misc/unwind-link.c

   1 /* Dynamic loading of the libgcc unwinder.
   2    Copyright (C) 2021-2022 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
  27 /* Statically allocate the object, so that we do not have to deal with
  28    malloc failure.  __libc_unwind_link_get must not fail if libgcc_s
  29    has already been loaded by other means.  */
  30 static struct unwind_link global;
  31
  32 /* dlopen handle.  Also used for the double-checked locking idiom.  */
  33 static void *global_libgcc_handle;
  34
  35 /* We cannot use __libc_once because the pthread_once implementation
  36    may depend on unwinding.  */
  37 __libc_lock_define (static, lock);
  38
  39 struct unwind_link *
  40 __libc_unwind_link_get (void)
  41 {
  42   /* Double-checked locking idiom.  Synchronizes with the release MO
  43      store at the end of this function.  */
  44   if (atomic_load_acquire (&global_libgcc_handle) != NULL)
  45    return &global;
  46
  47   /* Initialize a copy of the data, so that we do not need about
  48      unlocking in case the dynamic loader somehow triggers
  49      unwinding.  */
  50   void *local_libgcc_handle = __libc_dlopen (LIBGCC_S_SO);
  51   if (local_libgcc_handle == NULL)
  52     {
  53       __libc_lock_unlock (lock);
  54       return NULL;
  55     }
  56
  57   struct unwind_link local;
  58   local.ptr__Unwind_Backtrace
  59     = __libc_dlsym (local_libgcc_handle, "_Unwind_Backtrace");
  60   local.ptr__Unwind_ForcedUnwind
  61     = __libc_dlsym (local_libgcc_handle, "_Unwind_ForcedUnwind");
  62   local.ptr__Unwind_GetCFA
  63     = __libc_dlsym (local_libgcc_handle, "_Unwind_GetCFA");
  64 #if UNWIND_LINK_GETIP
  65   local.ptr__Unwind_GetIP
  66     = __libc_dlsym (local_libgcc_handle, "_Unwind_GetIP");
  67 #endif
  68   local.ptr__Unwind_Resume
  69     = __libc_dlsym (local_libgcc_handle, "_Unwind_Resume");
  70 #if UNWIND_LINK_FRAME_STATE_FOR
  71   local.ptr___frame_state_for
  72     = __libc_dlsym (local_libgcc_handle, "__frame_state_for");
  73 #endif
  74   local.ptr_personality
  75     = __libc_dlsym (local_libgcc_handle, "__gcc_personality_v0");
  76   UNWIND_LINK_EXTRA_INIT
  77
  78   /* If a symbol is missing, libgcc_s has somehow been corrupted.  */
  79   assert (local.ptr__Unwind_Backtrace != NULL);
  80   assert (local.ptr__Unwind_ForcedUnwind != NULL);
  81   assert (local.ptr__Unwind_GetCFA != NULL);
  82 #if UNWIND_LINK_GETIP
  83   assert (local.ptr__Unwind_GetIP != NULL);
  84 #endif
  85   assert (local.ptr__Unwind_Resume != NULL);
  86   assert (local.ptr_personality != NULL);
  87
  88 #ifdef PTR_MANGLE
  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 #endif
 101
 102   __libc_lock_lock (lock);
 103   if (atomic_load_relaxed (&global_libgcc_handle) != NULL)
 104     /* This thread lost the race.  Clean up.  */
 105     __libc_dlclose (local_libgcc_handle);
 106   else
 107     {
 108       global = local;
 109
 110       /* Completes the double-checked locking idiom.  */
 111       atomic_store_release (&global_libgcc_handle, local_libgcc_handle);
 112     }
 113
 114   __libc_lock_unlock (lock);
 115   return &global;
 116 }
 117 libc_hidden_def (__libc_unwind_link_get)
 118
 119 void
 120 __libc_unwind_link_after_fork (void)
 121 {
 122   if (__libc_lock_trylock (lock) == 0)
 123     /* The lock was not acquired during the fork.  This covers both
 124        the initialized and uninitialized case.  */
 125     __libc_lock_unlock (lock);
 126   else
 127     {
 128       /* Initialization was in progress in another thread.
 129          Reinitialize the lock.  */
 130       __libc_lock_init (lock);
 131       global_libgcc_handle = NULL;
 132     }
 133 }
 134
 135 void __libc_freeres_fn_section
 136 __libc_unwind_link_freeres (void)
 137 {
 138   if (global_libgcc_handle != NULL)
 139     {
 140       __libc_dlclose (global_libgcc_handle );
 141       global_libgcc_handle = NULL;
 142     }
 143 }
 144
 145 #endif /* SHARED */