontext(ucontext_t *ucp, void (*func)(), int argc, ...)
  77 {
  78         long *sp;
  79         long *tsp;
  80         va_list ap;
  81         size_t size;
  82
  83         ucp->uc_mcontext.gregs[EIP] = (greg_t)func;
  84
  85         size = sizeof (long) * (argc + 1);
  86
  87         tsp = (long *)(((uintptr_t)ucp->uc_stack.ss_sp +
  88             ucp->uc_stack.ss_size - size) & ~(STACK_ALIGN - 1));
  89
  90         /*
  91          * Since we're emulating the call instruction, we must push the
  92          * return address (which involves adjusting the stack pointer to
  93          * have the proper 4-byte bias).
  94          */
  95         sp = tsp - 1;
  96
  97         *sp = (long)resumecontext;              /* return address */
  98
  99         ucp->uc_mcontext.gregs[UESP] = (greg_t)sp;
 100
 101         /*
 102          * "push" all the arguments
 103          */
 104         va_start(ap, argc);
 105         while (argc-- > 0)
 106                 *tsp++ = va_arg(ap, long);
 107         va_end(ap);
 108 }
 109
 110
 111 static void
 112 resumecontext(void)
 113 {
 114         ucontext_t uc;
 115
 116         (void) getcontext(&uc);
 117         (void) setcontext(uc.uc_link);
 118 }