src/cmd/link/internal/loadpe/ldpe.go

   1 // Copyright 2010 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 // Package loadpe implements a PE/COFF file reader.
   6 package loadpe
   7
   8 import (
   9         "cmd/internal/bio"
  10         "cmd/internal/objabi"
  11         "cmd/internal/sys"
  12         "cmd/link/internal/loader"
  13         "cmd/link/internal/sym"
  14         "debug/pe"
  15         "encoding/binary"
  16         "errors"
  17         "fmt"
  18         "io"
  19         "strings"
  20 )
  21
  22 const (
  23         // TODO: the Microsoft doco says IMAGE_SYM_DTYPE_ARRAY is 3 (same with IMAGE_SYM_DTYPE_POINTER and IMAGE_SYM_DTYPE_FUNCTION)
  24         IMAGE_SYM_UNDEFINED              = 0
  25         IMAGE_SYM_ABSOLUTE               = -1
  26         IMAGE_SYM_DEBUG                  = -2
  27         IMAGE_SYM_TYPE_NULL              = 0
  28         IMAGE_SYM_TYPE_VOID              = 1
  29         IMAGE_SYM_TYPE_CHAR              = 2
  30         IMAGE_SYM_TYPE_SHORT             = 3
  31         IMAGE_SYM_TYPE_INT               = 4
  32         IMAGE_SYM_TYPE_LONG              = 5
  33         IMAGE_SYM_TYPE_FLOAT             = 6
  34         IMAGE_SYM_TYPE_DOUBLE            = 7
  35         IMAGE_SYM_TYPE_STRUCT            = 8
  36         IMAGE_SYM_TYPE_UNION             = 9
  37         IMAGE_SYM_TYPE_ENUM              = 10
  38         IMAGE_SYM_TYPE_MOE               = 11
  39         IMAGE_SYM_TYPE_BYTE              = 12
  40         IMAGE_SYM_TYPE_WORD              = 13
  41         IMAGE_SYM_TYPE_UINT              = 14
  42         IMAGE_SYM_TYPE_DWORD             = 15
  43         IMAGE_SYM_TYPE_PCODE             = 32768
  44         IMAGE_SYM_DTYPE_NULL             = 0
  45         IMAGE_SYM_DTYPE_POINTER          = 0x10
  46         IMAGE_SYM_DTYPE_FUNCTION         = 0x20
  47         IMAGE_SYM_DTYPE_ARRAY            = 0x30
  48         IMAGE_SYM_CLASS_END_OF_FUNCTION  = -1
  49         IMAGE_SYM_CLASS_NULL             = 0
  50         IMAGE_SYM_CLASS_AUTOMATIC        = 1
  51         IMAGE_SYM_CLASS_EXTERNAL         = 2
  52         IMAGE_SYM_CLASS_STATIC           = 3
  53         IMAGE_SYM_CLASS_REGISTER         = 4
  54         IMAGE_SYM_CLASS_EXTERNAL_DEF     = 5
  55         IMAGE_SYM_CLASS_LABEL            = 6
  56         IMAGE_SYM_CLASS_UNDEFINED_LABEL  = 7
  57         IMAGE_SYM_CLASS_MEMBER_OF_STRUCT = 8
  58         IMAGE_SYM_CLASS_ARGUMENT         = 9
  59         IMAGE_SYM_CLASS_STRUCT_TAG       = 10
  60         IMAGE_SYM_CLASS_MEMBER_OF_UNION  = 11
  61         IMAGE_SYM_CLASS_UNION_TAG        = 12
  62         IMAGE_SYM_CLASS_TYPE_DEFINITION  = 13
  63         IMAGE_SYM_CLASS_UNDEFINED_STATIC = 14
  64         IMAGE_SYM_CLASS_ENUM_TAG         = 15
  65         IMAGE_SYM_CLASS_MEMBER_OF_ENUM   = 16
  66         IMAGE_SYM_CLASS_REGISTER_PARAM   = 17
  67         IMAGE_SYM_CLASS_BIT_FIELD        = 18
  68         IMAGE_SYM_CLASS_FAR_EXTERNAL     = 68 /* Not in PECOFF v8 spec */
  69         IMAGE_SYM_CLASS_BLOCK            = 100
  70         IMAGE_SYM_CLASS_FUNCTION         = 101
  71         IMAGE_SYM_CLASS_END_OF_STRUCT    = 102
  72         IMAGE_SYM_CLASS_FILE             = 103
  73         IMAGE_SYM_CLASS_SECTION          = 104
  74         IMAGE_SYM_CLASS_WEAK_EXTERNAL    = 105
  75         IMAGE_SYM_CLASS_CLR_TOKEN        = 107
  76         IMAGE_REL_I386_ABSOLUTE          = 0x0000
