summaryrefslogtreecommitdiff
path: root/absl/debugging/internal/elf_mem_image.cc
blob: a2bea919957eaae78277495522d3d2ce063179c0 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
// Copyright 2017 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

// Allow dynamic symbol lookup in an in-memory Elf image.
//

#include "absl/debugging/internal/elf_mem_image.h"

#ifdef ABSL_HAVE_ELF_MEM_IMAGE  // defined in elf_mem_image.h

#include <string.h>
#include <cassert>
#include <cstddef>
#include "absl/base/internal/raw_logging.h"

// From binutils/include/elf/common.h (this doesn't appear to be documented
// anywhere else).
//
//   /* This flag appears in a Versym structure.  It means that the symbol
//      is hidden, and is only visible with an explicit version number.
//      This is a GNU extension.  */
//   #define VERSYM_HIDDEN           0x8000
//
//   /* This is the mask for the rest of the Versym information.  */
//   #define VERSYM_VERSION          0x7fff

#define VERSYM_VERSION 0x7fff

namespace absl {
inline namespace lts_2018_06_20 {
namespace debugging_internal {

namespace {

#if __WORDSIZE == 32
const int kElfClass = ELFCLASS32;
int ElfBind(const ElfW(Sym) *symbol) { return ELF32_ST_BIND(symbol->st_info); }
int ElfType(const ElfW(Sym) *symbol) { return ELF32_ST_TYPE(symbol->st_info); }
#elif __WORDSIZE == 64
const int kElfClass = ELFCLASS64;
int ElfBind(const ElfW(Sym) *symbol) { return ELF64_ST_BIND(symbol->st_info); }
int ElfType(const ElfW(Sym) *symbol) { return ELF64_ST_TYPE(symbol->st_info); }
#else
const int kElfClass = -1;
int ElfBind(const ElfW(Sym) *) {
  ABSL_RAW_LOG(FATAL, "Unexpected word size");
  return 0;
}
int ElfType(const ElfW(Sym) *) {
  ABSL_RAW_LOG(FATAL, "Unexpected word size");
  return 0;
}
#endif

// Extract an element from one of the ELF tables, cast it to desired type.
// This is just a simple arithmetic and a glorified cast.
// Callers are responsible for bounds checking.
template <typename T>
const T *GetTableElement(const ElfW(Ehdr) * ehdr, ElfW(Off) table_offset,
                         ElfW(Word) element_size, size_t index) {
  return reinterpret_cast<const T*>(reinterpret_cast<const char *>(ehdr)
                                    + table_offset
                                    + index * element_size);
}

}  // namespace

// The value of this variable doesn't matter; it's used only for its
// unique address.
const int ElfMemImage::kInvalidBaseSentinel = 0;

ElfMemImage::ElfMemImage(const void *base) {
  ABSL_RAW_CHECK(base != kInvalidBase, "bad pointer");
  Init(base);
}

int ElfMemImage::GetNumSymbols() const {
  if (!hash_) {
    return 0;
  }
  // See http://www.caldera.com/developers/gabi/latest/ch5.dynamic.html#hash
  return hash_[1];
}

const ElfW(Sym) *ElfMemImage::GetDynsym(int index) const {
  ABSL_RAW_CHECK(index < GetNumSymbols(), "index out of range");
  return dynsym_ + index;
}

const ElfW(Versym) *ElfMemImage::GetVersym(int index) const {
  ABSL_RAW_CHECK(index < GetNumSymbols(), "index out of range");
  return versym_ + index;
}

const ElfW(Phdr) *ElfMemImage::GetPhdr(int index) const {
  ABSL_RAW_CHECK(index < ehdr_->e_phnum, "index out of range");
  return GetTableElement<ElfW(Phdr)>(ehdr_,
                                     ehdr_->e_phoff,
                                     ehdr_->e_phentsize,
                                     index);
}

const char *ElfMemImage::GetDynstr(ElfW(Word) offset) const {
  ABSL_RAW_CHECK(offset < strsize_, "offset out of range");
  return dynstr_ + offset;
}

const void *ElfMemImage::GetSymAddr(const ElfW(Sym) *sym) const {
  if (sym->st_shndx == SHN_UNDEF || sym->st_shndx >= SHN_LORESERVE) {
    // Symbol corresponds to "special" (e.g. SHN_ABS) section.
    return reinterpret_cast<const void *>(sym->st_value);
  }
  ABSL_RAW_CHECK(link_base_ < sym->st_value, "symbol out of range");
  return GetTableElement<char>(ehdr_, 0, 1, sym->st_value - link_base_);
}

