xref: /illumos-gate/usr/src/lib/libc/i386/gen/makectxt.c (revision 3e76f9d6)
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/i386/crti.s
68  *  - lib/crt/i386/crt1.s
69  */
70 #undef	STACK_ALIGN
71 #define	STACK_ALIGN	16
72 
73 static void resumecontext(void);
74 
75 void
makecontext(ucontext_t * ucp,void (* func)(),int argc,...)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
resumecontext(void)112 resumecontext(void)
113 {
114 	ucontext_t uc;
115 
116 	(void) getcontext(&uc);
117 	(void) setcontext(uc.uc_link);
118 }
119