1 /* Create new context.
2 Copyright (C) 2002-2023 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
9
10 The GNU C Library is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, see
17 <https://www.gnu.org/licenses/>. */
18
19 #include <sysdep.h>
20 #include <stdarg.h>
21 #include <stdint.h>
22 #include <ucontext.h>
23 #if SHSTK_ENABLED
24 # include <pthread.h>
25 # include <libc-pointer-arith.h>
26 # include <sys/prctl.h>
27 #endif
28
29 #include "ucontext_i.h"
30
31 /* This implementation can handle any ARGC value but only
32 normal integer parameters.
33 makecontext sets up a stack and the registers for the
34 user context. The stack looks like this:
35 +-----------------------+
36 | next context |
37 +-----------------------+
38 | parameter 7-n |
39 +-----------------------+
40 | trampoline address |
41 %rsp -> +-----------------------+
42
43 The registers are set up like this:
44 %rdi,%rsi,%rdx,%rcx,%r8,%r9: parameter 1 to 6
45 %rbx : address of next context
46 %rsp : stack pointer.
47 */
48
49 /* XXX: This implementation currently only handles integer arguments.
50 To handle long int and pointer arguments the va_arg arguments needs
51 to be changed to long and also the stdlib/tst-setcontext.c file needs
52 to be changed to pass long arguments to makecontext. */
53
54
55 void
56 __makecontext (ucontext_t *ucp, void (*func) (void), int argc, ...)
57 {
58 extern void __start_context (void) attribute_hidden;
59 extern void __push___start_context (ucontext_t *)
60 attribute_hidden;
61 greg_t *sp;
62 unsigned int idx_uc_link;
63 va_list ap;
64 int i;
65
66 /* Generate room on stack for parameter if needed and uc_link. */
67 sp = (greg_t *) ((uintptr_t) ucp->uc_stack.ss_sp
68 + ucp->uc_stack.ss_size);
69 sp -= (argc > 6 ? argc - 6 : 0) + 1;
70 /* Align stack and make space for trampoline address. */
71 sp = (greg_t *) ((((uintptr_t) sp) & -16L) - 8);
72
73 idx_uc_link = (argc > 6 ? argc - 6 : 0) + 1;
74
75 /* Setup context ucp. */
76 /* Address to jump to. */
77 ucp->uc_mcontext.gregs[REG_RIP] = (uintptr_t) func;
78 /* Setup rbx.*/
79 ucp->uc_mcontext.gregs[REG_RBX] = (uintptr_t) &sp[idx_uc_link];
80 ucp->uc_mcontext.gregs[REG_RSP] = (uintptr_t) sp;
81
82 /* Setup stack. */
83 #if SHSTK_ENABLED
84 struct pthread *self = THREAD_SELF;
85 unsigned int feature_1 = THREAD_GETMEM (self, header.feature_1);
86 /* NB: We must check feature_1 before accessing __ssp since caller
87 may be compiled against ucontext_t without __ssp. */
88 if ((feature_1 & X86_FEATURE_1_SHSTK) != 0)
89 {
90 /* Shadow stack is enabled. We need to allocate a new shadow
91 stack. */
92 unsigned long ssp_size = (((uintptr_t) sp
93 - (uintptr_t) ucp->uc_stack.ss_sp)
94 >> STACK_SIZE_TO_SHADOW_STACK_SIZE_SHIFT);
95 /* Align shadow stack to 8 bytes. */
96 ssp_size = ALIGN_UP (ssp_size, 8);
97
98 ucp->__ssp[1] = ssp_size;
99 ucp->__ssp[2] = ssp_size;
100
101 /* Call __push___start_context to allocate a new shadow stack,
102 push __start_context onto the new stack as well as the new
103 shadow stack. NB: After __push___start_context returns,
104 ucp->__ssp[0]: The new shadow stack pointer.
105 ucp->__ssp[1]: The base address of the new shadow stack.
106 ucp->__ssp[2]: The size of the new shadow stack.
107 */
108 __push___start_context (ucp);
109 }
110 else
111 #endif
112 sp[0] = (uintptr_t) &__start_context;
113 sp[idx_uc_link] = (uintptr_t) ucp->uc_link;
114
115 va_start (ap, argc);
116 /* Handle arguments.
117
118 The standard says the parameters must all be int values. This is
119 an historic accident and would be done differently today. For
120 x86-64 all integer values are passed as 64-bit values and
121 therefore extending the API to copy 64-bit values instead of
122 32-bit ints makes sense. It does not break existing
123 functionality and it does not violate the standard which says
124 that passing non-int values means undefined behavior. */
125 for (i = 0; i < argc; ++i)
126 switch (i)
127 {
128 case 0:
129 ucp->uc_mcontext.gregs[REG_RDI] = va_arg (ap, greg_t);
130 break;
131 case 1:
132 ucp->uc_mcontext.gregs[REG_RSI] = va_arg (ap, greg_t);
133 break;
134 case 2:
135 ucp->uc_mcontext.gregs[REG_RDX] = va_arg (ap, greg_t);
136 break;
137 case 3:
138 ucp->uc_mcontext.gregs[REG_RCX] = va_arg (ap, greg_t);
139 break;
140 case 4:
141 ucp->uc_mcontext.gregs[REG_R8] = va_arg (ap, greg_t);
142 break;
143 case 5:
144 ucp->uc_mcontext.gregs[REG_R9] = va_arg (ap, greg_t);
145 break;
146 default:
147 /* Put value on stack. */
148 sp[i - 5] = va_arg (ap, greg_t);
149 break;
150 }
151 va_end (ap);
152
153 }
154
155
156 weak_alias (__makecontext, makecontext)