elf: Add _dl_find_object function

It can be used to speed up the libgcc unwinder, and the internal
_dl_find_dso_for_object function (which is used for caller
identification in dlopen and related functions, and in dladdr).

_dl_find_object is in the internal namespace due to bug 28503.
If libgcc switches to _dl_find_object, this namespace issue will
be fixed.  It is located in libc for two reasons: it is necessary
to forward the call to the static libc after static dlopen, and
there is a link ordering issue with -static-libgcc and libgcc_eh.a
because libc.so is not a linker script that includes ld.so in the
glibc build tree (so that GCC's internal -lc after libgcc_eh.a does
not pick up ld.so).

It is necessary to do the i386 customization in the
sysdeps/x86/bits/dl_find_object.h header shared with x86-64 because
otherwise, multilib installations are broken.

The implementation uses software transactional memory, as suggested
by Torvald Riegel.  Two copies of the supporting data structures are
used, also achieving full async-signal-safety.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

1 files changed, 115 insertions, 0 deletions

diff --git a/elf/dl-find_object.h b/elf/dl-find_object.h
new file mode 100644
index 0000000000..f899905e09
--- /dev/null
+++ b/elf/dl-find_object.h
@@ -0,0 +1,115 @@
+/* Locating objects in the process image.  ld.so implementation.
+   Copyright (C) 2021 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/>.  */
+
+#ifndef _DL_FIND_EH_FRAME_H
+#define _DL_FIND_EH_FRAME_H
+
+#include <assert.h>
+#include <dlfcn.h>
+#include <ldsodefs.h>
+#include <stdbool.h>
+#include <stdint.h>
+
+/* Internal version of struct dl_find_object.  Does not include the
+   (yet unused) flags member.  We need to make a copy of data also in
+   struct link_map to support non-contiguous mappings, and to support
+   software transactional memory (the link map is not covered by
+   transactions).  */
+struct dl_find_object_internal
+{
+  uintptr_t map_start;
+  uintptr_t map_end;            /* Set to map_start by dlclose.  */
+  struct link_map *map;         /* Set to NULL by dlclose.  */
+  void *eh_frame;
+#if DLFO_STRUCT_HAS_EH_DBASE
+  void *eh_dbase;
+#endif
+#if DLFO_STRUCT_HAS_EH_COUNT
+  int eh_count;
+#endif
+};
+
+static inline void
+_dl_find_object_to_external (struct dl_find_object_internal *internal,
+                             struct dl_find_object *external)
+{
+  external->dlfo_flags = 0;
+  external->dlfo_map_start = (void *) internal->map_start;
+  external->dlfo_map_end = (void *) internal->map_end;
+  external->dlfo_link_map = internal->map;
+  external->dlfo_eh_frame = internal->eh_frame;
+# if DLFO_STRUCT_HAS_EH_DBASE
+  external->dlfo_eh_dbase = internal->eh_dbase;
+# endif
+# if DLFO_STRUCT_HAS_EH_COUNT
+  external->dlfo_eh_count = internal->eh_count;
+# endif
+}
+
+/* Extract the object location data from a link map and writes it to
+   *RESULT.  */
+static void __attribute__ ((unused))
+_dl_find_object_from_map (struct link_map *l,
+                          struct dl_find_object_internal *result)
+{
+  result->map_start = (uintptr_t) l->l_map_start;
+  result->map_end = (uintptr_t) l->l_map_end;
+  result->map = l;
+
+#if DLFO_STRUCT_HAS_EH_DBASE
+  result->eh_dbase = (void *) l->l_info[DT_PLTGOT];
+#endif
+
+  for (const ElfW(Phdr) *ph = l->l_phdr, *ph_end = l->l_phdr + l->l_phnum;
+       ph < ph_end; ++ph)
+    if (ph->p_type == DLFO_EH_SEGMENT_TYPE)
+      {
+        result->eh_frame = (void *) (ph->p_vaddr + l->l_addr);
+#if DLFO_STRUCT_HAS_EH_COUNT
+        result->eh_count = ph->p_memsz / 8;
+#endif
+        return;
+      }
+
+  /* Object has no exception handling segment.  */
+  result->eh_frame = NULL;
+#if DLFO_STRUCT_HAS_EH_COUNT
+  result->eh_count = 0;
+#endif
+}
+
+/* Called by the dynamic linker to set up the data structures for the
+   initially loaded objects.  This creates a few persistent
+   allocations, so it should be called with the minimal malloc.  */
+void _dl_find_object_init (void) attribute_hidden;
+
+/* Called by dlopen/dlmopen to add new objects to the DWARF EH frame
+   data structures.  NEW_MAP is the dlopen'ed link map.  Link maps on
+   the l_next list are added if l_object_processed is 0.  Needs to
+   be protected by loader write lock.  Returns true on success, false
+   on malloc failure.  */
+bool _dl_find_object_update (struct link_map *new_map) attribute_hidden;
+
+/* Called by dlclose to remove the link map from the DWARF EH frame
+   data structures.  Needs to be protected by loader write lock.  */
+void _dl_find_object_dlclose (struct link_map *l) attribute_hidden;
+
+/* Called from __libc_freeres to deallocate malloc'ed memory.  */
+void _dl_find_object_freeres (void) attribute_hidden;
+
+#endif /* _DL_FIND_OBJECT_H */