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seh.go
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seh.go
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// Copyright 2023 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package ld
import (
"cmd/internal/sys"
"cmd/link/internal/loader"
"cmd/link/internal/sym"
)
var sehp struct {
pdata []sym.LoaderSym
xdata []sym.LoaderSym
}
func writeSEH(ctxt *Link) {
switch ctxt.Arch.Family {
case sys.AMD64:
writeSEHAMD64(ctxt)
}
}
func writeSEHAMD64(ctxt *Link) {
ldr := ctxt.loader
mkSecSym := func(name string, kind sym.SymKind) *loader.SymbolBuilder {
s := ldr.CreateSymForUpdate(name, 0)
s.SetType(kind)
s.SetAlign(4)
return s
}
pdata := mkSecSym(".pdata", sym.SSEHSECT)
xdata := mkSecSym(".xdata", sym.SSEHSECT)
// The .xdata entries have very low cardinality
// as it only contains frame pointer operations,
// which are very similar across functions.
// These are referenced by .pdata entries using
// an RVA, so it is possible, and binary-size wise,
// to deduplicate .xdata entries.
uwcache := make(map[string]int64) // aux symbol name --> .xdata offset
for _, s := range ctxt.Textp {
if fi := ldr.FuncInfo(s); !fi.Valid() {
continue
}
uw := ldr.SEHUnwindSym(s)
if uw == 0 {
continue
}
name := ctxt.SymName(uw)
off, cached := uwcache[name]
if !cached {
off = xdata.Size()
uwcache[name] = off
xdata.AddBytes(ldr.Data(uw))
// The SEH unwind data can contain relocations,
// make sure those are copied over.
rels := ldr.Relocs(uw)
for i := 0; i < rels.Count(); i++ {
r := rels.At(i)
rel, _ := xdata.AddRel(r.Type())
rel.SetOff(int32(off) + r.Off())
rel.SetSiz(r.Siz())
rel.SetSym(r.Sym())
rel.SetAdd(r.Add())
}
}
// Reference:
// https://learn.microsoft.com/en-us/cpp/build/exception-handling-x64#struct-runtime_function
pdata.AddPEImageRelativeAddrPlus(ctxt.Arch, s, 0)
pdata.AddPEImageRelativeAddrPlus(ctxt.Arch, s, ldr.SymSize(s))
pdata.AddPEImageRelativeAddrPlus(ctxt.Arch, xdata.Sym(), off)
}
sehp.pdata = append(sehp.pdata, pdata.Sym())
sehp.xdata = append(sehp.xdata, xdata.Sym())
}