  77         IMAGE_REL_I386_DIR16             = 0x0001
  78         IMAGE_REL_I386_REL16             = 0x0002
  79         IMAGE_REL_I386_DIR32             = 0x0006
  80         IMAGE_REL_I386_DIR32NB           = 0x0007
  81         IMAGE_REL_I386_SEG12             = 0x0009
  82         IMAGE_REL_I386_SECTION           = 0x000A
  83         IMAGE_REL_I386_SECREL            = 0x000B
  84         IMAGE_REL_I386_TOKEN             = 0x000C
  85         IMAGE_REL_I386_SECREL7           = 0x000D
  86         IMAGE_REL_I386_REL32             = 0x0014
  87         IMAGE_REL_AMD64_ABSOLUTE         = 0x0000
  88         IMAGE_REL_AMD64_ADDR64           = 0x0001
  89         IMAGE_REL_AMD64_ADDR32           = 0x0002
  90         IMAGE_REL_AMD64_ADDR32NB         = 0x0003
  91         IMAGE_REL_AMD64_REL32            = 0x0004
  92         IMAGE_REL_AMD64_REL32_1          = 0x0005
  93         IMAGE_REL_AMD64_REL32_2          = 0x0006
  94         IMAGE_REL_AMD64_REL32_3          = 0x0007
  95         IMAGE_REL_AMD64_REL32_4          = 0x0008
  96         IMAGE_REL_AMD64_REL32_5          = 0x0009
  97         IMAGE_REL_AMD64_SECTION          = 0x000A
  98         IMAGE_REL_AMD64_SECREL           = 0x000B
  99         IMAGE_REL_AMD64_SECREL7          = 0x000C
 100         IMAGE_REL_AMD64_TOKEN            = 0x000D
 101         IMAGE_REL_AMD64_SREL32           = 0x000E
 102         IMAGE_REL_AMD64_PAIR             = 0x000F
 103         IMAGE_REL_AMD64_SSPAN32          = 0x0010
 104         IMAGE_REL_ARM_ABSOLUTE           = 0x0000
 105         IMAGE_REL_ARM_ADDR32             = 0x0001
 106         IMAGE_REL_ARM_ADDR32NB           = 0x0002
 107         IMAGE_REL_ARM_BRANCH24           = 0x0003
 108         IMAGE_REL_ARM_BRANCH11           = 0x0004
 109         IMAGE_REL_ARM_SECTION            = 0x000E
 110         IMAGE_REL_ARM_SECREL             = 0x000F
 111         IMAGE_REL_ARM_MOV32              = 0x0010
 112         IMAGE_REL_THUMB_MOV32            = 0x0011
 113         IMAGE_REL_THUMB_BRANCH20         = 0x0012
 114         IMAGE_REL_THUMB_BRANCH24         = 0x0014
 115         IMAGE_REL_THUMB_BLX23            = 0x0015
 116         IMAGE_REL_ARM_PAIR               = 0x0016
 117 )
 118
 119 // TODO(crawshaw): de-duplicate these symbols with cmd/internal/ld, ideally in debug/pe.
 120 const (
 121         IMAGE_SCN_CNT_CODE               = 0x00000020
 122         IMAGE_SCN_CNT_INITIALIZED_DATA   = 0x00000040
 123         IMAGE_SCN_CNT_UNINITIALIZED_DATA = 0x00000080
 124         IMAGE_SCN_MEM_DISCARDABLE        = 0x02000000
 125         IMAGE_SCN_MEM_EXECUTE            = 0x20000000
 126         IMAGE_SCN_MEM_READ               = 0x40000000
 127         IMAGE_SCN_MEM_WRITE              = 0x80000000
 128 )
 129
 130 // TODO(brainman): maybe just add ReadAt method to bio.Reader instead of creating peBiobuf
 131
 132 // peBiobuf makes bio.Reader look like io.ReaderAt.
 133 type peBiobuf bio.Reader
 134
 135 func (f *peBiobuf) ReadAt(p []byte, off int64) (int, error) {
 136         ret := ((*bio.Reader)(f)).MustSeek(off, 0)
 137         if ret < 0 {
 138                 return 0, errors.New("fail to seek")
 139         }
 140         n, err := f.Read(p)
 141         if err != nil {
 142                 return 0, err
 143         }
 144         return n, nil
 145 }
 146
 147 // makeUpdater creates a loader.SymbolBuilder if one hasn't been created previously.
