acbaad31e8
As explained in Hans Boehm, Can Seqlocks Get Along with Programming Language Memory Models?, an acquire fence is needed in _dlfo_read_success. The lack of a fence resulted in an observable bug on powerpc64le compile-time load reordering. The fence in _dlfo_mappings_begin_update has been reordered, turning the fence/store sequence into a release MO store equivalent. Relaxed MO loads are used on the reader side, and relaxed MO stores on the writer side for the shared data, to avoid formal data races. This is just to be conservative; it should not actually be necessary given how the data is used. This commit also fixes the test run time. The intent was to run it for 3 seconds, but 0.3 seconds was enough to uncover the bug very occasionally (while 3 seconds did not reliably show the bug on every test run). Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
142 lines
5.2 KiB
C
142 lines
5.2 KiB
C
/* Locating objects in the process image. ld.so implementation.
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Copyright (C) 2021-2022 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<https://www.gnu.org/licenses/>. */
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#ifndef _DL_FIND_EH_FRAME_H
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#define _DL_FIND_EH_FRAME_H
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#include <assert.h>
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#include <atomic.h>
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#include <dlfcn.h>
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#include <ldsodefs.h>
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#include <stdbool.h>
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#include <stdint.h>
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/* Internal version of struct dl_find_object. Does not include the
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(yet unused) flags member. We need to make a copy of data also in
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struct link_map to support non-contiguous mappings, and to support
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software transactional memory (the link map is not covered by
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transactions). */
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struct dl_find_object_internal
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{
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uintptr_t map_start;
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uintptr_t map_end; /* Set to map_start by dlclose. */
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struct link_map *map; /* Set to NULL by dlclose. */
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void *eh_frame;
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#if DLFO_STRUCT_HAS_EH_DBASE
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void *eh_dbase;
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#endif
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#if DLFO_STRUCT_HAS_EH_COUNT
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int eh_count;
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#endif
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};
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/* Create a copy of *SOURCE in *COPY using relaxed MO loads and
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stores. */
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static inline void
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_dl_find_object_internal_copy (const struct dl_find_object_internal *source,
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struct dl_find_object_internal *copy)
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{
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atomic_store_relaxed (©->map_start,
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atomic_load_relaxed (&source->map_start));
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atomic_store_relaxed (©->map_end,
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atomic_load_relaxed (&source->map_end));
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atomic_store_relaxed (©->map,
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atomic_load_relaxed (&source->map));
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atomic_store_relaxed (©->eh_frame,
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atomic_load_relaxed (&source->eh_frame));
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#if DLFO_STRUCT_HAS_EH_DBASE
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atomic_store_relaxed (©->eh_dbase,
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atomic_load_relaxed (&source->eh_dbase));
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#endif
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#if DLFO_STRUCT_HAS_EH_COUNT
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atomic_store_relaxed (©->eh_count,
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atomic_load_relaxed (&source->eh_count));
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#endif
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}
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static inline void
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_dl_find_object_to_external (struct dl_find_object_internal *internal,
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struct dl_find_object *external)
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{
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external->dlfo_flags = 0;
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external->dlfo_map_start = (void *) internal->map_start;
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external->dlfo_map_end = (void *) internal->map_end;
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external->dlfo_link_map = internal->map;
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external->dlfo_eh_frame = internal->eh_frame;
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# if DLFO_STRUCT_HAS_EH_DBASE
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external->dlfo_eh_dbase = internal->eh_dbase;
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# endif
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# if DLFO_STRUCT_HAS_EH_COUNT
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external->dlfo_eh_count = internal->eh_count;
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# endif
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}
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/* Extract the object location data from a link map and writes it to
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*RESULT using relaxed MO stores. */
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static void __attribute__ ((unused))
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_dl_find_object_from_map (struct link_map *l,
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struct dl_find_object_internal *result)
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{
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atomic_store_relaxed (&result->map_start, (uintptr_t) l->l_map_start);
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atomic_store_relaxed (&result->map_end, (uintptr_t) l->l_map_end);
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atomic_store_relaxed (&result->map, l);
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#if DLFO_STRUCT_HAS_EH_DBASE
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atomic_store_relaxed (&result->eh_dbase, (void *) l->l_info[DT_PLTGOT]);
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#endif
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for (const ElfW(Phdr) *ph = l->l_phdr, *ph_end = l->l_phdr + l->l_phnum;
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ph < ph_end; ++ph)
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if (ph->p_type == DLFO_EH_SEGMENT_TYPE)
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{
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atomic_store_relaxed (&result->eh_frame,
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(void *) (ph->p_vaddr + l->l_addr));
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#if DLFO_STRUCT_HAS_EH_COUNT
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atomic_store_relaxed (&result->eh_count, ph->p_memsz / 8);
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#endif
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return;
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}
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/* Object has no exception handling segment. */
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atomic_store_relaxed (&result->eh_frame, NULL);
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#if DLFO_STRUCT_HAS_EH_COUNT
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atomic_store_relaxed (&result->eh_count, 0);
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#endif
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}
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/* Called by the dynamic linker to set up the data structures for the
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initially loaded objects. This creates a few persistent
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allocations, so it should be called with the minimal malloc. */
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void _dl_find_object_init (void) attribute_hidden;
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/* Called by dlopen/dlmopen to add new objects to the DWARF EH frame
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data structures. NEW_MAP is the dlopen'ed link map. Link maps on
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the l_next list are added if l_object_processed is 0. Needs to
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be protected by loader write lock. Returns true on success, false
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on malloc failure. */
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bool _dl_find_object_update (struct link_map *new_map) attribute_hidden;
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/* Called by dlclose to remove the link map from the DWARF EH frame
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data structures. Needs to be protected by loader write lock. */
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void _dl_find_object_dlclose (struct link_map *l) attribute_hidden;
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/* Called from __libc_freeres to deallocate malloc'ed memory. */
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void _dl_find_object_freeres (void) attribute_hidden;
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#endif /* _DL_FIND_OBJECT_H */
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