gold/layout.cc

   1 // layout.cc -- lay out output file sections for gold
   2
   3 #include "gold.h"
   4
   5 #include <cassert>
   6 #include <cstring>
   7 #include <iostream>
   8 #include <utility>
   9
  10 #include "output.h"
  11 #include "layout.h"
  12
  13 namespace gold
  14 {
  15
  16 // Layout_task methods.
  17
  18 Layout_task::~Layout_task()
  19 {
  20 }
  21
  22 // This task can be run when it is unblocked.
  23
  24 Task::Is_runnable_type
  25 Layout_task::is_runnable(Workqueue*)
  26 {
  27   if (this->this_blocker_->is_blocked())
  28     return IS_BLOCKED;
  29   return IS_RUNNABLE;
  30 }
  31
  32 // We don't need to hold any locks for the duration of this task.  In
  33 // fact this task will be the only one running.
  34
  35 Task_locker*
  36 Layout_task::locks(Workqueue*)
  37 {
  38   return NULL;
  39 }
  40
  41 // Lay out the sections.  This is called after all the input objects
  42 // have been read.
  43
  44 void
  45 Layout_task::run(Workqueue*)
  46 {
  47   Layout layout(this->options_);
  48   for (Object_list::const_iterator p = this->input_objects_->begin();
  49        p != this->input_objects_->end();
  50        ++p)
  51     (*p)->layout(&layout);
  52 }
  53
  54 // Layout methods.
  55
  56 // Hash a key we use to look up an output section mapping.
  57
  58 size_t
  59 Layout::Hash_key::operator()(const Layout::Key& k) const
  60 {
  61  return reinterpret_cast<size_t>(k.first) + k.second.first + k.second.second;
  62 }
  63
  64 // Whether to include this section in the link.
  65
  66 template<int size, bool big_endian>
  67 bool
  68 Layout::include_section(Object*, const char*,
  69                         const elfcpp::Shdr<size, big_endian>& shdr)
  70 {
  71   // Some section types are never linked.  Some are only linked when
  72   // doing a relocateable link.
  73   switch (shdr.get_sh_type())
  74     {
  75     case elfcpp::SHT_NULL:
  76     case elfcpp::SHT_SYMTAB:
  77     case elfcpp::SHT_DYNSYM:
  78     case elfcpp::SHT_STRTAB:
  79     case elfcpp::SHT_HASH:
  80     case elfcpp::SHT_DYNAMIC:
  81     case elfcpp::SHT_SYMTAB_SHNDX:
  82       return false;
  83
  84     case elfcpp::SHT_RELA:
  85     case elfcpp::SHT_REL:
  86     case elfcpp::SHT_GROUP:
  87       return this->options_.is_relocatable();
  88
  89     default:
  90       // FIXME: Handle stripping debug sections here.
  91       return true;
  92     }
  93 }
  94
  95 // Return the output section to use for input section NAME, with
  96 // header HEADER, from object OBJECT.  Set *OFF to the offset of this
  97 // input section without the output section.
  98
  99 template<int size, bool big_endian>
 100 Output_section*
 101 Layout::layout(Object* object, const char* name,
 102                const elfcpp::Shdr<size, big_endian>& shdr, off_t* off)
 103 {
 104   if (!this->include_section(object, name, shdr))
 105     return NULL;
 106
 107   // Unless we are doing a relocateable link, .gnu.linkonce sections
 108   // are laid out as though they were named for the sections are
 109   // placed into.
 110   if (!this->options_.is_relocatable() && Layout::is_linkonce(name))
 111     name = Layout::linkonce_output_name(name);
 112
 113   // FIXME: Handle SHF_OS_NONCONFORMING here.
 114
 115   // Canonicalize the section name.
 116   name = this->namepool_.add(name);
 117
 118   // Find the output section.  The output section is selected based on
 119   // the section name, type, and flags.
 120
 121   // FIXME: If we want to do relaxation, we need to modify this
 122   // algorithm.  We also build a list of input sections for each
 123   // output section.  Then we relax all the input sections.  Then we
 124   // walk down the list and adjust all the offsets.
