cmd/internal/ld: do not depend on local symbols to read a type's gcdata

We already read the address of a gcmask/gcprog out of the type data, but I
didn't know how many bytes to read. But it turns out that it's easy to
calculate, so change to do that. This means that we no longer depend on the
local symbols being present, allowing me to strip the shared libraries for
distribution and make them a lot smaller.

As a bonus, this makes LSym another 24 bytes smaller, down to 296 bytes now.

Change-Id: I379d359e28d63afae6753efd23efdf1fbb716992
Reviewed-on: https://go-review.googlesource.com/10377Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/testshared/src/dep/dep.go

@@ -2,6 +2,12 @@ package dep
 
 var V int = 1
 
+var HasMask []string = []string{"hi"}
+
+type HasProg struct {
+	array [1024]*byte
+}
+
 func F() int {
 	return V
 }

misc/cgo/testshared/src/dep2/dep2.go

@@ -4,6 +4,8 @@ import "dep"
 
 var W int = 1
 
+var hasProg dep.HasProg
+
 func G() int {
 	return dep.F() + 1
 }

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

@@ -1104,7 +1104,7 @@ func (p *GCProg) AddSym(s *LSym) {
 	// Copy program.
 	prog := decodetype_gcprog(typ)
 	p.w.ZeroUntil(s.Value / ptrsize)
-	p.w.Append(prog.P[4:prog.Size], nptr)
+	p.w.Append(prog[4:], nptr)
 }
 
 func growdatsize(datsizep *int64, s *LSym) {

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

@@ -4,7 +4,10 @@
 
 package ld
 
-import "cmd/internal/obj"
+import (
+	"cmd/internal/obj"
+	"debug/elf"
+)
 
 // Decoding the type.* symbols.	 This has to be in sync with
 // ../../runtime/type.go, or more specifically, with what
@@ -72,14 +75,38 @@ func decodetype_ptrdata(s *LSym) int64 {
 	return int64(decode_inuxi(s.P[Thearch.Ptrsize:], Thearch.Ptrsize)) // 0x8 / 0x10
 }
 
+// Find the elf.Section of a given shared library that contains a given address.
+func findShlibSection(path string, addr uint64) *elf.Section {
+	for _, shlib := range Ctxt.Shlibs {
+		if shlib.Path == path {
+			for _, sect := range shlib.File.Sections {
+				if sect.Addr <= addr && addr <= sect.Addr+sect.Size {
+					return sect
+				}
+			}
+		}
+	}
+	return nil
+}
+
 // Type.commonType.gc
-func decodetype_gcprog(s *LSym) *LSym {
+func decodetype_gcprog(s *LSym) []byte {
 	if s.Type == obj.SDYNIMPORT {
-		// The gcprog for "type.$name" is calle "type..gcprog.$name".
-		x := "type..gcprog." + s.Name[5:]
-		return Linklookup(Ctxt, x, 0)
+		addr := decodetype_gcprog_shlib(s)
+		sect := findShlibSection(s.File, addr)
+		if sect != nil {
+			// A gcprog is a 4-byte uint32 indicating length, followed by
+			// the actual program.
+			progsize := make([]byte, 4)
+			sect.ReadAt(progsize, int64(addr-sect.Addr))
+			progbytes := make([]byte, Ctxt.Arch.ByteOrder.Uint32(progsize))
+			sect.ReadAt(progbytes, int64(addr-sect.Addr+4))
+			return append(progsize, progbytes...)
+		}
+		Exitf("cannot find gcprog for %s", s.Name)
+		return nil
 	}
-	return decode_reloc_sym(s, 2*int32(Thearch.Ptrsize)+8+1*int32(Thearch.Ptrsize))
+	return decode_reloc_sym(s, 2*int32(Thearch.Ptrsize)+8+1*int32(Thearch.Ptrsize)).P
 }
 
 func decodetype_gcprog_shlib(s *LSym) uint64 {
@@ -88,9 +115,16 @@ func decodetype_gcprog_shlib(s *LSym) uint64 {
 
 func decodetype_gcmask(s *LSym) []byte {
 	if s.Type == obj.SDYNIMPORT {
-		// ldshlibsyms makes special efforts to read the value
-		// of gcmask for types defined in that shared library.
-		return s.gcmask
+		addr := decodetype_gcprog_shlib(s)
+		ptrdata := decodetype_ptrdata(s)
+		sect := findShlibSection(s.File, addr)
+		if sect != nil {
+			r := make([]byte, ptrdata/int64(Thearch.Ptrsize))
+			sect.ReadAt(r, int64(addr-sect.Addr))
+			return r
+		}
+		Exitf("cannot find gcmask for %s", s.Name)
+		return nil
 	}
 	mask := decode_reloc_sym(s, 2*int32(Thearch.Ptrsize)+8+1*int32(Thearch.Ptrsize))
 	return mask.P