const ElfW(Verdef) *ElfMemImage::GetVerdef(int index) const {
  ABSL_RAW_CHECK(0 <= index && static_cast<size_t>(index) <= verdefnum_,
                 "index out of range");
  const ElfW(Verdef) *version_definition = verdef_;
  while (version_definition->vd_ndx < index && version_definition->vd_next) {
    const char *const version_definition_as_char =
        reinterpret_cast<const char *>(version_definition);
    version_definition =
        reinterpret_cast<const ElfW(Verdef) *>(version_definition_as_char +
                                               version_definition->vd_next);
  }
  return version_definition->vd_ndx == index ? version_definition : nullptr;
}

const ElfW(Verdaux) *ElfMemImage::GetVerdefAux(
    const ElfW(Verdef) *verdef) const {
  return reinterpret_cast<const ElfW(Verdaux) *>(verdef+1);
}

const char *ElfMemImage::GetVerstr(ElfW(Word) offset) const {
  ABSL_RAW_CHECK(offset < strsize_, "offset out of range");
  return dynstr_ + offset;
}

void ElfMemImage::Init(const void *base) {
  ehdr_      = nullptr;
  dynsym_    = nullptr;
  dynstr_    = nullptr;
  versym_    = nullptr;
  verdef_    = nullptr;
  hash_      = nullptr;
  strsize_   = 0;
  verdefnum_ = 0;
  link_base_ = ~0L;  // Sentinel: PT_LOAD .p_vaddr can't possibly be this.
  if (!base) {
    return;
  }
  const char *const base_as_char = reinterpret_cast<const char *>(base);
  if (base_as_char[EI_MAG0] != ELFMAG0 || base_as_char[EI_MAG1] != ELFMAG1 ||
      base_as_char[EI_MAG2] != ELFMAG2 || base_as_char[EI_MAG3] != ELFMAG3) {
    assert(false);
    return;
  }
  int elf_class = base_as_char[EI_CLASS];
  if (elf_class != kElfClass) {
    assert(false);
    return;
  }
  switch (base_as_char[EI_DATA]) {
    case ELFDATA2LSB: {
      if (__LITTLE_ENDIAN != __BYTE_ORDER) {
        assert(false);
        return;
      }
      break;
    }
    case ELFDATA2MSB: {
      if (__BIG_ENDIAN != __BYTE_ORDER) {
        assert(false);
        return;
      }
      break;
    }
    default: {
      assert(false);
      return;
    }
  }

  ehdr_ = reinterpret_cast<const ElfW(Ehdr) *>(base);
  const ElfW(Phdr) *dynamic_program_header = nullptr;
  for (int i = 0; i < ehdr_->e_phnum; ++i) {
    const ElfW(Phdr) *const program_header = GetPhdr(i);
    switch (program_header->p_type) {
      case PT_LOAD:
        if (!~link_base_) {
          link_base_ = program_header->p_vaddr;
        }
        break;
      case PT_DYNAMIC:
        dynamic_program_header = program_header;
        break;
    }
  }
  if (!~link_base_ || !dynamic_program_header) {
    assert(false);
    // Mark this image as not present. Can not recur infinitely.
    Init(nullptr);
    return;
  }
  ptrdiff_t relocation =
      base_as_char - reinterpret_cast<const char *>(link_base_);
  ElfW(Dyn) *dynamic_entry =
      reinterpret_cast<ElfW(Dyn) *>(dynamic_program_header->p_vaddr +
                                    relocation);
  for (; dynamic_entry->d_tag != DT_NULL; ++dynamic_entry) {
    const ElfW(Xword) value = dynamic_entry->d_un.d_val + relocation;
    switch (dynamic_entry->d_tag) {
      case DT_HASH:
        hash_ = reinterpret_cast<ElfW(Word) *>(value);
        break;
      case DT_SYMTAB:
        dynsym_ = reinterpret_cast<ElfW(Sym) *>(value);
        break;
      case DT_STRTAB:
        dynstr_ = reinterpret_cast<const char *>(value);
        break;
      case DT_VERSYM:
        versym_ = reinterpret_cast<ElfW(Versym) *>(value);
        break;
      case DT_VERDEF:
        verdef_ = reinterpret_cast<ElfW(Verdef) *>(value);
        break;
      case DT_VERDEFNUM:
        verdefnum_ = dynamic_entry->d_un.d_val;
        break;
      case DT_STRSZ:
        strsize_ = dynamic_entry->d_un.d_val;
        break;
      default:
        // Unrecognized entries explicitly ignored.
        break;
    }
  }
  if (!hash_ || !dynsym_ || !dynstr_ || !versym_ ||
      !verdef_ || !verdefnum_ || !strsize_) {
    assert(false);  // invalid VDSO
    // Mark this image as not present. Can not recur infinitely.
    Init(nullptr);
    return;
  }
}