 125
 126   elfcpp::Elf_Word type = shdr.get_sh_type();
 127   elfcpp::Elf_Xword flags = shdr.get_sh_flags();
 128   const Key key(name, std::make_pair(type, flags));
 129   const std::pair<Key, Output_section*> v(key, NULL);
 130   std::pair<Section_name_map::iterator, bool> ins(
 131     this->section_name_map_.insert(v));
 132
 133   Output_section* os;
 134   if (!ins.second)
 135     os = ins.first->second;
 136   else
 137     {
 138       // This is the first time we've seen this name/type/flags
 139       // combination.
 140       os = this->make_output_section(name, type, flags);
 141       ins.first->second = os;
 142     }
 143
 144   // FIXME: Handle SHF_LINK_ORDER somewhere.
 145
 146   *off = os->add_input_section(object, name, shdr);
 147
 148   return os;
 149 }
 150
 151 // Return whether SEG1 should be before SEG2 in the output file.  This
 152 // is based entirely on the segment type and flags.  When this is
 153 // called the segment addresses has normally not yet been set.
 154
 155 bool
 156 Layout::segment_precedes(const Output_segment* seg1,
 157                          const Output_segment* seg2)
 158 {
 159   elfcpp::Elf_Word type1 = seg1->type();
 160   elfcpp::Elf_Word type2 = seg2->type();
 161
 162   // The single PT_PHDR segment is required to precede any loadable
 163   // segment.  We simply make it always first.
 164   if (type1 == elfcpp::PT_PHDR)
 165     {
 166       assert(type2 != elfcpp::PT_PHDR);
 167       return true;
 168     }
 169   if (type2 == elfcpp::PT_PHDR)
 170     return false;
 171
 172   // The single PT_INTERP segment is required to precede any loadable
 173   // segment.  We simply make it always second.
 174   if (type1 == elfcpp::PT_INTERP)
 175     {
 176       assert(type2 != elfcpp::PT_INTERP);
 177       return true;
 178     }
 179   if (type2 == elfcpp::PT_INTERP)
 180     return false;
 181
 182   // We then put PT_LOAD segments before any other segments.
 183   if (type1 == elfcpp::PT_LOAD && type2 != elfcpp::PT_LOAD)
 184     return true;
 185   if (type2 == elfcpp::PT_LOAD && type1 != elfcpp::PT_LOAD)
 186     return false;
 187
 188   const elfcpp::Elf_Word flags1 = seg1->flags();
 189   const elfcpp::Elf_Word flags2 = seg2->flags();
 190
 191   // The order of non-PT_LOAD segments is unimportant.  We simply sort
 192   // by the numeric segment type and flags values.  There should not
 193   // be more than one segment with the same type and flags.
 194   if (type1 != elfcpp::PT_LOAD)
 195     {
 196       if (type1 != type2)
 197         return type1 < type2;
 198       assert(flags1 != flags2);
 199       return flags1 < flags2;
 200     }
 201
 202   // We sort PT_LOAD segments based on the flags.  Readonly segments
 203   // come before writable segments.  Then executable segments come
 204   // before non-executable segments.  Then the unlikely case of a
 205   // non-readable segment comes before the normal case of a readable
 206   // segment.  If there are multiple segments with the same type and
 207   // flags, we require that the address be set, and we sort by
 208   // virtual address and then physical address.
 209   if ((flags1 & elfcpp::PF_W) != (flags2 & elfcpp::PF_W))
 210     return (flags1 & elfcpp::PF_W) == 0;
 211   if ((flags1 & elfcpp::PF_X) != (flags2 & elfcpp::PF_X))
 212     return (flags1 & elfcpp::PF_X) != 0;
 213   if ((flags1 & elfcpp::PF_R) != (flags2 & elfcpp::PF_R))
 214     return (flags1 & elfcpp::PF_R) == 0;
 215
 216   uint64_t vaddr1 = seg1->vaddr();
 217   uint64_t vaddr2 = seg2->vaddr();
 218   if (vaddr1 != vaddr2)
 219     return vaddr1 < vaddr2;
 220
 221   uint64_t paddr1 = seg1->paddr();
 222   uint64_t paddr2 = seg2->paddr();
 223   assert(paddr1 != paddr2);
 224   return paddr1 < paddr2;
 225 }
 226
 227 // Map section flags to segment flags.
