elf/dl-runtime.c

   1 /* On-demand PLT fixup for shared objects.
   2    Copyright (C) 1995-2022 Free Software Foundation, Inc.
   3    This file is part of the GNU C Library.
   4
   5    The GNU C Library is free software; you can redistribute it and/or
   6    modify it under the terms of the GNU Lesser General Public
   7    License as published by the Free Software Foundation; either
   8    version 2.1 of the License, or (at your option) any later version.
   9
  10    The GNU C Library is distributed in the hope that it will be useful,
  11    but WITHOUT ANY WARRANTY; without even the implied warranty of
  12    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13    Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser General Public
  16    License along with the GNU C Library; if not, see
  17    <https://www.gnu.org/licenses/>.  */
  18
  19 #include <alloca.h>
  20 #include <assert.h>
  21 #include <stdlib.h>
  22 #include <unistd.h>
  23 #include <sys/param.h>
  24 #include <ldsodefs.h>
  25 #include <sysdep-cancel.h>
  26 #include "dynamic-link.h"
  27 #include <tls.h>
  28 #include <dl-irel.h>
  29 #include <dl-runtime.h>
  30
  31
  32 /* This function is called through a special trampoline from the PLT the
  33    first time each PLT entry is called.  We must perform the relocation
  34    specified in the PLT of the given shared object, and return the resolved
  35    function address to the trampoline, which will restart the original call
  36    to that address.  Future calls will bounce directly from the PLT to the
  37    function.  */
  38
  39 DL_FIXUP_VALUE_TYPE
  40 attribute_hidden __attribute ((noinline)) DL_ARCH_FIXUP_ATTRIBUTE
  41 _dl_fixup (
  42 # ifdef ELF_MACHINE_RUNTIME_FIXUP_ARGS
  43            ELF_MACHINE_RUNTIME_FIXUP_ARGS,
  44 # endif
  45            struct link_map *l, ElfW(Word) reloc_arg)
  46 {
  47   const ElfW(Sym) *const symtab
  48     = (const void *) D_PTR (l, l_info[DT_SYMTAB]);
  49   const char *strtab = (const void *) D_PTR (l, l_info[DT_STRTAB]);
  50
  51   const uintptr_t pltgot = (uintptr_t) D_PTR (l, l_info[DT_PLTGOT]);
  52
  53   const PLTREL *const reloc
  54     = (const void *) (D_PTR (l, l_info[DT_JMPREL])
  55                       + reloc_offset (pltgot, reloc_arg));
  56   const ElfW(Sym) *sym = &symtab[ELFW(R_SYM) (reloc->r_info)];
  57   const ElfW(Sym) *refsym = sym;
  58   void *const rel_addr = (void *)(l->l_addr + reloc->r_offset);
  59   lookup_t result;
  60   DL_FIXUP_VALUE_TYPE value;
  61
  62   /* Sanity check that we're really looking at a PLT relocation.  */
  63   assert (ELFW(R_TYPE)(reloc->r_info) == ELF_MACHINE_JMP_SLOT);
  64
  65    /* Look up the target symbol.  If the normal lookup rules are not
  66       used don't look in the global scope.  */
  67   if (__builtin_expect (ELFW(ST_VISIBILITY) (sym->st_other), 0) == 0)
  68     {
  69       const struct r_found_version *version = NULL;
  70
  71       if (l->l_info[VERSYMIDX (DT_VERSYM)] != NULL)
  72         {
  73           const ElfW(Half) *vernum =
  74             (const void *) D_PTR (l, l_info[VERSYMIDX (DT_VERSYM)]);
  75           ElfW(Half) ndx = vernum[ELFW(R_SYM) (reloc->r_info)] & 0x7fff;
  76           version = &l->l_versions[ndx];
  77           if (version->hash == 0)
  78             version = NULL;
  79         }
  80
  81       /* We need to keep the scope around so do some locking.  This is
  82          not necessary for objects which cannot be unloaded or when
  83          we are not using any threads (yet).  */
  84       int flags = DL_LOOKUP_ADD_DEPENDENCY;
  85       if (!RTLD_SINGLE_THREAD_P)
  86         {
  87           THREAD_GSCOPE_SET_FLAG ();
  88           flags |= DL_LOOKUP_GSCOPE_LOCK;
  89         }
  90
  91 #ifdef RTLD_ENABLE_FOREIGN_CALL
  92       RTLD_ENABLE_FOREIGN_CALL;
  93 #endif
  94
  95       result = _dl_lookup_symbol_x (strtab + sym->st_name, l, &sym, l->l_scope,
