about summary refs log tree commit diff
path: root/misc/unwind-link.c
blob: 45b7886b46d85a25a5b80d52f8f2e23d6a448c80 (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
/* Dynamic loading of the libgcc unwinder.
   Copyright (C) 2021-2022 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   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/>.  */

#ifdef SHARED

#include <assert.h>
#include <dlfcn.h>
#include <gnu/lib-names.h>
#include <unwind-link.h>
#include <libc-lock.h>
#include <pointer_guard.h>

/* Statically allocate the object, so that we do not have to deal with
   malloc failure.  __libc_unwind_link_get must not fail if libgcc_s
   has already been loaded by other means.  */
static struct unwind_link global;

/* dlopen handle.  Also used for the double-checked locking idiom.  */
static void *global_libgcc_handle;

/* We cannot use __libc_once because the pthread_once implementation
   may depend on unwinding.  */
__libc_lock_define (static, lock);

struct unwind_link *
__libc_unwind_link_get (void)
{
  /* Double-checked locking idiom.  Synchronizes with the release MO
     store at the end of this function.  */
  if (atomic_load_acquire (&global_libgcc_handle) != NULL)
   return &global;

  /* Initialize a copy of the data, so that we do not need about
     unlocking in case the dynamic loader somehow triggers
     unwinding.  */
  void *local_libgcc_handle = __libc_dlopen (LIBGCC_S_SO);
  if (local_libgcc_handle == NULL)
    {
      __libc_lock_unlock (lock);
      return NULL;
    }

  struct unwind_link local;
  local.ptr__Unwind_Backtrace
    = __libc_dlsym (local_libgcc_handle, "_Unwind_Backtrace");
  local.ptr__Unwind_ForcedUnwind
    = __libc_dlsym (local_libgcc_handle, "_Unwind_ForcedUnwind");
  local.ptr__Unwind_GetCFA
    = __libc_dlsym (local_libgcc_handle, "_Unwind_GetCFA");
#if UNWIND_LINK_GETIP
  local.ptr__Unwind_GetIP
    = __libc_dlsym (local_libgcc_handle, "_Unwind_GetIP");
#endif
  local.ptr__Unwind_Resume
    = __libc_dlsym (local_libgcc_handle, "_Unwind_Resume");
#if UNWIND_LINK_FRAME_STATE_FOR
  local.ptr___frame_state_for
    = __libc_dlsym (local_libgcc_handle, "__frame_state_for");
#endif
  local.ptr_personality
    = __libc_dlsym (local_libgcc_handle, "__gcc_personality_v0");
  UNWIND_LINK_EXTRA_INIT

  /* If a symbol is missing, libgcc_s has somehow been corrupted.  */
  assert (local.ptr__Unwind_Backtrace != NULL);
  assert (local.ptr__Unwind_ForcedUnwind != NULL);
  assert (local.ptr__Unwind_GetCFA != NULL);
#if UNWIND_LINK_GETIP
  assert (local.ptr__Unwind_GetIP != NULL);
#endif
  assert (local.ptr__Unwind_Resume != NULL);
  assert (local.ptr_personality != NULL);

#ifdef PTR_MANGLE
  PTR_MANGLE (local.ptr__Unwind_Backtrace);
  PTR_MANGLE (local.ptr__Unwind_ForcedUnwind);
  PTR_MANGLE (local.ptr__Unwind_GetCFA);
# if UNWIND_LINK_GETIP
  PTR_MANGLE (local.ptr__Unwind_GetIP);
# endif
  PTR_MANGLE (local.ptr__Unwind_Resume);
# if UNWIND_LINK_FRAME_STATE_FOR
  PTR_MANGLE (local.ptr___frame_state_for);
# endif
  PTR_MANGLE (local.ptr_personality);
#endif

  __libc_lock_lock (lock);
  if (atomic_load_relaxed (&global_libgcc_handle) != NULL)
    /* This thread lost the race.  Clean up.  */
    __libc_dlclose (local_libgcc_handle);
  else
    {
      global = local;

      /* Completes the double-checked locking idiom.  */
      atomic_store_release (&global_libgcc_handle, local_libgcc_handle);
    }

  __libc_lock_unlock (lock);
  return &global;
}
libc_hidden_def (__libc_unwind_link_get)

void
__libc_unwind_link_after_fork (void)
{
  if (__libc_lock_trylock (lock) == 0)
    /* The lock was not acquired during the fork.  This covers both
       the initialized and uninitialized case.  */
    __libc_lock_unlock (lock);
  else
    {
      /* Initialization was in progress in another thread.
         Reinitialize the lock.  */
      __libc_lock_init (lock);
      global_libgcc_handle = NULL;
    }
}

void __libc_freeres_fn_section
__libc_unwind_link_freeres (void)
{
  if (global_libgcc_handle != NULL)
    {
      __libc_dlclose (global_libgcc_handle );
      global_libgcc_handle = NULL;
    }
}

#endif /* SHARED */