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/* Dynamic loading of the libgcc unwinder.
Copyright (C) 2021-2025 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);
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);
__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_unwind_link_freeres (void)
{
if (global_libgcc_handle != NULL)
{
__libc_dlclose (global_libgcc_handle );
global_libgcc_handle = NULL;
}
}
#endif /* SHARED */
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