  96                                     version, ELF_RTYPE_CLASS_PLT, flags, NULL);
  97
  98       /* We are done with the global scope.  */
  99       if (!RTLD_SINGLE_THREAD_P)
 100         THREAD_GSCOPE_RESET_FLAG ();
 101
 102 #ifdef RTLD_FINALIZE_FOREIGN_CALL
 103       RTLD_FINALIZE_FOREIGN_CALL;
 104 #endif
 105
 106       /* Currently result contains the base load address (or link map)
 107          of the object that defines sym.  Now add in the symbol
 108          offset.  */
 109       value = DL_FIXUP_MAKE_VALUE (result,
 110                                    SYMBOL_ADDRESS (result, sym, false));
 111     }
 112   else
 113     {
 114       /* We already found the symbol.  The module (and therefore its load
 115          address) is also known.  */
 116       value = DL_FIXUP_MAKE_VALUE (l, SYMBOL_ADDRESS (l, sym, true));
 117       result = l;
 118     }
 119
 120   /* And now perhaps the relocation addend.  */
 121   value = elf_machine_plt_value (l, reloc, value);
 122
 123   if (sym != NULL
 124       && __builtin_expect (ELFW(ST_TYPE) (sym->st_info) == STT_GNU_IFUNC, 0))
 125     value = elf_ifunc_invoke (DL_FIXUP_VALUE_ADDR (value));
 126
 127 #ifdef SHARED
 128   /* Auditing checkpoint: we have a new binding.  Provide the auditing
 129      libraries the possibility to change the value and tell us whether further
 130      auditing is wanted.
 131      The l_reloc_result is only allocated if there is an audit module which
 132      provides a la_symbind.  */
 133   if (l->l_reloc_result != NULL)
 134     {
 135       /* This is the address in the array where we store the result of previous
 136          relocations.  */
 137       struct reloc_result *reloc_result
 138         = &l->l_reloc_result[reloc_index (pltgot, reloc_arg, sizeof (PLTREL))];
 139       unsigned int init = atomic_load_acquire (&reloc_result->init);
 140       if (init == 0)
 141         {
 142           _dl_audit_symbind (l, reloc_result, sym, &value, result);
 143
 144           /* Store the result for later runs.  */
 145           if (__glibc_likely (! GLRO(dl_bind_not)))
 146             {
 147               reloc_result->addr = value;
 148               /* Guarantee all previous writes complete before init is
 149                  updated.  See CONCURRENCY NOTES below.  */
 150               atomic_store_release (&reloc_result->init, 1);
 151             }
 152         }
 153       else
 154         value = reloc_result->addr;
 155     }
 156 #endif
 157
 158   /* Finally, fix up the plt itself.  */
 159   if (__glibc_unlikely (GLRO(dl_bind_not)))
 160     return value;
 161
 162   return elf_machine_fixup_plt (l, result, refsym, sym, reloc, rel_addr, value);
 163 }
 164
 165 #ifndef PROF
 166 DL_FIXUP_VALUE_TYPE
 167 __attribute ((noinline))
 168 DL_ARCH_FIXUP_ATTRIBUTE
 169 _dl_profile_fixup (
 170 #ifdef ELF_MACHINE_RUNTIME_FIXUP_ARGS
 171                    ELF_MACHINE_RUNTIME_FIXUP_ARGS,
 172 #endif
 173                    struct link_map *l, ElfW(Word) reloc_arg,
 174                    ElfW(Addr) retaddr, void *regs, long int *framesizep)
 175 {
 176   void (*mcount_fct) (ElfW(Addr), ElfW(Addr)) = _dl_mcount;
 177
 178   if (l->l_reloc_result == NULL)
 179     {
 180       /* BZ #14843: ELF_DYNAMIC_RELOCATE is called before l_reloc_result
 181          is allocated.  We will get here if ELF_DYNAMIC_RELOCATE calls a
 182          resolver function to resolve an IRELATIVE relocation and that
 183          resolver calls a function that is not yet resolved (lazy).  For
 184          example, the resolver in x86-64 libm.so calls __get_cpu_features
 185          defined in libc.so.  Skip audit and resolve the external function
 186          in this case.  */
 187       *framesizep = -1;
 188       return _dl_fixup (
 189 # ifdef ELF_MACHINE_RUNTIME_FIXUP_ARGS
 190 #  ifndef ELF_MACHINE_RUNTIME_FIXUP_PARAMS
 191 #   error Please define ELF_MACHINE_RUNTIME_FIXUP_PARAMS.
