cmd/link: generate .pdata PE section

This CL adds a .pdata section to the PE file generated by the Go linker.

The .pdata section is a standard section [1] that contains an array of
function table entries that are used for stack unwinding.
The table entries layout is taken from [2].

This CL just generates the table entries without any unwinding
information, which is enough to start doing some E2E tests
between the Go linker and the Win32 APIs.

The goal of the .pdata table is to allow Windows retrieve
unwind information for a function at a given PC. It does so by doing
a binary search on the table, looking for an entry that meets
BeginAddress >= PC < EndAddress.

Each table entry takes 12 bytes and only non-leaf functions with
frame pointer needs an entry on the .pdata table.
The result is that PE binaries will be ~0.7% bigger due to the unwind
information, a reasonable amount considering the benefits in
debuggability.

Updates #57302

[1] https://learn.microsoft.com/en-us/windows/win32/debug/pe-format#the-pdata-section
[2] https://learn.microsoft.com/en-us/cpp/build/exception-handling-x64#struct-runtime_function

Change-Id: If675d10c64452946dbab76709da20569651e3e9f
Reviewed-on: https://go-review.googlesource.com/c/go/+/461738
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Alex Brainman <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>
Run-TryBot: Quim Muntal <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

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

@@ -60,6 +60,10 @@ func asmb(ctxt *Link) {
 
 	writeParallel(&wg, dwarfblk, ctxt, Segdwarf.Fileoff, Segdwarf.Vaddr, Segdwarf.Filelen)
 
+	if Segpdata.Filelen > 0 {
+		writeParallel(&wg, pdatablk, ctxt, Segpdata.Fileoff, Segpdata.Vaddr, Segpdata.Filelen)
+	}
+
 	wg.Wait()
 }
 

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

@@ -1154,6 +1154,10 @@ func dwarfblk(ctxt *Link, out *OutBuf, addr int64, size int64) {
 	writeBlocks(ctxt, out, ctxt.outSem, ctxt.loader, syms, addr, size, zeros[:])
 }
 
+func pdatablk(ctxt *Link, out *OutBuf, addr int64, size int64) {
+	writeBlocks(ctxt, out, ctxt.outSem, ctxt.loader, []loader.Sym{sehp.pdata}, addr, size, zeros[:])
+}
+
 var covCounterDataStartOff, covCounterDataLen uint64
 
 var zeros [512]byte
@@ -1649,6 +1653,8 @@ func (ctxt *Link) dodata(symGroupType []sym.SymKind) {
 	// data/rodata (and related) symbols.
 	state.allocateDataSections(ctxt)
 
+	state.allocateSEHSections(ctxt)
+
 	// Create *sym.Section objects and assign symbols to sections for
 	// DWARF symbols.
 	state.allocateDwarfSections(ctxt)
@@ -1676,6 +1682,10 @@ func (ctxt *Link) dodata(symGroupType []sym.SymKind) {
 		sect.Extnum = n
 		n++
 	}
+	for _, sect := range Segpdata.Sections {
+		sect.Extnum = n
+		n++
+	}
 }
 
 // allocateDataSectionForSym creates a new sym.Section into which a
@@ -2148,6 +2158,16 @@ func (state *dodataState) allocateDwarfSections(ctxt *Link) {
 	}
 }
 
+// allocateSEHSections allocate a sym.Section object for SEH
+// symbols, and assigns symbols to sections.
+func (state *dodataState) allocateSEHSections(ctxt *Link) {
+	if sehp.pdata > 0 {
+		sect := state.allocateDataSectionForSym(&Segpdata, sehp.pdata, 04)
+		state.assignDsymsToSection(sect, []loader.Sym{sehp.pdata}, sym.SRODATA, aligndatsize)
+		state.checkdatsize(sym.SPDATASECT)
+	}
+}
+
 type symNameSize struct {
 	name string
 	sz   int64
@@ -2684,6 +2704,21 @@ func (ctxt *Link) address() []*sym.Segment {
 	// simply because right now we know where the BSS starts.
 	Segdata.Filelen = bss.Vaddr - Segdata.Vaddr
 
+	if len(Segpdata.Sections) > 0 {
+		va = uint64(Rnd(int64(va), int64(*FlagRound)))
+		order = append(order, &Segpdata)
+		Segpdata.Rwx = 04
+		Segpdata.Vaddr = va
+		// Segpdata.Sections is intended to contain just one section.
+		// Loop through the slice anyway for consistency.
+		for _, s := range Segpdata.Sections {
+			va = uint64(Rnd(int64(va), int64(s.Align)))
+			s.Vaddr = va
+			va += s.Length
+		}
+		Segpdata.Length = va - Segpdata.Vaddr
+	}
+
 	va = uint64(Rnd(int64(va), int64(*FlagRound)))
 	order = append(order, &Segdwarf)
 	Segdwarf.Rwx = 06
@@ -2735,6 +2770,10 @@ func (ctxt *Link) address() []*sym.Segment {
 		}
 	}
 