 148 // We use this to lazily make SymbolBuilders as we don't always need a builder, and creating them for all symbols might be an error.
 149 func makeUpdater(l *loader.Loader, bld *loader.SymbolBuilder, s loader.Sym) *loader.SymbolBuilder {
 150         if bld != nil {
 151                 return bld
 152         }
 153         bld = l.MakeSymbolUpdater(s)
 154         return bld
 155 }
 156
 157 // Load loads the PE file pn from input.
 158 // Symbols are written into syms, and a slice of the text symbols is returned.
 159 // If an .rsrc section is found, its symbol is returned as rsrc.
 160 func Load(l *loader.Loader, arch *sys.Arch, localSymVersion int, input *bio.Reader, pkg string, length int64, pn string) (textp []loader.Sym, rsrc loader.Sym, err error) {
 161         lookup := func(name string, version int) (*loader.SymbolBuilder, loader.Sym) {
 162                 s := l.LookupOrCreateSym(name, version)
 163                 sb := l.MakeSymbolUpdater(s)
 164                 return sb, s
 165         }
 166         sectsyms := make(map[*pe.Section]loader.Sym)
 167         sectdata := make(map[*pe.Section][]byte)
 168
 169         // Some input files are archives containing multiple of
 170         // object files, and pe.NewFile seeks to the start of
 171         // input file and get confused. Create section reader
 172         // to stop pe.NewFile looking before current position.
 173         sr := io.NewSectionReader((*peBiobuf)(input), input.Offset(), 1<<63-1)
 174
 175         // TODO: replace pe.NewFile with pe.Load (grep for "add Load function" in debug/pe for details)
 176         f, err := pe.NewFile(sr)
 177         if err != nil {
 178                 return nil, 0, err
 179         }
 180         defer f.Close()
 181
 182         // TODO return error if found .cormeta
 183
 184         // create symbols for mapped sections
 185         for _, sect := range f.Sections {
 186                 if sect.Characteristics&IMAGE_SCN_MEM_DISCARDABLE != 0 {
 187                         continue
 188                 }
 189
 190                 if sect.Characteristics&(IMAGE_SCN_CNT_CODE|IMAGE_SCN_CNT_INITIALIZED_DATA|IMAGE_SCN_CNT_UNINITIALIZED_DATA) == 0 {
 191                         // This has been seen for .idata sections, which we
 192                         // want to ignore. See issues 5106 and 5273.
 193                         continue
 194                 }
 195
 196                 name := fmt.Sprintf("%s(%s)", pkg, sect.Name)
 197                 bld, s := lookup(name, localSymVersion)
 198
 199                 switch sect.Characteristics & (IMAGE_SCN_CNT_UNINITIALIZED_DATA | IMAGE_SCN_CNT_INITIALIZED_DATA | IMAGE_SCN_MEM_READ | IMAGE_SCN_MEM_WRITE | IMAGE_SCN_CNT_CODE | IMAGE_SCN_MEM_EXECUTE) {
 200                 case IMAGE_SCN_CNT_INITIALIZED_DATA | IMAGE_SCN_MEM_READ: //.rdata
 201                         bld.SetType(sym.SRODATA)
 202
 203                 case IMAGE_SCN_CNT_UNINITIALIZED_DATA | IMAGE_SCN_MEM_READ | IMAGE_SCN_MEM_WRITE: //.bss
 204                         bld.SetType(sym.SNOPTRBSS)
 205
 206                 case IMAGE_SCN_CNT_INITIALIZED_DATA | IMAGE_SCN_MEM_READ | IMAGE_SCN_MEM_WRITE: //.data
 207                         bld.SetType(sym.SNOPTRDATA)
 208
 209                 case IMAGE_SCN_CNT_CODE | IMAGE_SCN_MEM_EXECUTE | IMAGE_SCN_MEM_READ: //.text
 210                         bld.SetType(sym.STEXT)
 211
 212                 default:
 213                         return nil, 0, fmt.Errorf("unexpected flags %#06x for PE section %s", sect.Characteristics, sect.Name)
 214                 }
 215
 216                 if bld.Type() != sym.SNOPTRBSS {
 217                         data, err := sect.Data()
 218                         if err != nil {
 219                                 return nil, 0, err
 220                         }
 221                         sectdata[sect] = data
 222                         bld.SetData(data)
 223                 }
 224                 bld.SetSize(int64(sect.Size))
 225                 sectsyms[sect] = s
 226                 if sect.Name == ".rsrc" {
 227                         rsrc = s
 228                 }
 229         }
 230
 231         // load relocations
 232         for _, rsect := range f.Sections {
 233                 if _, found := sectsyms[rsect]; !found {
 234                         continue
 235                 }
 236                 if rsect.NumberOfRelocations == 0 {
 237                         continue
 238                 }
 239                 if rsect.Characteristics&IMAGE_SCN_MEM_DISCARDABLE != 0 {
 240                         continue
 241                 }
 242                 if rsect.Characteristics&(IMAGE_SCN_CNT_CODE|IMAGE_SCN_CNT_INITIALIZED_DATA|IMAGE_SCN_CNT_UNINITIALIZED_DATA) == 0 {
 243                         // This has been seen for .idata sections, which we
 244                         // want to ignore. See issues 5106 and 5273.