bool ElfMemImage::LookupSymbol(const char *name,
                               const char *version,
                               int type,
                               SymbolInfo *info_out) const {
  for (const SymbolInfo& info : *this) {
    if (strcmp(info.name, name) == 0 && strcmp(info.version, version) == 0 &&
        ElfType(info.symbol) == type) {
      if (info_out) {
        *info_out = info;
      }
      return true;
    }
  }
  return false;
}

bool ElfMemImage::LookupSymbolByAddress(const void *address,
                                        SymbolInfo *info_out) const {
  for (const SymbolInfo& info : *this) {
    const char *const symbol_start =
        reinterpret_cast<const char *>(info.address);
    const char *const symbol_end = symbol_start + info.symbol->st_size;
    if (symbol_start <= address && address < symbol_end) {
      if (info_out) {
        // Client wants to know details for that symbol (the usual case).
        if (ElfBind(info.symbol) == STB_GLOBAL) {
          // Strong symbol; just return it.
          *info_out = info;
          return true;
        } else {
          // Weak or local. Record it, but keep looking for a strong one.
          *info_out = info;
        }
      } else {
        // Client only cares if there is an overlapping symbol.
        return true;
      }
    }
  }
  return false;
}

ElfMemImage::SymbolIterator::SymbolIterator(const void *const image, int index)
    : index_(index), image_(image) {
}

const ElfMemImage::SymbolInfo *ElfMemImage::SymbolIterator::operator->() const {
  return &info_;
}

const ElfMemImage::SymbolInfo& ElfMemImage::SymbolIterator::operator*() const {
  return info_;
}

bool ElfMemImage::SymbolIterator::operator==(const SymbolIterator &rhs) const {
  return this->image_ == rhs.image_ && this->index_ == rhs.index_;
}

bool ElfMemImage::SymbolIterator::operator!=(const SymbolIterator &rhs) const {
  return !(*this == rhs);
}

ElfMemImage::SymbolIterator &ElfMemImage::SymbolIterator::operator++() {
  this->Update(1);
  return *this;
}

ElfMemImage::SymbolIterator ElfMemImage::begin() const {
  SymbolIterator it(this, 0);
  it.Update(0);
  return it;
}

ElfMemImage::SymbolIterator ElfMemImage::end() const {
  return SymbolIterator(this, GetNumSymbols());
}

void ElfMemImage::SymbolIterator::Update(int increment) {
  const ElfMemImage *image = reinterpret_cast<const ElfMemImage *>(image_);
  ABSL_RAW_CHECK(image->IsPresent() || increment == 0, "");
  if (!image->IsPresent()) {
    return;
  }
  index_ += increment;
  if (index_ >= image->GetNumSymbols()) {
    index_ = image->GetNumSymbols();
    return;
  }
  const ElfW(Sym)    *symbol = image->GetDynsym(index_);
  const ElfW(Versym) *version_symbol = image->GetVersym(index_);
  ABSL_RAW_CHECK(symbol && version_symbol, "");
  const char *const symbol_name = image->GetDynstr(symbol->st_name);
  const ElfW(Versym) version_index = version_symbol[0] & VERSYM_VERSION;
  const ElfW(Verdef) *version_definition = nullptr;
  const char *version_name = "";
  if (symbol->st_shndx == SHN_UNDEF) {
    // Undefined symbols reference DT_VERNEED, not DT_VERDEF, and
    // version_index could well be greater than verdefnum_, so calling
    // GetVerdef(version_index) may trigger assertion.
  } else {
    version_definition = image->GetVerdef(version_index);
  }
  if (version_definition) {
    // I am expecting 1 or 2 auxiliary entries: 1 for the version itself,
    // optional 2nd if the version has a parent.
    ABSL_RAW_CHECK(
        version_definition->vd_cnt == 1 || version_definition->vd_cnt == 2,
        "wrong number of entries");
    const ElfW(Verdaux) *version_aux = image->GetVerdefAux(version_definition);
    version_name = image->GetVerstr(version_aux->vda_name);
  }
  info_.name    = symbol_name;
  info_.version = version_name;
  info_.address = image->GetSymAddr(symbol);
  info_.symbol  = symbol;
}

}  // namespace debugging_internal
}  // inline namespace lts_2018_06_20
}  // namespace absl

#endif  // ABSL_HAVE_ELF_MEM_IMAGE