cmd/internal/obj, cmd/link: record ABIs and aliases in Go obj files

This repurposes the "version" field of a symbol reference in the Go
object file format to be an ABI field. Currently, this is just 0 or 1
depending on whether the symbol is static (the linker turns it into a
different internal version number), so it's already only tenuously a
symbol version. We change this to be -1 for static symbols and
otherwise by the ABI number.

This also adds a separate list of ABI alias symbols to be recorded in
the object file. The ABI aliases must be a separate list and not just
part of the symbol definitions because it's possible to have a symbol
defined in one package and the alias "defined" in a different package.
For example, this can happen if a symbol is defined in assembly in one
package and stubbed in a different package. The stub triggers the
generation of the ABI alias, but in a different package from the
definition.

For #27539.

Change-Id: I015c9fe54690c027de6ef77e22b5585976a01587
Reviewed-on: https://go-review.googlesource.com/c/147157
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/internal/goobj/read.go

@@ -288,18 +288,31 @@ func (r *objReader) readSymID() SymID {
 }
 
 func (r *objReader) readRef() {
-	name, vers := r.readString(), r.readInt()
+	name, abiOrStatic := r.readString(), r.readInt()
 
 	// In a symbol name in an object file, "". denotes the
 	// prefix for the package in which the object file has been found.
 	// Expand it.
 	name = strings.ReplaceAll(name, `"".`, r.pkgprefix)
 
-	// An individual object file only records version 0 (extern) or 1 (static).
-	// To make static symbols unique across all files being read, we
-	// replace version 1 with the version corresponding to the current
-	// file number. The number is incremented on each call to parseObject.
-	if vers != 0 {
+	// The ABI field records either the ABI or -1 for static symbols.
+	//
+	// To distinguish different static symbols with the same name,
+	// we use the symbol "version". Version 0 corresponds to
+	// global symbols, and each file has a unique version > 0 for
+	// all of its static symbols. The version is incremented on
+	// each call to parseObject.
+	//
+	// For global symbols, we currently ignore the ABI.
+	//
+	// TODO(austin): Record the ABI in SymID. Since this is a
+	// public API, we'll have to keep Version as 0 and record the
+	// ABI in a new field (which differs from how the linker does
+	// this, but that's okay). Show the ABI in things like
+	// objdump.
+	var vers int64
+	if abiOrStatic == -1 {
+		// Static symbol
 		vers = r.p.MaxVersion
 	}
 	r.p.SymRefs = append(r.p.SymRefs, SymID{name, vers})
@@ -487,7 +500,7 @@ func (r *objReader) parseObject(prefix []byte) error {
 	// TODO: extract OS + build ID if/when we need it
 
 	r.readFull(r.tmp[:8])
-	if !bytes.Equal(r.tmp[:8], []byte("\x00\x00go19ld")) {
+	if !bytes.Equal(r.tmp[:8], []byte("\x00go112ld")) {
 		return r.error(errCorruptObject)
 	}
 
@@ -602,7 +615,7 @@ func (r *objReader) parseObject(prefix []byte) error {
 	}
 
 	r.readFull(r.tmp[:7])
-	if !bytes.Equal(r.tmp[:7], []byte("\xffgo19ld")) {
+	if !bytes.Equal(r.tmp[:7], []byte("go112ld")) {
 		return r.error(errCorruptObject)
 	}
 

src/cmd/internal/obj/link.go

@@ -432,7 +432,7 @@ const (
 )
 
 // Attribute is a set of symbol attributes.
-type Attribute int16
+type Attribute uint16
 
 const (
 	AttrDuplicateOK Attribute = 1 << iota
@@ -468,6 +468,13 @@ const (
 	// For function symbols; indicates that the specified function was the
 	// target of an inline during compilation
 	AttrWasInlined
+
+	// attrABIBase is the value at which the ABI is encoded in
+	// Attribute. This must be last; all bits after this are
+	// assumed to be an ABI value.
+	//
+	// MUST BE LAST since all bits above this comprise the ABI.
+	attrABIBase
 )
 
 func (a Attribute) DuplicateOK() bool   { return a&AttrDuplicateOK != 0 }
@@ -493,6 +500,12 @@ func (a *Attribute) Set(flag Attribute, value bool) {
 	}
 }
 
+func (a Attribute) ABI() ABI { return ABI(a / attrABIBase) }
+func (a *Attribute) SetABI(abi ABI) {
+	const mask = 1 // Only one ABI bit for now.
+	*a = (*a &^ (mask * attrABIBase)) | Attribute(abi)*attrABIBase
+}
+
 var textAttrStrings = [...]struct {
 	bit Attribute
 	s   string
@@ -524,6 +537,12 @@ func (a Attribute) TextAttrString() string {
 			a &^= x.bit
 		}
 	}
+	switch a.ABI() {
+	case ABI0:
+	case ABIInternal:
+		s += "ABIInternal|"
+		a.SetABI(0) // Clear ABI so we don't print below.
+	}
 	if a != 0 {
 		s += fmt.Sprintf("UnknownAttribute(%d)|", a)
 	}
@@ -606,6 +625,16 @@ type Link struct {
 	// state for writing objects
 	Text []*LSym
 	Data []*LSym
+
+	// ABIAliases are text symbols that should be aliased to all
+	// ABIs. These symbols may only be referenced and not defined
+	// by this object, since the need for an alias may appear in a
+	// different object than the definition. Hence, this
+	// information can't be carried in the symbol definition.
+	//
+	// TODO(austin): Replace this with ABI wrappers once the ABIs
+	// actually diverge.
+	ABIAliases []*LSym
 }
 
 func (ctxt *Link) Diag(format string, args ...interface{}) {

src/cmd/internal/obj/objfile.go

@@ -82,7 +82,7 @@ func WriteObjFile(ctxt *Link, b *bufio.Writer) {
 	w := newObjWriter(ctxt, b)
 