+	if sect := ldr.SymSect(sehp.pdata); sect != nil {
+		ldr.AddToSymValue(sehp.pdata, int64(sect.Vaddr))
+	}
+
 	if ctxt.BuildMode == BuildModeShared {
 		s := ldr.LookupOrCreateSym("go:link.abihashbytes", 0)
 		sect := ldr.SymSect(ldr.LookupOrCreateSym(".note.go.abihash", 0))

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

@@ -328,8 +328,9 @@ var (
 	Segrelrodata sym.Segment
 	Segdata      sym.Segment
 	Segdwarf     sym.Segment
+	Segpdata     sym.Segment // windows-only
 
-	Segments = []*sym.Segment{&Segtext, &Segrodata, &Segrelrodata, &Segdata, &Segdwarf}
+	Segments = []*sym.Segment{&Segtext, &Segrodata, &Segrelrodata, &Segdata, &Segdwarf, &Segpdata}
 )
 
 const pkgdef = "__.PKGDEF"

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

@@ -433,6 +433,7 @@ type peFile struct {
 	dataSect       *peSection
 	bssSect        *peSection
 	ctorsSect      *peSection
+	pdataSect      *peSection
 	nextSectOffset uint32
 	nextFileOffset uint32
 	symtabOffset   int64 // offset to the start of symbol table
@@ -498,6 +499,25 @@ func (f *peFile) addDWARF() {
 	}
 }
 
+// addSEH adds SEH information to the COFF file f.
+func (f *peFile) addSEH(ctxt *Link) {
+	if Segpdata.Length == 0 {
+		return
+	}
+	d := pefile.addSection(".pdata", int(Segpdata.Length), int(Segpdata.Length))
+	d.characteristics = IMAGE_SCN_CNT_INITIALIZED_DATA | IMAGE_SCN_MEM_READ
+	if ctxt.LinkMode == LinkExternal {
+		// Some gcc versions don't honor the default alignment for the .pdata section.
+		d.characteristics |= IMAGE_SCN_ALIGN_4BYTES
+	}
+	pefile.pdataSect = d
+	d.checkSegment(&Segpdata)
+	// TODO: remove extraSize once the dummy unwind info is removed from the .pdata section.
+	const extraSize = 12
+	pefile.dataDirectory[pe.IMAGE_DIRECTORY_ENTRY_EXCEPTION].VirtualAddress = d.virtualAddress + extraSize
+	pefile.dataDirectory[pe.IMAGE_DIRECTORY_ENTRY_EXCEPTION].Size = d.virtualSize - extraSize
+}
+
 // addInitArray adds .ctors COFF section to the file f.
 func (f *peFile) addInitArray(ctxt *Link) *peSection {
 	// The size below was determined by the specification for array relocations,
@@ -593,15 +613,19 @@ func (f *peFile) emitRelocations(ctxt *Link) {
 		return int(sect.Rellen / relocLen)
 	}
 
-	sects := []struct {
+	type relsect struct {
 		peSect *peSection
 		seg    *sym.Segment
 		syms   []loader.Sym
-	}{
+	}
+	sects := []relsect{
 		{f.textSect, &Segtext, ctxt.Textp},
 		{f.rdataSect, &Segrodata, ctxt.datap},
 		{f.dataSect, &Segdata, ctxt.datap},
 	}
+	if sehp.pdata != 0 {
+		sects = append(sects, relsect{f.pdataSect, &Segpdata, []loader.Sym{sehp.pdata}})
+	}
 	for _, s := range sects {
 		s.peSect.emitRelocations(ctxt.Out, func() int {
 			var n int
@@ -1595,6 +1619,7 @@ func addPEBaseReloc(ctxt *Link) {
 func (ctxt *Link) dope() {
 	initdynimport(ctxt)
 	initdynexport(ctxt)
+	writeSEH(ctxt)
 }
 
 func setpersrc(ctxt *Link, syms []loader.Sym) {
@@ -1689,6 +1714,7 @@ func asmbPe(ctxt *Link) {
 		pefile.bssSect = b
 	}
 
+	pefile.addSEH(ctxt)
 	pefile.addDWARF()
 
 	if ctxt.LinkMode == LinkExternal {

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

@@ -0,0 +1,54 @@
+// 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 loader.Sym
+}
+
+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.SPDATASECT)
+	// TODO: the following 12 bytes represent a dummy unwind info,
+	// remove once unwind infos are encoded in the .xdata section.
+	pdata.AddUint64(ctxt.Arch, 0)
+	pdata.AddUint32(ctxt.Arch, 0)
+	for _, s := range ctxt.Textp {
+		if fi := ldr.FuncInfo(s); !fi.Valid() || fi.TopFrame() {
+			continue
+		}
+		uw := ldr.SEHUnwindSym(s)
+		if uw == 0 {
+			continue
+		}
+
+		// 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))
+		// TODO: reference the .xdata symbol.
+		pdata.AddPEImageRelativeAddrPlus(ctxt.Arch, pdata.Sym(), 0)
+	}
+	sehp.pdata = pdata.Sym()
+}

src/cmd/link/internal/loader/loader.go

@@ -1646,6 +1646,16 @@ func (l *Loader) WasmImportSym(fnSymIdx Sym) (Sym, bool) {
 	return 0, false
 }
 
