[dev.link] cmd/link: use fingerprint as package hash

Now the compiler-generated fingerprint is a hash of the export
data. We don't need to hash it ourselves in the linker. And the
linker doesn't need to read export data at all.

Fixes #33820.

Change-Id: I54bf3ebfd0f0c72aa43a352d7b2e0575dd62970d
Reviewed-on: https://go-review.googlesource.com/c/go/+/236119
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

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

@@ -12,7 +12,6 @@ import (
 	"crypto/sha1"
 	"encoding/binary"
 	"encoding/hex"
-	"io"
 	"path/filepath"
 	"sort"
 	"strings"
@@ -1713,7 +1712,7 @@ func (ctxt *Link) doelf() {
 		sort.Sort(byPkg(ctxt.Library))
 		h := sha1.New()
 		for _, l := range ctxt.Library {
-			io.WriteString(h, l.Hash)
+			h.Write(l.Fingerprint[:])
 		}
 		addgonote(ctxt, ".note.go.abihash", ELF_NOTE_GOABIHASH_TAG, h.Sum([]byte{}))
 		addgonote(ctxt, ".note.go.pkg-list", ELF_NOTE_GOPKGLIST_TAG, pkglistfornote)

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

@@ -48,7 +48,6 @@ import (
 	"debug/macho"
 	"encoding/base64"
 	"encoding/binary"
-	"encoding/hex"
 	"fmt"
 	"io"
 	"io/ioutil"
@@ -785,16 +784,6 @@ func (ctxt *Link) linksetup() {
 	ctxt.loader.SetAttrReachable(moduledata, true)
 	ctxt.Moduledata = moduledata
 
-	// If package versioning is required, generate a hash of the
-	// packages used in the link.
-	if ctxt.BuildMode == BuildModeShared || ctxt.BuildMode == BuildModePlugin || ctxt.CanUsePlugins() {
-		for _, lib := range ctxt.Library {
-			if lib.Shlib == "" {
-				genhash(ctxt, lib)
-			}
-		}
-	}
-
 	if ctxt.Arch == sys.Arch386 && ctxt.HeadType != objabi.Hwindows {
 		if (ctxt.BuildMode == BuildModeCArchive && ctxt.IsELF) || ctxt.BuildMode == BuildModeCShared || ctxt.BuildMode == BuildModePIE || ctxt.DynlinkingGo() {
 			got := ctxt.loader.LookupOrCreateSym("_GLOBAL_OFFSET_TABLE_", 0)
@@ -919,67 +908,6 @@ func nextar(bp *bio.Reader, off int64, a *ArHdr) int64 {
 	return arsize + SAR_HDR
 }
 
-func genhash(ctxt *Link, lib *sym.Library) {
-	f, err := bio.Open(lib.File)
-	if err != nil {
-		Errorf(nil, "cannot open file %s for hash generation: %v", lib.File, err)
-		return
-	}
-	defer f.Close()
-
-	var magbuf [len(ARMAG)]byte
-	if _, err := io.ReadFull(f, magbuf[:]); err != nil {
-		Exitf("file %s too short", lib.File)
-	}
-
-	if string(magbuf[:]) != ARMAG {
-		Exitf("%s is not an archive file", lib.File)
-	}
-
-	var arhdr ArHdr
-	l := nextar(f, f.Offset(), &arhdr)
-	if l <= 0 {
-		Errorf(nil, "%s: short read on archive file symbol header", lib.File)
-		return
-	}
-	if arhdr.name != pkgdef {
-		Errorf(nil, "%s: missing package data entry", lib.File)
-		return
-	}
-
-	h := sha1.New()
-
-	// To compute the hash of a package, we hash the first line of
-	// __.PKGDEF (which contains the toolchain version and any
-	// GOEXPERIMENT flags) and the export data (which is between
-	// the first two occurrences of "\n$$").
-
-	pkgDefBytes := make([]byte, atolwhex(arhdr.size))
-	_, err = io.ReadFull(f, pkgDefBytes)
-	if err != nil {
-		Errorf(nil, "%s: error reading package data: %v", lib.File, err)
-		return
-	}
-	firstEOL := bytes.IndexByte(pkgDefBytes, '\n')
-	if firstEOL < 0 {
-		Errorf(nil, "cannot parse package data of %s for hash generation, no newline found", lib.File)
-		return
-	}
-	firstDoubleDollar := bytes.Index(pkgDefBytes, []byte("\n$$"))
-	if firstDoubleDollar < 0 {
-		Errorf(nil, "cannot parse package data of %s for hash generation, no \\n$$ found", lib.File)
-		return
-	}
-	secondDoubleDollar := bytes.Index(pkgDefBytes[firstDoubleDollar+1:], []byte("\n$$"))
-	if secondDoubleDollar < 0 {
-		Errorf(nil, "cannot parse package data of %s for hash generation, only one \\n$$ found", lib.File)
-		return
-	}
-	h.Write(pkgDefBytes[0:firstEOL])
-	h.Write(pkgDefBytes[firstDoubleDollar : firstDoubleDollar+secondDoubleDollar])
-	lib.Hash = hex.EncodeToString(h.Sum(nil))
-}
-
 func loadobjfile(ctxt *Link, lib *sym.Library) {
 	pkg := objabi.PathToPrefix(lib.Pkg)
 

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

@@ -591,13 +591,13 @@ func (ctxt *Link) symtab() []sym.SymKind {
 			s := ldr.CreateSymForUpdate("go.link.pkghashbytes."+l.Pkg, 0)
 			s.SetReachable(true)
 			s.SetType(sym.SRODATA)
-			s.SetSize(int64(len(l.Hash)))
-			s.SetData([]byte(l.Hash))
+			s.SetSize(int64(len(l.Fingerprint)))
+			s.SetData(l.Fingerprint[:])
 			str := ldr.CreateSymForUpdate("go.link.pkghash."+l.Pkg, 0)
 			str.SetReachable(true)
 			str.SetType(sym.SRODATA)
 			str.AddAddr(ctxt.Arch, s.Sym())
-			str.AddUint(ctxt.Arch, uint64(len(l.Hash)))
+			str.AddUint(ctxt.Arch, uint64(len(l.Fingerprint)))
 		}
 	}
 
@@ -698,7 +698,7 @@ func (ctxt *Link) symtab() []sym.SymKind {
 			// pkghashes[i].name
 			addgostring(ctxt, ldr, pkghashes, fmt.Sprintf("go.link.pkgname.%d", i), l.Pkg)
 			// pkghashes[i].linktimehash
-			addgostring(ctxt, ldr, pkghashes, fmt.Sprintf("go.link.pkglinkhash.%d", i), l.Hash)
+			addgostring(ctxt, ldr, pkghashes, fmt.Sprintf("go.link.pkglinkhash.%d", i), string(l.Fingerprint[:]))
 			// pkghashes[i].runtimehash
 			hash := ldr.Lookup("go.link.pkghash."+l.Pkg, 0)
 			pkghashes.AddAddr(ctxt.Arch, hash)

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

@@ -12,7 +12,6 @@ type Library struct {
 	File        string
 	Pkg         string
 	Shlib       string
-	Hash        string
 	Fingerprint goobj2.FingerprintType
 	Autolib     []goobj2.ImportedPkg
 	Imports     []*Library