 	// Magic header
-	w.wr.WriteString("\x00\x00go19ld")
+	w.wr.WriteString("\x00go112ld")
 
 	// Version
 	w.wr.WriteByte(1)
@@ -102,6 +102,10 @@ func WriteObjFile(ctxt *Link, b *bufio.Writer) {
 		w.writeRefs(s)
 		w.addLengths(s)
 	}
+	for _, s := range ctxt.ABIAliases {
+		w.writeRefs(s)
+		w.addLengths(s)
+	}
 	// End symbol references
 	w.wr.WriteByte(0xff)
 
@@ -137,9 +141,12 @@ func WriteObjFile(ctxt *Link, b *bufio.Writer) {
 	for _, s := range ctxt.Data {
 		w.writeSym(s)
 	}
+	for _, s := range ctxt.ABIAliases {
+		w.writeSym(s)
+	}
 
 	// Magic footer
-	w.wr.WriteString("\xff\xffgo19ld")
+	w.wr.WriteString("\xffgo112ld")
 }
 
 // Symbols are prefixed so their content doesn't get confused with the magic footer.
@@ -155,8 +162,12 @@ func (w *objWriter) writeRef(s *LSym, isPath bool) {
 	} else {
 		w.writeString(s.Name)
 	}
-	// Write "version".
-	w.writeBool(s.Static())
+	// Write ABI/static information.
+	abi := int64(s.ABI())
+	if s.Static() {
+		abi = -1
+	}
+	w.writeInt(abi)
 	w.nRefs++
 	s.RefIdx = w.nRefs
 }

src/cmd/internal/objabi/doc.go

@@ -22,7 +22,7 @@
 //
 // The file format is:
 //
-//	- magic header: "\x00\x00go19ld"
+//	- magic header: "\x00go112ld"
 //	- byte 1 - version number
 //	- sequence of strings giving dependencies (imported packages)
 //	- empty string (marks end of sequence)
@@ -38,7 +38,7 @@
 //	- data, the content of the defined symbols
 //	- sequence of defined symbols
 //	- byte 0xff (marks end of sequence)
-//	- magic footer: "\xff\xffgo19ld"
+//	- magic footer: "\xffgo112ld"
 //
 // All integers are stored in a zigzag varint format.
 // See golang.org/s/go12symtab for a definition.
@@ -46,7 +46,7 @@
 // Data blocks and strings are both stored as an integer
 // followed by that many bytes.
 //
-// A symbol reference is a string name followed by a version.
+// A symbol reference is a string name followed by an ABI or -1 for static.
 //
 // A symbol points to other symbols using an index into the symbol
 // reference sequence. Index 0 corresponds to a nil symbol pointer.
@@ -57,7 +57,7 @@
 //
 //	- byte 0xfe (sanity check for synchronization)
 //	- type [byte]
-//	- name & version [symref index]
+//	- name & ABI [symref index]
 //	- flags [int]
 //		1<<0 dupok
 //		1<<1 local

src/cmd/link/internal/objfile/objfile.go

@@ -13,6 +13,7 @@ import (
 	"bytes"
 	"cmd/internal/bio"
 	"cmd/internal/dwarf"
+	"cmd/internal/obj"
 	"cmd/internal/objabi"
 	"cmd/internal/sys"
 	"cmd/link/internal/sym"
@@ -23,8 +24,8 @@ import (
 )
 
 const (
-	startmagic = "\x00\x00go19ld"
-	endmagic   = "\xff\xffgo19ld"
+	startmagic = "\x00go112ld"
+	endmagic   = "\xffgo112ld"
 )
 
 var emptyPkg = []byte(`"".`)
@@ -382,17 +383,20 @@ func (r *objReader) readRef() {
 		log.Fatalf("readSym out of sync")
 	}
 	name := r.readSymName()
-	v := r.readInt()
-	if v != 0 && v != 1 {
-		log.Fatalf("invalid symbol version for %q: %d", name, v)
-	}
-	if v == 1 {
+	var v int
+	if abi := r.readInt(); abi == -1 {
+		// Static
 		v = r.localSymVersion
+	} else if abiver := sym.ABIToVersion(obj.ABI(abi)); abiver != -1 {
+		// Note that data symbols are "ABI0", which maps to version 0.
+		v = abiver
+	} else {
+		log.Fatalf("invalid symbol ABI for %q: %d", name, abi)
 	}
 	s := r.syms.Lookup(name, v)
 	r.refs = append(r.refs, s)
 
-	if s == nil || v != 0 {
+	if s == nil || v == r.localSymVersion {
 		return
 	}
 	if s.Name[0] == '$' && len(s.Name) > 5 && s.Type == 0 && len(s.P) == 0 {

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

@@ -5,6 +5,7 @@
 package sym
 
 import (
+	"cmd/internal/obj"
 	"cmd/internal/objabi"
 	"cmd/internal/sys"
 	"debug/elf"
@@ -52,9 +53,21 @@ type AuxSymbol struct {
 }
 
 const (
+	SymVerABI0        = 0
+	SymVerABIInternal = 1
 	SymVerStatic      = 10 // Minimum version used by static (file-local) syms
 )
 
+func ABIToVersion(abi obj.ABI) int {
+	switch abi {
+	case obj.ABI0:
+		return SymVerABI0
+	case obj.ABIInternal:
+		return SymVerABIInternal
+	}
+	return -1
+}
+
 func (s *Symbol) String() string {
 	if s.Version == 0 {
 		return s.Name