dlfcn.cpp revision 22d629211d72adaf46f3fc48f59540f8e5798b1e 
1/*
2 * Copyright (C) 2007 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *      http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17#include <dlfcn.h>
18#include <pthread.h>
19#include <stdio.h>
20
21#include <private/ScopedPthreadMutexLocker.h>
22
23#include "linker.h"
24#include "linker_format.h"
25
26/* This file hijacks the symbols stubbed out in libdl.so. */
27
28static char dl_err_buf[1024];
29static const char* dl_err_str;
30
31#define likely(expr)   __builtin_expect (expr, 1)
32#define unlikely(expr) __builtin_expect (expr, 0)
33
34static pthread_mutex_t gDlMutex = PTHREAD_RECURSIVE_MUTEX_INITIALIZER;
35
36static void set_dlerror(const char* msg, const char* detail) {
37  if (detail != NULL) {
38    format_buffer(dl_err_buf, sizeof(dl_err_buf), "%s: %s", msg, detail);
39  } else {
40    format_buffer(dl_err_buf, sizeof(dl_err_buf), "%s", msg);
41  }
42  dl_err_str = (const char*) &dl_err_buf[0];
43}
44
45void *dlopen(const char* filename, int flag) {
46  ScopedPthreadMutexLocker locker(&gDlMutex);
47  soinfo* result = find_library(filename);
48  if (result == NULL) {
49    set_dlerror("dlopen failed", linker_get_error());
50    return NULL;
51  }
52  soinfo_call_constructors(result);
53  result->refcount++;
54  return result;
55}
56
57const char* dlerror() {
58  const char* old_value = dl_err_str;
59  dl_err_str = NULL;
60  return (const char*) old_value;
61}
62
63void* dlsym(void* handle, const char* symbol) {
64  ScopedPthreadMutexLocker locker(&gDlMutex);
65
66  if (unlikely(handle == 0)) {
67    set_dlerror("dlsym library handle is null", NULL);
68    return NULL;
69  }
70  if (unlikely(symbol == 0)) {
71    set_dlerror("dlsym symbol name is null", NULL);
72    return NULL;
73  }
74
75  soinfo* found = NULL;
76  Elf32_Sym* sym = NULL;
77  if (handle == RTLD_DEFAULT) {
78    sym = lookup(symbol, &found, NULL);
79  } else if (handle == RTLD_NEXT) {
80    void* ret_addr = __builtin_return_address(0);
81    soinfo* si = find_containing_library(ret_addr);
82
83    sym = NULL;
84    if (si && si->next) {
85      sym = lookup(symbol, &found, si->next);
86    }
87  } else {
88    found = (soinfo*) handle;
89    sym = soinfo_lookup(found, symbol);
90  }
91
92  if (likely(sym != 0)) {
93    unsigned bind = ELF32_ST_BIND(sym->st_info);
94
95    if (likely((bind == STB_GLOBAL) && (sym->st_shndx != 0))) {
96      unsigned ret = sym->st_value + found->load_bias;
97      return (void*) ret;
98    }
99
100    set_dlerror("symbol found but not global", symbol);
101    return NULL;
102  } else {
103    set_dlerror("undefined symbol", symbol);
104    return NULL;
105  }
106}
107
108int dladdr(const void* addr, Dl_info* info) {
109  ScopedPthreadMutexLocker locker(&gDlMutex);
110
111  // Determine if this address can be found in any library currently mapped.
112  soinfo* si = find_containing_library(addr);
113  if (si == NULL) {
114    return 0;
115  }
116
117  memset(info, 0, sizeof(Dl_info));
118
119  info->dli_fname = si->name;
120  // Address at which the shared object is loaded.
121  info->dli_fbase = (void*) si->base;
122
123  // Determine if any symbol in the library contains the specified address.
124  Elf32_Sym *sym = soinfo_find_symbol(si, addr);
125  if (sym != NULL) {
126    info->dli_sname = si->strtab + sym->st_name;
127    info->dli_saddr = (void*)(si->load_bias + sym->st_value);
128  }
129
130  return 1;
131}
132
133int dlclose(void* handle) {
134  ScopedPthreadMutexLocker locker(&gDlMutex);
135  return soinfo_unload((soinfo*) handle);
136}
137
138#if defined(ANDROID_ARM_LINKER)
139//                     0000000 00011111 111112 22222222 2333333 333344444444445555555
140//                     0123456 78901234 567890 12345678 9012345 678901234567890123456
141#define ANDROID_LIBDL_STRTAB \
142                      "dlopen\0dlclose\0dlsym\0dlerror\0dladdr\0dl_unwind_find_exidx\0"
143
144#elif defined(ANDROID_X86_LINKER) || defined(ANDROID_MIPS_LINKER)
145//                     0000000 00011111 111112 22222222 2333333 3333444444444455
146//                     0123456 78901234 567890 12345678 9012345 6789012345678901
147#define ANDROID_LIBDL_STRTAB \
148                      "dlopen\0dlclose\0dlsym\0dlerror\0dladdr\0dl_iterate_phdr\0"
149#else
150#error Unsupported architecture. Only ARM, MIPS, and x86 are presently supported.