 192 #  endif
 193                         ELF_MACHINE_RUNTIME_FIXUP_PARAMS,
 194 # endif
 195                         l, reloc_arg);
 196     }
 197
 198   const uintptr_t pltgot = (uintptr_t) D_PTR (l, l_info[DT_PLTGOT]);
 199
 200   /* This is the address in the array where we store the result of previous
 201      relocations.  */
 202   struct reloc_result *reloc_result
 203     = &l->l_reloc_result[reloc_index (pltgot, reloc_arg, sizeof (PLTREL))];
 204
 205  /* CONCURRENCY NOTES:
 206
 207   Multiple threads may be calling the same PLT sequence and with
 208   LD_AUDIT enabled they will be calling into _dl_profile_fixup to
 209   update the reloc_result with the result of the lazy resolution.
 210   The reloc_result guard variable is reloc_init, and we use
 211   acquire/release loads and store to it to ensure that the results of
 212   the structure are consistent with the loaded value of the guard.
 213   This does not fix all of the data races that occur when two or more
 214   threads read reloc_result->reloc_init with a value of zero and read
 215   and write to that reloc_result concurrently.  The expectation is
 216   generally that while this is a data race it works because the
 217   threads write the same values.  Until the data races are fixed
 218   there is a potential for problems to arise from these data races.
 219   The reloc result updates should happen in parallel but there should
 220   be an atomic RMW which does the final update to the real result
 221   entry (see bug 23790).
 222
 223   The following code uses reloc_result->init set to 0 to indicate if it is
 224   the first time this object is being relocated, otherwise 1 which
 225   indicates the object has already been relocated.
 226
 227   Reading/Writing from/to reloc_result->reloc_init must not happen
 228   before previous writes to reloc_result complete as they could
 229   end-up with an incomplete struct.  */
 230   DL_FIXUP_VALUE_TYPE value;
 231   unsigned int init = atomic_load_acquire (&reloc_result->init);
 232
 233   if (init == 0)
 234     {
 235       /* This is the first time we have to relocate this object.  */
 236       const ElfW(Sym) *const symtab
 237         = (const void *) D_PTR (l, l_info[DT_SYMTAB]);
 238       const char *strtab = (const char *) D_PTR (l, l_info[DT_STRTAB]);
 239
 240       const uintptr_t pltgot = (uintptr_t) D_PTR (l, l_info[DT_PLTGOT]);
 241
 242       const PLTREL *const reloc
 243         = (const void *) (D_PTR (l, l_info[DT_JMPREL])
 244                           + reloc_offset (pltgot, reloc_arg));
 245       const ElfW(Sym) *refsym = &symtab[ELFW(R_SYM) (reloc->r_info)];
 246       const ElfW(Sym) *defsym = refsym;
 247       lookup_t result;
 248
 249       /* Sanity check that we're really looking at a PLT relocation.  */
 250       assert (ELFW(R_TYPE)(reloc->r_info) == ELF_MACHINE_JMP_SLOT);
 251
 252       /* Look up the target symbol.  If the symbol is marked STV_PROTECTED
 253          don't look in the global scope.  */
 254       if (__builtin_expect (ELFW(ST_VISIBILITY) (refsym->st_other), 0) == 0)
 255         {
 256           const struct r_found_version *version = NULL;
 257
 258           if (l->l_info[VERSYMIDX (DT_VERSYM)] != NULL)
 259             {
 260               const ElfW(Half) *vernum =
 261                 (const void *) D_PTR (l, l_info[VERSYMIDX (DT_VERSYM)]);
 262               ElfW(Half) ndx = vernum[ELFW(R_SYM) (reloc->r_info)] & 0x7fff;
 263               version = &l->l_versions[ndx];
