src/cmd/link/internal/ld/seh.go

   1 // Copyright 2023 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 ld
   6
   7 import (
   8         "cmd/internal/sys"
   9         "cmd/link/internal/loader"
  10         "cmd/link/internal/sym"
  11 )
  12
  13 var sehp struct {
  14         pdata []sym.LoaderSym
  15         xdata []sym.LoaderSym
  16 }
  17
  18 func writeSEH(ctxt *Link) {
  19         switch ctxt.Arch.Family {
  20         case sys.AMD64:
  21                 writeSEHAMD64(ctxt)
  22         }
  23 }
  24
  25 func writeSEHAMD64(ctxt *Link) {
  26         ldr := ctxt.loader
  27         mkSecSym := func(name string, kind sym.SymKind) *loader.SymbolBuilder {
  28                 s := ldr.CreateSymForUpdate(name, 0)
  29                 s.SetType(kind)
  30                 s.SetAlign(4)
  31                 return s
  32         }
  33         pdata := mkSecSym(".pdata", sym.SSEHSECT)
  34         xdata := mkSecSym(".xdata", sym.SSEHSECT)
  35         // The .xdata entries have very low cardinality
  36         // as it only contains frame pointer operations,
  37         // which are very similar across functions.
  38         // These are referenced by .pdata entries using
  39         // an RVA, so it is possible, and binary-size wise,
  40         // to deduplicate .xdata entries.
  41         uwcache := make(map[string]int64) // aux symbol name --> .xdata offset
  42         for _, s := range ctxt.Textp {
  43                 if fi := ldr.FuncInfo(s); !fi.Valid() {
  44                         continue
  45                 }
  46                 uw := ldr.SEHUnwindSym(s)
  47                 if uw == 0 {
  48                         continue
  49                 }
  50                 name := ctxt.SymName(uw)
  51                 off, cached := uwcache[name]
  52                 if !cached {
  53                         off = xdata.Size()
  54                         uwcache[name] = off
  55                         xdata.AddBytes(ldr.Data(uw))
  56                         // The SEH unwind data can contain relocations,
  57                         // make sure those are copied over.
  58                         rels := ldr.Relocs(uw)
  59                         for i := 0; i < rels.Count(); i++ {
  60                                 r := rels.At(i)
  61                                 rel, _ := xdata.AddRel(r.Type())
  62                                 rel.SetOff(int32(off) + r.Off())
  63                                 rel.SetSiz(r.Siz())
  64                                 rel.SetSym(r.Sym())
  65                                 rel.SetAdd(r.Add())
  66                         }
  67                 }
  68
  69                 // Reference:
  70                 // https://learn.microsoft.com/en-us/cpp/build/exception-handling-x64#struct-runtime_function
  71                 pdata.AddPEImageRelativeAddrPlus(ctxt.Arch, s, 0)
  72                 pdata.AddPEImageRelativeAddrPlus(ctxt.Arch, s, ldr.SymSize(s))
  73                 pdata.AddPEImageRelativeAddrPlus(ctxt.Arch, xdata.Sym(), off)
  74         }
  75         sehp.pdata = append(sehp.pdata, pdata.Sym())
  76         sehp.xdata = append(sehp.xdata, xdata.Sym())
  77 }