about summary refs log tree commit diff
path: root/sysdeps/unix/sysv/linux/x86_64/makecontext.c
blob: ceb5c02bad4bf01c02c19da3a733dc793dead79d (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
/* Create new context.
   Copyright (C) 2002-2021 Free Software Foundation, Inc.
   This file is part of the GNU C Library.
   Contributed by Andreas Jaeger <aj@suse.de>, 2002.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

#include <sysdep.h>
#include <stdarg.h>
#include <stdint.h>
#include <ucontext.h>
#if SHSTK_ENABLED
# include <pthread.h>
# include <libc-pointer-arith.h>
# include <sys/prctl.h>
#endif

#include "ucontext_i.h"

/* This implementation can handle any ARGC value but only
   normal integer parameters.
   makecontext sets up a stack and the registers for the
   user context. The stack looks like this:
               +-----------------------+
               | next context          |
               +-----------------------+
               | parameter 7-n         |
	       +-----------------------+
	       | trampoline address    |
    %rsp ->    +-----------------------+

   The registers are set up like this:
     %rdi,%rsi,%rdx,%rcx,%r8,%r9: parameter 1 to 6
     %rbx   : address of next context
     %rsp   : stack pointer.
*/

/* XXX: This implementation currently only handles integer arguments.
   To handle long int and pointer arguments the va_arg arguments needs
   to be changed to long and also the stdlib/tst-setcontext.c file needs
   to be changed to pass long arguments to makecontext.  */


void
__makecontext (ucontext_t *ucp, void (*func) (void), int argc, ...)
{
  extern void __start_context (void) attribute_hidden;
  extern void __push___start_context (ucontext_t *)
    attribute_hidden;
  greg_t *sp;
  unsigned int idx_uc_link;
  va_list ap;
  int i;

  /* Generate room on stack for parameter if needed and uc_link.  */
  sp = (greg_t *) ((uintptr_t) ucp->uc_stack.ss_sp
		   + ucp->uc_stack.ss_size);
  sp -= (argc > 6 ? argc - 6 : 0) + 1;
  /* Align stack and make space for trampoline address.  */
  sp = (greg_t *) ((((uintptr_t) sp) & -16L) - 8);

  idx_uc_link = (argc > 6 ? argc - 6 : 0) + 1;

  /* Setup context ucp.  */
  /* Address to jump to.  */
  ucp->uc_mcontext.gregs[REG_RIP] = (uintptr_t) func;
  /* Setup rbx.*/
  ucp->uc_mcontext.gregs[REG_RBX] = (uintptr_t) &sp[idx_uc_link];
  ucp->uc_mcontext.gregs[REG_RSP] = (uintptr_t) sp;

  /* Setup stack.  */
#if SHSTK_ENABLED
  struct pthread *self = THREAD_SELF;
  unsigned int feature_1 = THREAD_GETMEM (self, header.feature_1);
  /* NB: We must check feature_1 before accessing __ssp since caller
	 may be compiled against ucontext_t without __ssp.  */
  if ((feature_1 & X86_FEATURE_1_SHSTK) != 0)
    {
      /* Shadow stack is enabled.  We need to allocate a new shadow
         stack.  */
      unsigned long ssp_size = (((uintptr_t) sp
				 - (uintptr_t) ucp->uc_stack.ss_sp)
				>> STACK_SIZE_TO_SHADOW_STACK_SIZE_SHIFT);
      /* Align shadow stack to 8 bytes.  */
      ssp_size = ALIGN_UP (ssp_size, 8);

      ucp->__ssp[1] = ssp_size;
      ucp->__ssp[2] = ssp_size;

      /* Call __push___start_context to allocate a new shadow stack,
	 push __start_context onto the new stack as well as the new
	 shadow stack.  NB: After __push___start_context returns,
	   ucp->__ssp[0]: The new shadow stack pointer.
	   ucp->__ssp[1]: The base address of the new shadow stack.
	   ucp->__ssp[2]: The size of the new shadow stack.
       */
      __push___start_context (ucp);
    }
  else
#endif
    sp[0] = (uintptr_t) &__start_context;
  sp[idx_uc_link] = (uintptr_t) ucp->uc_link;

  va_start (ap, argc);
  /* Handle arguments.

     The standard says the parameters must all be int values.  This is
     an historic accident and would be done differently today.  For
     x86-64 all integer values are passed as 64-bit values and
     therefore extending the API to copy 64-bit values instead of
     32-bit ints makes sense.  It does not break existing
     functionality and it does not violate the standard which says
     that passing non-int values means undefined behavior.  */
  for (i = 0; i < argc; ++i)
    switch (i)
      {
      case 0:
	ucp->uc_mcontext.gregs[REG_RDI] = va_arg (ap, greg_t);
	break;
      case 1:
	ucp->uc_mcontext.gregs[REG_RSI] = va_arg (ap, greg_t);
	break;
      case 2:
	ucp->uc_mcontext.gregs[REG_RDX] = va_arg (ap, greg_t);
	break;
      case 3:
	ucp->uc_mcontext.gregs[REG_RCX] = va_arg (ap, greg_t);
	break;
      case 4:
	ucp->uc_mcontext.gregs[REG_R8] = va_arg (ap, greg_t);
	break;
      case 5:
	ucp->uc_mcontext.gregs[REG_R9] = va_arg (ap, greg_t);
	break;
      default:
	/* Put value on stack.  */
	sp[i - 5] = va_arg (ap, greg_t);
	break;
      }
  va_end (ap);

}


weak_alias (__makecontext, makecontext)