 245                         continue
 246                 }
 247
 248                 sb := l.MakeSymbolUpdater(sectsyms[rsect])
 249                 for j, r := range rsect.Relocs {
 250                         if int(r.SymbolTableIndex) >= len(f.COFFSymbols) {
 251                                 return nil, 0, fmt.Errorf("relocation number %d symbol index idx=%d cannot be large then number of symbols %d", j, r.SymbolTableIndex, len(f.COFFSymbols))
 252                         }
 253                         pesym := &f.COFFSymbols[r.SymbolTableIndex]
 254                         _, gosym, err := readpesym(l, arch, l.LookupOrCreateSym, f, pesym, sectsyms, localSymVersion)
 255                         if err != nil {
 256                                 return nil, 0, err
 257                         }
 258                         if gosym == 0 {
 259                                 name, err := pesym.FullName(f.StringTable)
 260                                 if err != nil {
 261                                         name = string(pesym.Name[:])
 262                                 }
 263                                 return nil, 0, fmt.Errorf("reloc of invalid sym %s idx=%d type=%d", name, r.SymbolTableIndex, pesym.Type)
 264                         }
 265
 266                         rSym := gosym
 267                         rSize := uint8(4)
 268                         rOff := int32(r.VirtualAddress)
 269                         var rAdd int64
 270                         var rType objabi.RelocType
 271                         switch arch.Family {
 272                         default:
 273                                 return nil, 0, fmt.Errorf("%s: unsupported arch %v", pn, arch.Family)
 274                         case sys.I386, sys.AMD64:
 275                                 switch r.Type {
 276                                 default:
 277                                         return nil, 0, fmt.Errorf("%s: %v: unknown relocation type %v", pn, sectsyms[rsect], r.Type)
 278
 279                                 case IMAGE_REL_I386_REL32, IMAGE_REL_AMD64_REL32,
 280                                         IMAGE_REL_AMD64_ADDR32, // R_X86_64_PC32
 281                                         IMAGE_REL_AMD64_ADDR32NB:
 282                                         rType = objabi.R_PCREL
 283
 284                                         rAdd = int64(int32(binary.LittleEndian.Uint32(sectdata[rsect][rOff:])))
 285
 286                                 case IMAGE_REL_I386_DIR32NB, IMAGE_REL_I386_DIR32:
 287                                         rType = objabi.R_ADDR
 288
 289                                         // load addend from image
 290                                         rAdd = int64(int32(binary.LittleEndian.Uint32(sectdata[rsect][rOff:])))
 291
 292                                 case IMAGE_REL_AMD64_ADDR64: // R_X86_64_64
 293                                         rSize = 8
 294
 295                                         rType = objabi.R_ADDR
 296
 297                                         // load addend from image
 298                                         rAdd = int64(binary.LittleEndian.Uint64(sectdata[rsect][rOff:]))
 299                                 }
 300
 301                         case sys.ARM:
 302                                 switch r.Type {
 303                                 default:
 304                                         return nil, 0, fmt.Errorf("%s: %v: unknown ARM relocation type %v", pn, sectsyms[rsect], r.Type)
 305
 306                                 case IMAGE_REL_ARM_SECREL:
 307                                         rType = objabi.R_PCREL
 308
 309                                         rAdd = int64(int32(binary.LittleEndian.Uint32(sectdata[rsect][rOff:])))
 310
 311                                 case IMAGE_REL_ARM_ADDR32:
 312                                         rType = objabi.R_ADDR
 313
 314                                         rAdd = int64(int32(binary.LittleEndian.Uint32(sectdata[rsect][rOff:])))
 315
 316                                 case IMAGE_REL_ARM_BRANCH24:
 317                                         rType = objabi.R_CALLARM
 318
 319                                         rAdd = int64(int32(binary.LittleEndian.Uint32(sectdata[rsect][rOff:])))
 320                                 }
 321                         }
 322
 323                         // ld -r could generate multiple section symbols for the
 324                         // same section but with different values, we have to take
 325                         // that into account
 326                         if issect(pesym) {
 327                                 rAdd += int64(pesym.Value)
 328                         }
 329
 330                         rel, _ := sb.AddRel(rType)
 331                         rel.SetOff(rOff)
 332                         rel.SetSiz(rSize)
 333                         rel.SetSym(rSym)
 334                         rel.SetAdd(rAdd)
 335                 }
 336
 337                 sb.SortRelocs()
 338         }
 339
 340         // enter sub-symbols into symbol table.
