1/* Hash table for TLS descriptors.
2 Copyright (C) 2005-2021 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4 Contributed by Alexandre Oliva <aoliva@redhat.com>
5
6 The GNU C Library is free software; you can redistribute it and/or
7 modify it under the terms of the GNU Lesser General Public
8 License as published by the Free Software Foundation; either
9 version 2.1 of the License, or (at your option) any later version.
10
11 The GNU C Library is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 Lesser General Public License for more details.
15
16 You should have received a copy of the GNU Lesser General Public
17 License along with the GNU C Library; if not, see
18 <https://www.gnu.org/licenses/>. */
19
20#ifndef TLSDESCHTAB_H
21# define TLSDESCHTAB_H 1
22
23#include <atomic.h>
24
25# ifdef SHARED
26
27# include <inline-hashtab.h>
28
29inline static int
30hash_tlsdesc (void *p)
31{
32 struct tlsdesc_dynamic_arg *td = p;
33
34 /* We know all entries are for the same module, so ti_offset is the
35 only distinguishing entry. */
36 return td->tlsinfo.ti_offset;
37}
38
39inline static int
40eq_tlsdesc (void *p, void *q)
41{
42 struct tlsdesc_dynamic_arg *tdp = p, *tdq = q;
43
44 return tdp->tlsinfo.ti_offset == tdq->tlsinfo.ti_offset;
45}
46
47inline static size_t
48map_generation (struct link_map *map)
49{
50 size_t idx = map->l_tls_modid;
51 struct dtv_slotinfo_list *listp = GL(dl_tls_dtv_slotinfo_list);
52
53 /* Find the place in the dtv slotinfo list. */
54 do
55 {
56 /* Does it fit in the array of this list element? */
57 if (idx < listp->len)
58 {
59 /* We should never get here for a module in static TLS, so
60 we can assume that, if the generation count is zero, we
61 still haven't determined the generation count for this
62 module. */
63 if (listp->slotinfo[idx].map == map && listp->slotinfo[idx].gen)
64 return listp->slotinfo[idx].gen;
65 else
66 break;
67 }
68 idx -= listp->len;
69 listp = listp->next;
70 }
71 while (listp != NULL);
72
73 /* If we get to this point, the module still hasn't been assigned an
74 entry in the dtv slotinfo data structures, and it will when we're
75 done with relocations. At that point, the module will get a
76 generation number that is one past the current generation, so
77 return exactly that. */
78 return GL(dl_tls_generation) + 1;
79}
80
81void *
82_dl_make_tlsdesc_dynamic (struct link_map *map, size_t ti_offset)
83{
84 struct hashtab *ht;
85 void **entry;
86 struct tlsdesc_dynamic_arg *td, test;
87
88 /* FIXME: We could use a per-map lock here, but is it worth it? */
89 __rtld_lock_lock_recursive (GL(dl_load_lock));
90
91 ht = map->l_mach.tlsdesc_table;
92 if (! ht)
93 {
94 ht = htab_create ();
95 if (! ht)
96 {
97 __rtld_lock_unlock_recursive (GL(dl_load_lock));
98 return 0;
99 }
100 map->l_mach.tlsdesc_table = ht;
101 }
102
103 test.tlsinfo.ti_module = map->l_tls_modid;
104 test.tlsinfo.ti_offset = ti_offset;
105 entry = htab_find_slot (ht, &test, 1, hash_tlsdesc, eq_tlsdesc);
106 if (! entry)
107 {
108 __rtld_lock_unlock_recursive (GL(dl_load_lock));
109 return 0;
110 }
111
112 if (*entry)
113 {
114 td = *entry;
115 __rtld_lock_unlock_recursive (GL(dl_load_lock));
116 return td;
117 }
118
119 *entry = td = malloc (sizeof (struct tlsdesc_dynamic_arg));
120 /* This may be higher than the map's generation, but it doesn't
121 matter much. Worst case, we'll have one extra DTV update per
122 thread. */
123 td->gen_count = map_generation (map);
124 td->tlsinfo = test.tlsinfo;
125
126 __rtld_lock_unlock_recursive (GL(dl_load_lock));
127 return td;
128}
129
130# endif /* SHARED */
131
132/* The idea of the following two functions is to stop multiple threads
133 from attempting to resolve the same TLS descriptor without busy
134 waiting. Ideally, we should be able to release the lock right
135 after changing td->entry, and then using say a condition variable
136 or a futex wake to wake up any waiting threads, but let's try to
137 avoid introducing such dependencies. */
138
139static int
140__attribute__ ((unused))
141_dl_tlsdesc_resolve_early_return_p (struct tlsdesc volatile *td, void *caller)
142{
143 if (caller != atomic_load_relaxed (&td->entry))
144 return 1;
145
146 __rtld_lock_lock_recursive (GL(dl_load_lock));
147 if (caller != atomic_load_relaxed (&td->entry))
148 {
149 __rtld_lock_unlock_recursive (GL(dl_load_lock));
150 return 1;
151 }
152
153 atomic_store_relaxed (&td->entry, _dl_tlsdesc_resolve_hold);
154
155 return 0;
156}
157
158static void
159__attribute__ ((unused))
160_dl_tlsdesc_wake_up_held_fixups (void)
161{
162 __rtld_lock_unlock_recursive (GL(dl_load_lock));
163}
164
165#endif
166