misc/unwind-link.c

   1 /* Dynamic loading of the libgcc unwinder.
   2    Copyright (C) 2021-2024 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 #ifdef SHARED
  20
  21 #include <assert.h>
  22 #include <dlfcn.h>
  23 #include <gnu/lib-names.h>
  24 #include <unwind-link.h>
  25 #include <libc-lock.h>
  26 #include <pointer_guard.h>
  27
  28 /* Statically allocate the object, so that we do not have to deal with
  29    malloc failure.  __libc_unwind_link_get must not fail if libgcc_s
  30    has already been loaded by other means.  */
  31 static struct unwind_link global;
  32
  33 /* dlopen handle.  Also used for the double-checked locking idiom.  */
  34 static void *global_libgcc_handle;
  35
  36 /* We cannot use __libc_once because the pthread_once implementation
  37    may depend on unwinding.  */
  38 __libc_lock_define (static, lock);
  39
  40 struct unwind_link *
  41 __libc_unwind_link_get (void)
  42 {
  43   /* Double-checked locking idiom.  Synchronizes with the release MO
  44      store at the end of this function.  */
  45   if (atomic_load_acquire (&global_libgcc_handle) != NULL)
  46    return &global;
  47
  48   /* Initialize a copy of the data, so that we do not need about
  49      unlocking in case the dynamic loader somehow triggers
  50      unwinding.  */
  51   void *local_libgcc_handle = __libc_dlopen (LIBGCC_S_SO);
  52   if (local_libgcc_handle == NULL)
  53     {
  54       __libc_lock_unlock (lock);
  55       return NULL;
  56     }
  57
  58   struct unwind_link local;
  59   local.ptr__Unwind_Backtrace
  60     = __libc_dlsym (local_libgcc_handle, "_Unwind_Backtrace");
  61   local.ptr__Unwind_ForcedUnwind
  62     = __libc_dlsym (local_libgcc_handle, "_Unwind_ForcedUnwind");
  63   local.ptr__Unwind_GetCFA
  64     = __libc_dlsym (local_libgcc_handle, "_Unwind_GetCFA");
  65 #if UNWIND_LINK_GETIP
  66   local.ptr__Unwind_GetIP
  67     = __libc_dlsym (local_libgcc_handle, "_Unwind_GetIP");
  68 #endif
  69   local.ptr__Unwind_Resume
  70     = __libc_dlsym (local_libgcc_handle, "_Unwind_Resume");
  71 #if UNWIND_LINK_FRAME_STATE_FOR
  72   local.ptr___frame_state_for
  73     = __libc_dlsym (local_libgcc_handle, "__frame_state_for");
  74 #endif
  75   local.ptr_personality
  76     = __libc_dlsym (local_libgcc_handle, "__gcc_personality_v0");
  77   UNWIND_LINK_EXTRA_INIT
  78
  79   /* If a symbol is missing, libgcc_s has somehow been corrupted.  */
  80   assert (local.ptr__Unwind_Backtrace != NULL);
  81   assert (local.ptr__Unwind_ForcedUnwind != NULL);
  82   assert (local.ptr__Unwind_GetCFA != NULL);
  83 #if UNWIND_LINK_GETIP
  84   assert (local.ptr__Unwind_GetIP != NULL);
  85 #endif
  86   assert (local.ptr__Unwind_Resume != NULL);
  87   assert (local.ptr_personality != NULL);
  88
  89   PTR_MANGLE (local.ptr__Unwind_Backtrace);
  90   PTR_MANGLE (local.ptr__Unwind_ForcedUnwind);
  91   PTR_MANGLE (local.ptr__Unwind_GetCFA);
  92 #if UNWIND_LINK_GETIP
  93   PTR_MANGLE (local.ptr__Unwind_GetIP);
  94 #endif
  95   PTR_MANGLE (local.ptr__Unwind_Resume);
  96 #if UNWIND_LINK_FRAME_STATE_FOR
  97   PTR_MANGLE (local.ptr___frame_state_for);
  98 #endif
  99   PTR_MANGLE (local.ptr_personality);
 100
 101   __libc_lock_lock (lock);
 102   if (atomic_load_relaxed (&global_libgcc_handle) != NULL)
 103     /* This thread lost the race.  Clean up.  */
 104     __libc_dlclose (local_libgcc_handle);
 105   else
 106     {
 107       global = local;
 108
 109       /* Completes the double-checked locking idiom.  */
 110       atomic_store_release (&global_libgcc_handle, local_libgcc_handle);
 111     }
 112
 113   __libc_lock_unlock (lock);
 114   return &global;
 115 }
 116 libc_hidden_def (__libc_unwind_link_get)
 117
 118 void
 119 __libc_unwind_link_after_fork (void)
 120 {
 121   if (__libc_lock_trylock (lock) == 0)
 122     /* The lock was not acquired during the fork.  This covers both
 123        the initialized and uninitialized case.  */
 124     __libc_lock_unlock (lock);
 125   else
 126     {
 127       /* Initialization was in progress in another thread.
 128          Reinitialize the lock.  */
 129       __libc_lock_init (lock);
 130       global_libgcc_handle = NULL;
 131     }
 132 }
 133
 134 void
 135 __libc_unwind_link_freeres (void)
 136 {
 137   if (global_libgcc_handle != NULL)
 138     {
 139       __libc_dlclose (global_libgcc_handle );
 140       global_libgcc_handle = NULL;
 141     }
 142 }
 143
 144 #endif /* SHARED */