 341         for i, numaux := 0, 0; i < len(f.COFFSymbols); i += numaux + 1 {
 342                 pesym := &f.COFFSymbols[i]
 343
 344                 numaux = int(pesym.NumberOfAuxSymbols)
 345
 346                 name, err := pesym.FullName(f.StringTable)
 347                 if err != nil {
 348                         return nil, 0, err
 349                 }
 350                 if name == "" {
 351                         continue
 352                 }
 353                 if issect(pesym) {
 354                         continue
 355                 }
 356                 if int(pesym.SectionNumber) > len(f.Sections) {
 357                         continue
 358                 }
 359                 if pesym.SectionNumber == IMAGE_SYM_DEBUG {
 360                         continue
 361                 }
 362                 var sect *pe.Section
 363                 if pesym.SectionNumber > 0 {
 364                         sect = f.Sections[pesym.SectionNumber-1]
 365                         if _, found := sectsyms[sect]; !found {
 366                                 continue
 367                         }
 368                 }
 369
 370                 bld, s, err := readpesym(l, arch, l.LookupOrCreateSym, f, pesym, sectsyms, localSymVersion)
 371                 if err != nil {
 372                         return nil, 0, err
 373                 }
 374
 375                 if pesym.SectionNumber == 0 { // extern
 376                         if l.SymType(s) == sym.SDYNIMPORT {
 377                                 bld = makeUpdater(l, bld, s)
 378                                 bld.SetPlt(-2) // flag for dynimport in PE object files.
 379                         }
 380                         if l.SymType(s) == sym.SXREF && pesym.Value > 0 { // global data
 381                                 bld = makeUpdater(l, bld, s)
 382                                 bld.SetType(sym.SNOPTRDATA)
 383                                 bld.SetSize(int64(pesym.Value))
 384                         }
 385
 386                         continue
 387                 } else if pesym.SectionNumber > 0 && int(pesym.SectionNumber) <= len(f.Sections) {
 388                         sect = f.Sections[pesym.SectionNumber-1]
 389                         if _, found := sectsyms[sect]; !found {
 390                                 return nil, 0, fmt.Errorf("%s: %v: missing sect.sym", pn, s)
 391                         }
 392                 } else {
 393                         return nil, 0, fmt.Errorf("%s: %v: sectnum < 0!", pn, s)
 394                 }
 395
 396                 if sect == nil {
 397                         return nil, 0, nil
 398                 }
 399
 400                 if l.OuterSym(s) != 0 {
 401                         if l.AttrDuplicateOK(s) {
 402                                 continue
 403                         }
 404                         outerName := l.SymName(l.OuterSym(s))
 405                         sectName := l.SymName(sectsyms[sect])
 406                         return nil, 0, fmt.Errorf("%s: duplicate symbol reference: %s in both %s and %s", pn, l.SymName(s), outerName, sectName)
 407                 }
 408
 409                 bld = makeUpdater(l, bld, s)
 410                 sectsym := sectsyms[sect]
 411                 bld.SetType(l.SymType(sectsym))
 412                 l.AddInteriorSym(sectsym, s)
 413                 bld.SetValue(int64(pesym.Value))
 414                 bld.SetSize(4)
 415                 if l.SymType(sectsym) == sym.STEXT {
 416                         if bld.External() && !bld.DuplicateOK() {
 417                                 return nil, 0, fmt.Errorf("%s: duplicate symbol definition", l.SymName(s))
 418                         }
 419                         bld.SetExternal(true)
 420                 }
 421         }
 422
 423         // Sort outer lists by address, adding to textp.