 228
 229 elfcpp::Elf_Word
 230 Layout::section_flags_to_segment(elfcpp::Elf_Xword flags)
 231 {
 232   elfcpp::Elf_Word ret = elfcpp::PF_R;
 233   if ((flags & elfcpp::SHF_WRITE) != 0)
 234     ret |= elfcpp::PF_W;
 235   if ((flags & elfcpp::SHF_EXECINSTR) != 0)
 236     ret |= elfcpp::PF_X;
 237   return ret;
 238 }
 239
 240 // Make a new Output_section, and attach it to segments as
 241 // appropriate.
 242
 243 Output_section*
 244 Layout::make_output_section(const char* name, elfcpp::Elf_Word type,
 245                             elfcpp::Elf_Xword flags)
 246 {
 247   Output_section* os = new Output_section(name, type, flags);
 248
 249   if ((flags & elfcpp::SHF_ALLOC) == 0)
 250     this->section_list_.push_back(os);
 251   else
 252     {
 253       // This output section goes into a PT_LOAD segment.
 254
 255       elfcpp::Elf_Word seg_flags = Layout::section_flags_to_segment(flags);
 256
 257       // The only thing we really care about for PT_LOAD segments is
 258       // whether or not they are writable, so that is how we search
 259       // for them.  People who need segments sorted on some other
 260       // basis will have to wait until we implement a mechanism for
 261       // them to describe the segments they want.
 262
 263       Segment_list::const_iterator p;
 264       for (p = this->segment_list_.begin();
 265            p != this->segment_list_.end();
 266            ++p)
 267         {
 268           if ((*p)->type() == elfcpp::PT_LOAD
 269               && ((*p)->flags() & elfcpp::PF_W) == (seg_flags & elfcpp::PF_W))
 270             {
 271               (*p)->add_output_section(os);
 272               if ((*p)->flags() != seg_flags)
 273                 (*p)->update_flags(seg_flags);
 274               break;
 275             }
 276         }
 277
 278       if (p == this->segment_list_.end())
 279         {
 280           Output_segment* oseg = new Output_segment(elfcpp::PT_LOAD,
 281                                                     seg_flags);
 282           this->segment_list_.push_back(oseg);
 283           oseg->add_output_section(os);
 284         }
 285
 286       // If we see a loadable SHT_NOTE section, we create a PT_NOTE
 287       // segment.
 288       if (type == elfcpp::SHT_NOTE)
 289         {
 290           // See if we already have an equivalent PT_NOTE segment.
 291           for (p = this->segment_list_.begin();
 292                p != segment_list_.end();
 293                ++p)
 294             {
 295               if ((*p)->type() == elfcpp::PT_NOTE
 296                   && (((*p)->flags() & elfcpp::PF_W)
 297                       == (seg_flags & elfcpp::PF_W)))
 298                 {
 299                   (*p)->add_output_section(os);
 300                   if ((*p)->flags() != seg_flags)
 301                     (*p)->update_flags(seg_flags);
 302                   break;
 303                 }
 304             }
 305
 306           if (p == this->segment_list_.end())
 307             {
 308               Output_segment* oseg = new Output_segment(elfcpp::PT_NOTE,
 309                                                         seg_flags);
 310               this->segment_list_.push_back(oseg);
 311               oseg->add_output_section(os);
 312             }
 313         }
 314     }
 315
 316   return os;
 317 }
 318
 319 // The mapping of .gnu.linkonce section names to real section names.