+// SEHUnwindSym returns the auxiliary SEH unwind symbol associated with
+// a given function symbol.
+func (l *Loader) SEHUnwindSym(fnSymIdx Sym) Sym {
+	if l.SymType(fnSymIdx) != sym.STEXT {
+		log.Fatalf("error: non-function sym %d/%s t=%s passed to SEHUnwindSym", fnSymIdx, l.SymName(fnSymIdx), l.SymType(fnSymIdx).String())
+	}
+
+	return l.aux1(fnSymIdx, goobj.AuxSehUnwindInfo)
+}
+
 // GetFuncDwarfAuxSyms collects and returns the auxiliary DWARF
 // symbols associated with a given function symbol.  Prior to the
 // introduction of the loader, this was done purely using name

src/cmd/link/internal/sym/symkind.go

@@ -126,6 +126,7 @@ const (
 
 	// SEH symbol types
 	SSEHUNWINDINFO
+	SPDATASECT
 )
 
 // AbiSymKindToSymKind maps values read from object files (which are

src/internal/syscall/windows/syscall_windows.go

@@ -399,3 +399,5 @@ type FILE_ID_BOTH_DIR_INFO struct {
 }
 
 //sys	GetVolumeInformationByHandle(file syscall.Handle, volumeNameBuffer *uint16, volumeNameSize uint32, volumeNameSerialNumber *uint32, maximumComponentLength *uint32, fileSystemFlags *uint32, fileSystemNameBuffer *uint16, fileSystemNameSize uint32) (err error) = GetVolumeInformationByHandleW
+
+//sys	RtlLookupFunctionEntry(pc uintptr, baseAddress *uintptr, table *byte) (ret uintptr) = kernel32.RtlLookupFunctionEntry

src/runtime/defs_windows_386.go

@@ -56,6 +56,9 @@ func (c *context) set_lr(x uintptr) {}
 func (c *context) set_ip(x uintptr) { c.eip = uint32(x) }
 func (c *context) set_sp(x uintptr) { c.esp = uint32(x) }
 
+// 386 does not have frame pointer register.
+func (c *context) set_fp(x uintptr) {}
+
 func prepareContextForSigResume(c *context) {
 	c.edx = c.esp
 	c.ecx = c.eip

src/runtime/defs_windows_amd64.go

@@ -69,6 +69,7 @@ func (c *context) set_lr(x uintptr) {}
 
 func (c *context) set_ip(x uintptr) { c.rip = uint64(x) }
 func (c *context) set_sp(x uintptr) { c.rsp = uint64(x) }
+func (c *context) set_fp(x uintptr) { c.rbp = uint64(x) }
 
 func prepareContextForSigResume(c *context) {
 	c.r8 = c.rsp

src/runtime/defs_windows_arm.go

@@ -58,6 +58,9 @@ func (c *context) set_ip(x uintptr) { c.pc = uint32(x) }
 func (c *context) set_sp(x uintptr) { c.spr = uint32(x) }
 func (c *context) set_lr(x uintptr) { c.lrr = uint32(x) }
 
+// arm does not have frame pointer register.
+func (c *context) set_fp(x uintptr) {}
+
 func prepareContextForSigResume(c *context) {
 	c.r0 = c.spr
 	c.r1 = c.pc

src/runtime/defs_windows_arm64.go

@@ -40,6 +40,7 @@ func (c *context) lr() uintptr { return uintptr(c.x[30]) }
 func (c *context) set_ip(x uintptr) { c.pc = uint64(x) }
 func (c *context) set_sp(x uintptr) { c.xsp = uint64(x) }
 func (c *context) set_lr(x uintptr) { c.x[30] = uint64(x) }
+func (c *context) set_fp(x uintptr) { c.x[29] = uint64(x) }
 
 func prepareContextForSigResume(c *context) {
 	c.x[0] = c.xsp

src/runtime/export_windows_test.go

@@ -20,3 +20,19 @@ func NumberOfProcessors() int32 {
 	stdcall1(_GetSystemInfo, uintptr(unsafe.Pointer(&info)))
 	return int32(info.dwnumberofprocessors)
 }
+
+type ContextStub struct {
+	context
+}
+
+func (c ContextStub) GetPC() uintptr {
+	return c.ip()
+}
+
+func NewContextStub() ContextStub {
+	var ctx context
+	ctx.set_ip(getcallerpc())
+	ctx.set_sp(getcallersp())
+	ctx.set_fp(getcallerfp())
+	return ContextStub{ctx}
+}