151#endif
152
153// name_offset: starting index of the name in libdl_info.strtab
154#define ELF32_SYM_INITIALIZER(name_offset, value, shndx) \
155    { name_offset, \
156      reinterpret_cast<Elf32_Addr>(reinterpret_cast<void*>(value)), \
157      /* st_size */ 0, \
158      (shndx == 0) ? 0 : (STB_GLOBAL << 4), \
159      /* st_other */ 0, \
160      shndx }
161
162static Elf32_Sym libdl_symtab[] = {
163  // Total length of libdl_info.strtab, including trailing 0.
164  // This is actually the STH_UNDEF entry. Technically, it's
165  // supposed to have st_name == 0, but instead, it points to an index
166  // in the strtab with a \0 to make iterating through the symtab easier.
167  ELF32_SYM_INITIALIZER(sizeof(ANDROID_LIBDL_STRTAB) - 1, NULL, 0),
168  ELF32_SYM_INITIALIZER( 0, &dlopen, 1),
169  ELF32_SYM_INITIALIZER( 7, &dlclose, 1),
170  ELF32_SYM_INITIALIZER(15, &dlsym, 1),
171  ELF32_SYM_INITIALIZER(21, &dlerror, 1),
172  ELF32_SYM_INITIALIZER(29, &dladdr, 1),
173#if defined(ANDROID_ARM_LINKER)
174  ELF32_SYM_INITIALIZER(36, &dl_unwind_find_exidx, 1),
175#elif defined(ANDROID_X86_LINKER) || defined(ANDROID_MIPS_LINKER)
176  ELF32_SYM_INITIALIZER(36, &dl_iterate_phdr, 1),
177#endif
178};
179
180// Fake out a hash table with a single bucket.
181// A search of the hash table will look through
182// libdl_symtab starting with index [1], then
183// use libdl_chains to find the next index to
184// look at.  libdl_chains should be set up to
185// walk through every element in libdl_symtab,
186// and then end with 0 (sentinel value).
187//
188// That is, libdl_chains should look like
189// { 0, 2, 3, ... N, 0 } where N is the number
190// of actual symbols, or nelems(libdl_symtab)-1
191// (since the first element of libdl_symtab is not
192// a real symbol).
193//
194// (see soinfo_elf_lookup())
195//
196// Note that adding any new symbols here requires
197// stubbing them out in libdl.
198static unsigned libdl_buckets[1] = { 1 };
199static unsigned libdl_chains[7] = { 0, 2, 3, 4, 5, 6, 0 };
200
201// This is used by the dynamic linker. Every process gets these symbols for free.
202soinfo libdl_info = {
203    name: "libdl.so",
204
205    phdr: 0, phnum: 0,
206    entry: 0, base: 0, size: 0,
207    unused: 0, dynamic: 0, unused2: 0, unused3: 0,
208    next: 0,
209
210    flags: FLAG_LINKED,
211
212    strtab: ANDROID_LIBDL_STRTAB,
213    symtab: libdl_symtab,
214
215    nbucket: 1,
216    nchain: 7,
217    bucket: libdl_buckets,
218    chain: libdl_chains,
219
220    plt_got: 0, plt_rel: 0, plt_rel_count: 0, rel: 0, rel_count: 0,
221    preinit_array: 0, preinit_array_count: 0, init_array: 0, init_array_count: 0,
222    fini_array: 0, fini_array_count: 0, init_func: 0, fini_func: 0,
223
224#if defined(ANDROID_ARM_LINKER)
225    ARM_exidx: 0, ARM_exidx_count: 0,
226#elif defined(ANDROID_MIPS_LINKER)
227    mips_symtabno: 0, mips_local_gotno: 0, mips_gotsym: 0,
228#endif
229
230    refcount: 0,
231    { l_addr: 0, l_name: 0, l_ld: 0, l_next: 0, l_prev: 0, },
232    constructors_called: 0, load_bias: 0, has_text_relocations: 0,
233};
234