1 /*-
2 * SPDX-License-Identifier: BSD-2-Clause
3 *
4 * Copyright (c) 2001 Daniel M. Eischen <deischen@freebsd.org>
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
29 #include <sys/param.h>
30 #include <sys/signal.h>
31 #include <sys/ucontext.h>
32
33 #include <errno.h>
34 #include <stdarg.h>
35 #include <stdlib.h>
36 #include <unistd.h>
37
38 /* Prototypes */
39 extern void _ctx_start(ucontext_t *, int argc, ...);
40
41
42 __weak_reference(__makecontext, makecontext);
43
44 void
_ctx_done(ucontext_t * ucp)45 _ctx_done (ucontext_t *ucp)
46 {
47 if (ucp->uc_link == NULL)
48 exit(0);
49 else {
50 /*
51 * Since this context has finished, don't allow it
52 * to be restarted without being reinitialized (via
53 * setcontext or swapcontext).
54 */
55 ucp->uc_mcontext.mc_len = 0;
56
57 /* Set context to next one in link */
58 /* XXX - what to do for error, abort? */
59 setcontext((const ucontext_t *)ucp->uc_link);
60 abort(); /* should never get here */
61 }
62 }
63
64 void
__makecontext(ucontext_t * ucp,void (* start)(void),int argc,...)65 __makecontext(ucontext_t *ucp, void (*start)(void), int argc, ...)
66 {
67 va_list ap;
68 char *stack_top;
69 intptr_t *argp;
70 int i;
71
72 if (ucp == NULL)
73 return;
74 else if ((ucp->uc_stack.ss_sp == NULL) ||
75 (ucp->uc_stack.ss_size < MINSIGSTKSZ)) {
76 /*
77 * This should really return -1 with errno set to ENOMEM
78 * or something, but the spec says that makecontext is
79 * a void function. At least make sure that the context
80 * isn't valid so it can't be used without an error.
81 */
82 ucp->uc_mcontext.mc_len = 0;
83 }
84 /* XXX - Do we want to sanity check argc? */
85 else if (argc < 0) {
86 ucp->uc_mcontext.mc_len = 0;
87 }
88 /* Make sure the context is valid. */
89 else if (ucp->uc_mcontext.mc_len == sizeof(mcontext_t)) {
90 /*
91 * Arrange the stack as follows:
92 *
93 * _ctx_start() - context start wrapper
94 * start() - user start routine
95 * arg1 - first argument, aligned(16)
96 * ...
97 * argn
98 * ucp - this context, %ebp points here
99 *
100 * When the context is started, control will return to
101 * the context start wrapper which will pop the user
102 * start routine from the top of the stack. After that,
103 * the top of the stack will be setup with all arguments
104 * necessary for calling the start routine. When the
105 * start routine returns, the context wrapper then sets
106 * the stack pointer to %ebp which was setup to point to
107 * the base of the stack (and where ucp is stored). It
108 * will then call _ctx_done() to swap in the next context
109 * (uc_link != 0) or exit the program (uc_link == 0).
110 */
111 stack_top = (char *)(ucp->uc_stack.ss_sp +
112 ucp->uc_stack.ss_size - sizeof(intptr_t));
113
114 /*
115 * Adjust top of stack to allow for 3 pointers (return
116 * address, _ctx_start, and ucp) and argc arguments.
117 * We allow the arguments to be pointers also. The first
118 * argument to the user function must be properly aligned.
119 */
120 stack_top = stack_top - (sizeof(intptr_t) * (1 + argc));
121 stack_top = (char *)((unsigned)stack_top & ~15);
122 stack_top = stack_top - (2 * sizeof(intptr_t));
123 argp = (intptr_t *)stack_top;
124
125 /*
126 * Setup the top of the stack with the user start routine
127 * followed by all of its aguments and the pointer to the
128 * ucontext. We need to leave a spare spot at the top of
129 * the stack because setcontext will move eip to the top
130 * of the stack before returning.
131 */
132 *argp = (intptr_t)_ctx_start; /* overwritten with same value */
133 argp++;
134 *argp = (intptr_t)start;
135 argp++;
136
137 /* Add all the arguments: */
138 va_start(ap, argc);
139 for (i = 0; i < argc; i++) {
140 *argp = va_arg(ap, intptr_t);
141 argp++;
142 }
143 va_end(ap);
144
145 /* The ucontext is placed at the bottom of the stack. */
146 *argp = (intptr_t)ucp;
147
148 /*
149 * Set the machine context to point to the top of the
150 * stack and the program counter to the context start
151 * wrapper. Note that setcontext() pushes the return
152 * address onto the top of the stack, so allow for this
153 * by adjusting the stack downward 1 slot. Also set
154 * %esi to point to the base of the stack where ucp
155 * is stored.
156 */
157 ucp->uc_mcontext.mc_esi = (int)argp;
158 ucp->uc_mcontext.mc_ebp = 0;
159 ucp->uc_mcontext.mc_esp = (int)stack_top + sizeof(caddr_t);
160 ucp->uc_mcontext.mc_eip = (int)_ctx_start;
161 }
162 }
163