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

@@ -1178,7 +1178,7 @@ func readelfsymboldata(f *elf.File, sym *elf.Symbol) []byte {
 	if sect.Type != elf.SHT_PROGBITS && sect.Type != elf.SHT_NOTE {
 		Diag("reading %s from non-data section", sym.Name)
 	}
-	n, err := sect.ReadAt(data, int64(sym.Value-sect.Offset))
+	n, err := sect.ReadAt(data, int64(sym.Value-sect.Addr))
 	if uint64(n) != sym.Size {
 		Diag("reading contents of %s: %v", sym.Name, err)
 	}
@@ -1265,7 +1265,6 @@ func ldshlibsyms(shlib string) {
 		Diag("cannot open shared library: %s", libpath)
 		return
 	}
-	defer f.Close()
 
 	hash, err := readnote(f, ELF_NOTE_GO_NAME, ELF_NOTE_GOABIHASH_TAG)
 	if err != nil {
@@ -1280,33 +1279,15 @@ func ldshlibsyms(shlib string) {
 	}
 	deps := strings.Split(string(depsbytes), "\n")
 
-	syms, err := f.Symbols()
+	syms, err := f.DynamicSymbols()
 	if err != nil {
 		Diag("cannot read symbols from shared library: %s", libpath)
 		return
 	}
-	// If a package has a global variable of a type defined in another shared
-	// library, we need to know the gcmask used by the type, if any.  To support
-	// this, we read all the runtime.gcbits.* symbols, keep a map of address to
-	// gcmask, and after we're read all the symbols, read the addresses of the
-	// gcmasks symbols out of the type data to look up the gcmask for each type.
-	// This depends on the fact that the runtime.gcbits.* symbols are local (so
-	// the address is actually present in the type data and we don't have to
-	// search all relocations to find the ones which correspond to gcmasks) and
-	// also that the shared library we are linking against has not had the symbol
-	// table removed.
-	gcmasks := make(map[uint64][]byte)
-	types := []*LSym{}
 	for _, s := range syms {
 		if elf.ST_TYPE(s.Info) == elf.STT_NOTYPE || elf.ST_TYPE(s.Info) == elf.STT_SECTION {
 			continue
 		}
-		if strings.HasPrefix(s.Name, "runtime.gcbits.") {
-			gcmasks[s.Value] = readelfsymboldata(f, &s)
-		}
-		if elf.ST_BIND(s.Info) != elf.STB_GLOBAL {
-			continue
-		}
 		lsym := Linklookup(Ctxt, s.Name, 0)
 		if lsym.Type != 0 && lsym.Type != obj.SDYNIMPORT && lsym.Dupok == 0 {
 			Diag(
@@ -1315,27 +1296,15 @@ func ldshlibsyms(shlib string) {
 		}
 		lsym.Type = obj.SDYNIMPORT
 		lsym.ElfType = elf.ST_TYPE(s.Info)
-		lsym.File = libpath
-		if strings.HasPrefix(lsym.Name, "type.") {
-			if f.Sections[s.Section].Type == elf.SHT_PROGBITS {
+		if s.Section != elf.SHN_UNDEF {
+			// Set .File for the library that actually defines the symbol.
+			lsym.File = libpath
+			// The decodetype_* functions in decodetype.go need access to
+			// the type data.
+			if strings.HasPrefix(lsym.Name, "type.") && !strings.HasPrefix(lsym.Name, "type..") {
 				lsym.P = readelfsymboldata(f, &s)
 			}
-			if !strings.HasPrefix(lsym.Name, "type..") {
-				types = append(types, lsym)
-			}
-		}
-	}
-
-	for _, t := range types {
-		if decodetype_noptr(t) != 0 || decodetype_usegcprog(t) != 0 {
-			continue
-		}
-		addr := decodetype_gcprog_shlib(t)
-		tgcmask, ok := gcmasks[addr]
-		if !ok {
-			Diag("bits not found for %s at %d", t.Name, addr)
 		}
-		t.gcmask = tgcmask
 	}
 
 	// We might have overwritten some functions above (this tends to happen for the
@@ -1365,7 +1334,7 @@ func ldshlibsyms(shlib string) {
 		Ctxt.Etextp = last
 	}
 
-	Ctxt.Shlibs = append(Ctxt.Shlibs, Shlib{Path: libpath, Hash: hash, Deps: deps})
+	Ctxt.Shlibs = append(Ctxt.Shlibs, Shlib{Path: libpath, Hash: hash, Deps: deps, File: f})
 }
 
 func mywhatsys() {

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

@@ -84,7 +84,6 @@ type LSym struct {
 	P           []byte
 	R           []Reloc
 	Local       bool
-	gcmask      []byte
 }
 
 func (s *LSym) String() string {
@@ -118,6 +117,7 @@ type Shlib struct {
 	Path string
 	Hash []byte
 	Deps []string
+	File *elf.File
 }
 
 type Link struct {