elf/dl-runtime.c

   1 /* On-demand PLT fixup for shared objects.
   2 Copyright (C) 1995, 1996 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 Library General Public License as
   7 published by the Free Software Foundation; either version 2 of the
   8 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 Library General Public License for more details.
  14
  15 You should have received a copy of the GNU Library General Public
  16 License along with the GNU C Library; see the file COPYING.LIB.  If
  17 not, write to the Free Software Foundation, Inc., 675 Mass Ave,
  18 Cambridge, MA 02139, USA.  */
  19
  20 #include <link.h>
  21 #include "dynamic-link.h"
  22
  23 /* Figure out the right type, Rel or Rela.  */
  24 #define elf_machine_rel 1
  25 #define elf_machine_rela 2
  26 #if elf_machine_relplt == elf_machine_rel
  27 #define PLTREL Elf32_Rel
  28 #elif elf_machine_relplt == elf_machine_rela
  29 #define PLTREL Elf32_Rela
  30 #else
  31 #error "dl-machine.h bug: elf_machine_relplt not rel or rela"
  32 #endif
  33 #undef elf_machine_rel
  34 #undef elf_machine_rela
  35
  36 /* We need to define the function as a local symbol so that the reference
  37    in the trampoline code will be a local PC-relative call.  Tell the
  38    compiler not to worry that the function appears not to be called.  */
  39
  40 static Elf32_Addr fixup (
  41 #ifdef ELF_MACHINE_RUNTIME_FIXUP_ARGS
  42                          ELF_MACHINE_RUNTIME_FIXUP_ARGS,
  43 #endif
  44                          struct link_map *l, Elf32_Word reloc_offset)
  45      __attribute__ ((unused));
  46
  47 /* This function is called through a special trampoline from the PLT the
  48    first time each PLT entry is called.  We must perform the relocation
  49    specified in the PLT of the given shared object, and return the resolved
  50    function address to the trampoline, which will restart the original call
  51    to that address.  Future calls will bounce directly from the PLT to the
  52    function.  */
  53
  54 static Elf32_Addr
  55 fixup (
  56 #ifdef ELF_MACHINE_RUNTIME_FIXUP_ARGS
  57        ELF_MACHINE_RUNTIME_FIXUP_ARGS,
  58 #endif
  59        struct link_map *l, Elf32_Word reloc_offset)
  60 {
  61   const Elf32_Sym *const symtab
  62     = (const Elf32_Sym *) (l->l_addr + l->l_info[DT_SYMTAB]->d_un.d_ptr);
  63   const char *strtab =
  64     (const char *) (l->l_addr + l->l_info[DT_STRTAB]->d_un.d_ptr);
  65
  66   const PLTREL *const reloc
  67     = (const void *) (l->l_addr + l->l_info[DT_JMPREL]->d_un.d_ptr +
  68                       reloc_offset);
  69
  70   const Elf32_Sym *definer;
  71   Elf32_Addr loadbase;
  72   struct link_map *scope, *real_next;
  73
  74   /* Look up the symbol's run-time value.  */
  75
  76   real_next = l->l_next;
  77   if (l->l_info[DT_SYMBOLIC])
  78     {
  79       l->l_next = _dl_loaded;
  80       if (l->l_prev)
  81         l->l_prev->l_next = real_next;
  82       scope = l;
  83     }
  84   else
  85     scope = _dl_loaded;
  86
  87   definer = &symtab[ELF32_R_SYM (reloc->r_info)];
  88   loadbase = _dl_lookup_symbol (strtab + definer->st_name, &definer,
  89                                 scope, l->l_name, 0);
  90
  91   /* Restore list frobnication done above for DT_SYMBOLIC.  */
  92   l->l_next = real_next;
  93   if (l->l_prev)
  94     l->l_prev->l_next = l;
  95
  96   /* Apply the relocation with that value.  */
  97   elf_machine_relplt (l, reloc, loadbase, definer);
  98
  99   return *(Elf32_Addr *) (l->l_addr + reloc->r_offset);
 100 }
 101
 102
 103 /* This macro is defined in dl-machine.h to define the entry point called
 104    by the PLT.  The `fixup' function above does the real work, but a little
 105    more twiddling is needed to get the stack right and jump to the address
 106    finally resolved.  */
 107
 108 ELF_MACHINE_RUNTIME_TRAMPOLINE