sysdeps/unix/sysv/linux/x86_64/makecontext.c

   1 /* Create new context.
   2    Copyright (C) 2002-2024 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 # include <allocate-shadow-stack.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
  56 void
  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.  NB:
  93            ucp->__ssp[0]: The new shadow stack pointer.
  94            ucp->__ssp[1]: The base address of the new shadow stack.
  95            ucp->__ssp[2]: The size of the new shadow stack.
  96        */
  97       long int ret
  98         = __allocate_shadow_stack (((uintptr_t) sp
  99                                     - (uintptr_t) ucp->uc_stack.ss_sp),
 100                                    &ucp->__ssp[1]);
 101       if (ret != 0)
 102         {
 103           /* FIXME: What should we do?  */
 104           abort ();
 105         }
 106
 107       ucp->__ssp[0] = ucp->__ssp[1] + ucp->__ssp[2] - 8;
 108       /* Call __push___start_context to push __start_context onto the new
 109          stack as well as the new shadow stack.  */
 110       __push___start_context (ucp);
 111     }
 112   else
 113 #endif
 114     sp[0] = (uintptr_t) &__start_context;
 115   sp[idx_uc_link] = (uintptr_t) ucp->uc_link;
 116
 117   va_start (ap, argc);
 118   /* Handle arguments.
 119
 120      The standard says the parameters must all be int values.  This is
 121      an historic accident and would be done differently today.  For
 122      x86-64 all integer values are passed as 64-bit values and
 123      therefore extending the API to copy 64-bit values instead of
 124      32-bit ints makes sense.  It does not break existing
 125      functionality and it does not violate the standard which says
 126      that passing non-int values means undefined behavior.  */
 127   for (i = 0; i < argc; ++i)
 128     switch (i)
 129       {
 130       case 0:
 131         ucp->uc_mcontext.gregs[REG_RDI] = va_arg (ap, greg_t);
 132         break;
 133       case 1:
 134         ucp->uc_mcontext.gregs[REG_RSI] = va_arg (ap, greg_t);
 135         break;
 136       case 2:
 137         ucp->uc_mcontext.gregs[REG_RDX] = va_arg (ap, greg_t);
 138         break;
 139       case 3:
 140         ucp->uc_mcontext.gregs[REG_RCX] = va_arg (ap, greg_t);
 141         break;
 142       case 4:
 143         ucp->uc_mcontext.gregs[REG_R8] = va_arg (ap, greg_t);
 144         break;
 145       case 5:
 146         ucp->uc_mcontext.gregs[REG_R9] = va_arg (ap, greg_t);
 147         break;
 148       default:
 149         /* Put value on stack.  */
 150         sp[i - 5] = va_arg (ap, greg_t);
 151         break;
 152       }
 153   va_end (ap);
 154
 155 }
 156
 157
 158 weak_alias (__makecontext, makecontext)