With this change, all control data for _dl_find_object is either
RELRO data, or in the protected area, or tightly constrained
(the version counter is always masked using & 1 before array
indexing).
This commit can serve as an example how to extend the protected
memory area.
---
elf/dl-find_object.c | 39 +++++++++++++++++++-------------------
sysdeps/generic/ldsodefs.h | 9 +++++++++
2 files changed, 29 insertions(+), 19 deletions(-)
On Thu, 7 Dec 2023, Florian Weimer wrote:
> With this change, all control data for _dl_find_object is either
> RELRO data, or in the protected area, or tightly constrained
> (the version counter is always masked using & 1 before array
> indexing).
>
> This commit can serve as an example how to extend the protected
> memory area.
OK.
@@ -120,13 +120,6 @@ struct dlfo_mappings_segment
struct dl_find_object_internal objects[]; /* Read in the TM region. */
};
-/* To achieve async-signal-safety, two copies of the data structure
- are used, so that a signal handler can still use this data even if
- dlopen or dlclose modify the other copy. The the least significant
- bit in _dlfo_loaded_mappings_version determines which array element
- is the currently active region. */
-static struct dlfo_mappings_segment *_dlfo_loaded_mappings[2];
-
/* Returns the number of actually used elements in all segments
starting at SEG. */
static inline size_t
@@ -192,10 +185,17 @@ _dlfo_mappings_segment_allocate (size_t size,
}
/* Monotonic counter for software transactional memory. The lowest
- bit indicates which element of the _dlfo_loaded_mappings contains
- up-to-date data. */
+ bit indicates which element of the GLPM (dlfo_loaded_mappings)
+ contains up-to-date data. This achieves async-signal-safety for
+ _dl_find_object: a signal handler can still use the
+ GLPM (dlfo_loaded_mappings) data even if dlopen or dlclose
+ modify the other copy. */
static __atomic_wide_counter _dlfo_loaded_mappings_version;
+#ifndef SHARED
+struct dlfo_mappings_segment *_dlfo_loaded_mappings[2];
+#endif
+
/* TM version at the start of the read operation. */
static inline uint64_t
_dlfo_read_start_version (void)
@@ -263,7 +263,7 @@ _dlfo_read_success (uint64_t start_version)
static struct dlfo_mappings_segment *
_dlfo_mappings_active_segment (uint64_t start_version)
{
- return _dlfo_loaded_mappings[start_version & 1];
+ return GLPM (dlfo_loaded_mappings)[start_version & 1];
}
/* Searches PC among the address-sorted array [FIRST1, FIRST1 +
@@ -472,10 +472,10 @@ _dlfo_process_initial (void)
}
else if (l->l_type == lt_loaded)
{
- if (_dlfo_loaded_mappings[0] != NULL)
+ if (GLPM (dlfo_loaded_mappings)[0] != NULL)
/* Second pass only. */
_dl_find_object_from_map
- (l, &_dlfo_loaded_mappings[0]->objects[loaded]);
+ (l, &GLPM (dlfo_loaded_mappings)[0]->objects[loaded]);
++loaded;
}
}
@@ -535,10 +535,10 @@ _dl_find_object_init (void)
= _dl_protmem_allocate (_dlfo_nodelete_mappings_size
* sizeof (*_dlfo_nodelete_mappings));
if (loaded_size > 0)
- _dlfo_loaded_mappings[0]
+ GLPM (dlfo_loaded_mappings)[0]
= _dlfo_mappings_segment_allocate (loaded_size, NULL);
if (_dlfo_nodelete_mappings == NULL
- || (loaded_size > 0 && _dlfo_loaded_mappings[0] == NULL))
+ || (loaded_size > 0 && GLPM (dlfo_loaded_mappings)[0] == NULL))
_dl_fatal_printf ("\
Fatal glibc error: cannot allocate memory for find-object data\n");
/* Fill in the data with the second call. */
@@ -554,8 +554,8 @@ Fatal glibc error: cannot allocate memory for find-object data\n");
_dlfo_nodelete_mappings_end = _dlfo_nodelete_mappings[last_idx].map_end;
}
if (loaded_size > 0)
- _dlfo_sort_mappings (_dlfo_loaded_mappings[0]->objects,
- _dlfo_loaded_mappings[0]->size);
+ _dlfo_sort_mappings (GLPM (dlfo_loaded_mappings)[0]->objects,
+ GLPM (dlfo_loaded_mappings)[0]->size);
}
static void
@@ -609,11 +609,11 @@ _dl_find_object_update_1 (struct link_map_private **loaded, size_t count)
int active_idx = _dlfo_read_version_locked () & 1;
struct dlfo_mappings_segment *current_seg
- = _dlfo_loaded_mappings[active_idx];
+ = GLPM (dlfo_loaded_mappings)[active_idx];
size_t current_used = _dlfo_mappings_segment_count_used (current_seg);
struct dlfo_mappings_segment *target_seg
- = _dlfo_loaded_mappings[!active_idx];
+ = GLPM (dlfo_loaded_mappings)[!active_idx];
size_t remaining_to_add = current_used + count;
/* Ensure that the new segment chain has enough space. */
@@ -634,7 +634,8 @@ _dl_find_object_update_1 (struct link_map_private **loaded, size_t count)
/* The barrier ensures that a concurrent TM read or fork does
not see a partially initialized segment. */
- atomic_store_release (&_dlfo_loaded_mappings[!active_idx], target_seg);
+ atomic_store_release (&GLPM (dlfo_loaded_mappings)[!active_idx],
+ target_seg);
}
else
/* Start update cycle without allocation. */
@@ -523,6 +523,8 @@ extern struct rtld_global _rtld_global __rtld_global_attribute__;
# undef __rtld_global_attribute__
#endif
+struct dlfo_mappings_segment;
+
#ifdef SHARED
/* Implementation structure for the protected memory area. In static
builds, the protected memory area is just regular (.data) memory,
@@ -532,6 +534,13 @@ struct rtld_protmem
{
/* Structure describing the dynamic linker itself. */
EXTERN struct link_map_private _dl_rtld_map;
+#endif /* SHARED */
+
+ /* Two copies of the data structures for _dl_find_object. See
+ _dlfo_loaded_mappings_version in dl-find_object.c. */
+ EXTERN struct dlfo_mappings_segment *_dlfo_loaded_mappings[2];
+
+#ifdef SHARED
};
#endif /* SHARED */