 320
 321 #define MAPPING_INIT(f, t) { f, sizeof(f) - 1, t }
 322 const Layout::Linkonce_mapping Layout::linkonce_mapping[] =
 323 {
 324   MAPPING_INIT("d.rel.ro", ".data.rel.ro"),     // Must be before "d".
 325   MAPPING_INIT("t", ".text"),
 326   MAPPING_INIT("r", ".rodata"),
 327   MAPPING_INIT("d", ".data"),
 328   MAPPING_INIT("b", ".bss"),
 329   MAPPING_INIT("s", ".sdata"),
 330   MAPPING_INIT("sb", ".sbss"),
 331   MAPPING_INIT("s2", ".sdata2"),
 332   MAPPING_INIT("sb2", ".sbss2"),
 333   MAPPING_INIT("wi", ".debug_info"),
 334   MAPPING_INIT("td", ".tdata"),
 335   MAPPING_INIT("tb", ".tbss"),
 336   MAPPING_INIT("lr", ".lrodata"),
 337   MAPPING_INIT("l", ".ldata"),
 338   MAPPING_INIT("lb", ".lbss"),
 339 };
 340 #undef MAPPING_INIT
 341
 342 const int Layout::linkonce_mapping_count =
 343   sizeof(Layout::linkonce_mapping) / sizeof(Layout::linkonce_mapping[0]);
 344
 345 // Return the name of the output section to use for a .gnu.linkonce
 346 // section.  This is based on the default ELF linker script of the old
 347 // GNU linker.  For example, we map a name like ".gnu.linkonce.t.foo"
 348 // to ".text".
 349
 350 const char*
 351 Layout::linkonce_output_name(const char* name)
 352 {
 353   const char* s = name + sizeof(".gnu.linkonce") - 1;
 354   if (*s != '.')
 355     return name;
 356   ++s;
 357   const Linkonce_mapping* plm = linkonce_mapping;
 358   for (int i = 0; i < linkonce_mapping_count; ++i, ++plm)
 359     {
 360       if (strncmp(s, plm->from, plm->fromlen) == 0 && s[plm->fromlen] == '.')
 361         return plm->to;
 362     }
 363   return name;
 364 }
 365
 366 // Record the signature of a comdat section, and return whether to
 367 // include it in the link.  If GROUP is true, this is a regular
 368 // section group.  If GROUP is false, this is a group signature
 369 // derived from the name of a linkonce section.  We want linkonce
 370 // signatures and group signatures to block each other, but we don't
 371 // want a linkonce signature to block another linkonce signature.
 372
 373 bool
 374 Layout::add_comdat(const char* signature, bool group)
 375 {
 376   std::string sig(signature);
 377   std::pair<Signatures::iterator, bool> ins(
 378     this->signatures_.insert(std::make_pair(signature, group)));
 379
 380   if (ins.second)
 381     {
 382       // This is the first time we've seen this signature.
 383       return true;
 384     }
 385
 386   if (ins.first->second)
 387     {
 388       // We've already seen a real section group with this signature.
 389       return false;
 390     }
 391   else if (group)
 392     {
 393       // This is a real section group, and we've already seen a
 394       // linkonce section with tihs signature.  Record that we've seen
 395       // a section group, and don't include this section group.
 396       ins.first->second = true;
 397       return false;
 398     }
 399   else
 400     {
 401       // We've already seen a linkonce section and this is a linkonce
 402       // section.  These don't block each other--this may be the same
 403       // symbol name with different section types.
 404       return true;
 405     }
 406 }
 407
 408 // Instantiate the templates we need.  We could use the configure
 409 // script to restrict this to only the ones for implemented targets.
 410
 411 template
 412 Output_section*
 413 Layout::layout<32, false>(Object* object, const char* name,
 414                           const elfcpp::Shdr<32, false>& shdr, off_t*);
 415
 416 template
 417 Output_section*
 418 Layout::layout<32, true>(Object* object, const char* name,
 419                          const elfcpp::Shdr<32, true>& shdr, off_t*);
 420
 421 template
 422 Output_section*
 423 Layout::layout<64, false>(Object* object, const char* name,
 424                           const elfcpp::Shdr<64, false>& shdr, off_t*);
 425
 426 template
 427 Output_section*
 428 Layout::layout<64, true>(Object* object, const char* name,
 429                          const elfcpp::Shdr<64, true>& shdr, off_t*);
 430
 431
 432 } // End namespace gold.