 264               if (version->hash == 0)
 265                 version = NULL;
 266             }
 267
 268           /* We need to keep the scope around so do some locking.  This is
 269              not necessary for objects which cannot be unloaded or when
 270              we are not using any threads (yet).  */
 271           int flags = DL_LOOKUP_ADD_DEPENDENCY;
 272           if (!RTLD_SINGLE_THREAD_P)
 273             {
 274               THREAD_GSCOPE_SET_FLAG ();
 275               flags |= DL_LOOKUP_GSCOPE_LOCK;
 276             }
 277
 278           result = _dl_lookup_symbol_x (strtab + refsym->st_name, l,
 279                                         &defsym, l->l_scope, version,
 280                                         ELF_RTYPE_CLASS_PLT, flags, NULL);
 281
 282           /* We are done with the global scope.  */
 283           if (!RTLD_SINGLE_THREAD_P)
 284             THREAD_GSCOPE_RESET_FLAG ();
 285
 286           /* Currently result contains the base load address (or link map)
 287              of the object that defines sym.  Now add in the symbol
 288              offset.  */
 289           value = DL_FIXUP_MAKE_VALUE (result,
 290                                        SYMBOL_ADDRESS (result, defsym, false));
 291
 292           if (defsym != NULL
 293               && __builtin_expect (ELFW(ST_TYPE) (defsym->st_info)
 294                                    == STT_GNU_IFUNC, 0))
 295             value = elf_ifunc_invoke (DL_FIXUP_VALUE_ADDR (value));
 296         }
 297       else
 298         {
 299           /* We already found the symbol.  The module (and therefore its load
 300              address) is also known.  */
 301           value = DL_FIXUP_MAKE_VALUE (l, SYMBOL_ADDRESS (l, refsym, true));
 302
 303           if (__builtin_expect (ELFW(ST_TYPE) (refsym->st_info)
 304                                 == STT_GNU_IFUNC, 0))
 305             value = elf_ifunc_invoke (DL_FIXUP_VALUE_ADDR (value));
 306
 307           result = l;
 308         }
 309       /* And now perhaps the relocation addend.  */
 310       value = elf_machine_plt_value (l, reloc, value);
 311
 312 #ifdef SHARED
 313       /* Auditing checkpoint: we have a new binding.  Provide the
 314          auditing libraries the possibility to change the value and
 315          tell us whether further auditing is wanted.  */
 316       if (defsym != NULL && GLRO(dl_naudit) > 0)
 317         _dl_audit_symbind (l, reloc_result, defsym, &value, result);
 318 #endif
 319
 320       /* Store the result for later runs.  */
 321       if (__glibc_likely (! GLRO(dl_bind_not)))
 322         {
 323           reloc_result->addr = value;
 324           /* Guarantee all previous writes complete before
 325              init is updated.  See CONCURRENCY NOTES earlier  */
 326           atomic_store_release (&reloc_result->init, 1);
 327         }
 328       init = 1;
 329     }
 330   else
 331     value = reloc_result->addr;
 332
 333   /* By default we do not call the pltexit function.  */
 334   long int framesize = -1;
 335
 336
 337 #ifdef SHARED
 338   /* Auditing checkpoint: report the PLT entering and allow the
 339      auditors to change the value.  */
 340   _dl_audit_pltenter (l, reloc_result, &value, regs, &framesize);
 341 #endif
 342
 343   /* Store the frame size information.  */
 344   *framesizep = framesize;
 345
 346   (*mcount_fct) (retaddr, DL_FIXUP_VALUE_CODE_ADDR (value));
 347
 348   return value;
 349 }
 350
 351 #endif /* PROF */