 424         // This keeps textp in increasing address order.
 425         for _, sect := range f.Sections {
 426                 s := sectsyms[sect]
 427                 if s == 0 {
 428                         continue
 429                 }
 430                 l.SortSub(s)
 431                 if l.SymType(s) == sym.STEXT {
 432                         for ; s != 0; s = l.SubSym(s) {
 433                                 if l.AttrOnList(s) {
 434                                         return nil, 0, fmt.Errorf("symbol %s listed multiple times", l.SymName(s))
 435                                 }
 436                                 l.SetAttrOnList(s, true)
 437                                 textp = append(textp, s)
 438                         }
 439                 }
 440         }
 441
 442         return textp, rsrc, nil
 443 }
 444
 445 func issect(s *pe.COFFSymbol) bool {
 446         return s.StorageClass == IMAGE_SYM_CLASS_STATIC && s.Type == 0 && s.Name[0] == '.'
 447 }
 448
 449 func readpesym(l *loader.Loader, arch *sys.Arch, lookup func(string, int) loader.Sym, f *pe.File, pesym *pe.COFFSymbol, sectsyms map[*pe.Section]loader.Sym, localSymVersion int) (*loader.SymbolBuilder, loader.Sym, error) {
 450         symname, err := pesym.FullName(f.StringTable)
 451         if err != nil {
 452                 return nil, 0, err
 453         }
 454         var name string
 455         if issect(pesym) {
 456                 name = l.SymName(sectsyms[f.Sections[pesym.SectionNumber-1]])
 457         } else {
 458                 name = symname
 459                 switch arch.Family {
 460                 case sys.AMD64:
 461                         if name == "__imp___acrt_iob_func" {
 462                                 // Do not rename __imp___acrt_iob_func into __acrt_iob_func,
 463                                 // because __imp___acrt_iob_func symbol is real
 464                                 // (see commit b295099 from git://git.code.sf.net/p/mingw-w64/mingw-w64 for details).
 465                         } else {
 466                                 name = strings.TrimPrefix(name, "__imp_") // __imp_Name => Name
 467                         }
 468                 case sys.I386:
 469                         if name == "__imp____acrt_iob_func" {
 470                                 // Do not rename __imp____acrt_iob_func into ___acrt_iob_func,
 471                                 // because __imp____acrt_iob_func symbol is real
 472                                 // (see commit b295099 from git://git.code.sf.net/p/mingw-w64/mingw-w64 for details).
 473                         } else {
 474                                 name = strings.TrimPrefix(name, "__imp_") // __imp_Name => Name
 475                         }
 476                         if name[0] == '_' {
 477                                 name = name[1:] // _Name => Name
 478                         }
 479                 }
 480         }
 481
 482         // remove last @XXX
 483         if i := strings.LastIndex(name, "@"); i >= 0 {
 484                 name = name[:i]
 485         }
 486
 487         var s loader.Sym
 488         var bld *loader.SymbolBuilder
 489         switch pesym.Type {
 490         default:
 491                 return nil, 0, fmt.Errorf("%s: invalid symbol type %d", symname, pesym.Type)
 492
 493         case IMAGE_SYM_DTYPE_FUNCTION, IMAGE_SYM_DTYPE_NULL:
 494                 switch pesym.StorageClass {
 495                 case IMAGE_SYM_CLASS_EXTERNAL: //global
 496                         s = lookup(name, 0)
 497
 498                 case IMAGE_SYM_CLASS_NULL, IMAGE_SYM_CLASS_STATIC, IMAGE_SYM_CLASS_LABEL:
 499                         s = lookup(name, localSymVersion)
 500                         bld = makeUpdater(l, bld, s)
 501                         bld.SetDuplicateOK(true)
 502
 503                 default:
 504                         return nil, 0, fmt.Errorf("%s: invalid symbol binding %d", symname, pesym.StorageClass)
 505                 }
 506         }
 507
 508         if s != 0 && l.SymType(s) == 0 && (pesym.StorageClass != IMAGE_SYM_CLASS_STATIC || pesym.Value != 0) {
 509                 bld = makeUpdater(l, bld, s)
 510                 bld.SetType(sym.SXREF)
 511         }
 512         if strings.HasPrefix(symname, "__imp_") {
 513                 bld = makeUpdater(l, bld, s)
 514                 bld.SetGot(-2) // flag for __imp_
 515         }
 516
 517         return bld, s, nil
 518 }