1/* Create new context.
2 Copyright (C) 2002-2021 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4 Contributed by Andreas Jaeger <aj@suse.de>, 2002.
5
6 The GNU C Library is free software; you can redistribute it and/or
7 modify it under the terms of the GNU Lesser General Public
8 License as published by the Free Software Foundation; either
9 version 2.1 of the License, or (at your option) any later version.
10
11 The GNU C Library is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 Lesser General Public License for more details.
15
16 You should have received a copy of the GNU Lesser General Public
17 License along with the GNU C Library; if not, see
18 <https://www.gnu.org/licenses/>. */
19
20#include <sysdep.h>
21#include <stdarg.h>
22#include <stdint.h>
23#include <ucontext.h>
24#if SHSTK_ENABLED
25# include <pthread.h>
26# include <libc-pointer-arith.h>
27# include <sys/prctl.h>
28#endif
29
30#include "ucontext_i.h"
31
32/* This implementation can handle any ARGC value but only
33 normal integer parameters.
34 makecontext sets up a stack and the registers for the
35 user context. The stack looks like this:
36 +-----------------------+
37 | next context |
38 +-----------------------+
39 | parameter 7-n |
40 +-----------------------+
41 | trampoline address |
42 %rsp -> +-----------------------+
43
44 The registers are set up like this:
45 %rdi,%rsi,%rdx,%rcx,%r8,%r9: parameter 1 to 6
46 %rbx : address of next context
47 %rsp : stack pointer.
48*/
49
50/* XXX: This implementation currently only handles integer arguments.
51 To handle long int and pointer arguments the va_arg arguments needs
52 to be changed to long and also the stdlib/tst-setcontext.c file needs
53 to be changed to pass long arguments to makecontext. */
54
55
56void
57__makecontext (ucontext_t *ucp, void (*func) (void), int argc, ...)
58{
59 extern void __start_context (void) attribute_hidden;
60 extern void __push___start_context (ucontext_t *)
61 attribute_hidden;
62 greg_t *sp;
63 unsigned int idx_uc_link;
64 va_list ap;
65 int i;
66
67 /* Generate room on stack for parameter if needed and uc_link. */
68 sp = (greg_t *) ((uintptr_t) ucp->uc_stack.ss_sp
69 + ucp->uc_stack.ss_size);
70 sp -= (argc > 6 ? argc - 6 : 0) + 1;
71 /* Align stack and make space for trampoline address. */
72 sp = (greg_t *) ((((uintptr_t) sp) & -16L) - 8);
73
74 idx_uc_link = (argc > 6 ? argc - 6 : 0) + 1;
75
76 /* Setup context ucp. */
77 /* Address to jump to. */
78 ucp->uc_mcontext.gregs[REG_RIP] = (uintptr_t) func;
79 /* Setup rbx.*/
80 ucp->uc_mcontext.gregs[REG_RBX] = (uintptr_t) &sp[idx_uc_link];
81 ucp->uc_mcontext.gregs[REG_RSP] = (uintptr_t) sp;
82
83 /* Setup stack. */
84#if SHSTK_ENABLED
85 struct pthread *self = THREAD_SELF;
86 unsigned int feature_1 = THREAD_GETMEM (self, header.feature_1);
87 /* NB: We must check feature_1 before accessing __ssp since caller
88 may be compiled against ucontext_t without __ssp. */
89 if ((feature_1 & X86_FEATURE_1_SHSTK) != 0)
90 {
91 /* Shadow stack is enabled. We need to allocate a new shadow
92 stack. */
93 unsigned long ssp_size = (((uintptr_t) sp
94 - (uintptr_t) ucp->uc_stack.ss_sp)
95 >> STACK_SIZE_TO_SHADOW_STACK_SIZE_SHIFT);
96 /* Align shadow stack to 8 bytes. */
97 ssp_size = ALIGN_UP (ssp_size, 8);
98
99 ucp->__ssp[1] = ssp_size;
100 ucp->__ssp[2] = ssp_size;
101
102 /* Call __push___start_context to allocate a new shadow stack,
103 push __start_context onto the new stack as well as the new
104 shadow stack. NB: After __push___start_context returns,
105 ucp->__ssp[0]: The new shadow stack pointer.
106 ucp->__ssp[1]: The base address of the new shadow stack.
107 ucp->__ssp[2]: The size of the new shadow stack.
108 */
109 __push___start_context (ucp);
110 }
111 else
112#endif
113 sp[0] = (uintptr_t) &__start_context;
114 sp[idx_uc_link] = (uintptr_t) ucp->uc_link;
115
116 va_start (ap, argc);
117 /* Handle arguments.
118
119 The standard says the parameters must all be int values. This is
120 an historic accident and would be done differently today. For
121 x86-64 all integer values are passed as 64-bit values and
122 therefore extending the API to copy 64-bit values instead of
123 32-bit ints makes sense. It does not break existing
124 functionality and it does not violate the standard which says
125 that passing non-int values means undefined behavior. */
126 for (i = 0; i < argc; ++i)
127 switch (i)
128 {
129 case 0:
130 ucp->uc_mcontext.gregs[REG_RDI] = va_arg (ap, greg_t);
131 break;
132 case 1:
133 ucp->uc_mcontext.gregs[REG_RSI] = va_arg (ap, greg_t);
134 break;
135 case 2:
136 ucp->uc_mcontext.gregs[REG_RDX] = va_arg (ap, greg_t);
137 break;
138 case 3:
139 ucp->uc_mcontext.gregs[REG_RCX] = va_arg (ap, greg_t);
140 break;
141 case 4:
142 ucp->uc_mcontext.gregs[REG_R8] = va_arg (ap, greg_t);
143 break;
144 case 5:
145 ucp->uc_mcontext.gregs[REG_R9] = va_arg (ap, greg_t);
146 break;
147 default:
148 /* Put value on stack. */
149 sp[i - 5] = va_arg (ap, greg_t);
150 break;
151 }
152 va_end (ap);
153
154}
155
156
157weak_alias (__makecontext, makecontext)
158