cmd/internal/ld: store the libraries a shared library was linked against in a note

The motivation for this is the innocuous looking test case that is added. This
creates a stack exe -> libdep2.so -> libdep.so -> libruntime.so. The problem
comes from the fact that a function from libdep.so gets inlined all the way
into exe. This (unsurprisingly) means that the object file for exe references
symbols from libdep.so, which means that -ldep needs to be passed when linking
exe and it isn't. The fix is simply to pass it -- there is no harm in passing
it when it's not needed.

The thing is, it's not clear at all in the current code to see how the linker
can know that libdep2 is linked against libdep. It could look through the
DT_NEEDED entries in libdep2 and try to guess which are Go libraries, but it
feels better to be explicit. So this adds another SHT_NOTE section that lists
the shared libraries a shared library was linked against, and makes sure the
complete set of depended upon shared libraries is passed to the external
linker.

Change-Id: I79aa6f98b4db4721d657a7eb7b7f062269bf49e2
Reviewed-on: https://go-review.googlesource.com/10376Reviewed-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/shared_test.go

@@ -368,6 +368,21 @@ func testABIHashNote(t *testing.T, f *elf.File, note *note) {
 	}
 }
 
+// A Go shared library contains a note indicating which other Go shared libraries it
+// was linked against in an unmapped section.
+func testDepsNote(t *testing.T, f *elf.File, note *note) {
+	if note.section.Flags != 0 {
+		t.Errorf("package list section has flags %v", note.section.Flags)
+	}
+	if isOffsetLoaded(f, note.section.Offset) {
+		t.Errorf("package list section contained in PT_LOAD segment")
+	}
+	// libdep.so just links against the lib containing the runtime.
+	if note.desc != soname {
+		t.Errorf("incorrect dependency list %q", note.desc)
+	}
+}
+
 // The shared library contains notes with defined contents; see above.
 func TestNotes(t *testing.T) {
 	goCmd(t, "install", "-buildmode=shared", "-linkshared", "dep")
@@ -382,6 +397,7 @@ func TestNotes(t *testing.T) {
 	}
 	pkgListNoteFound := false
 	abiHashNoteFound := false
+	depsNoteFound := false
 	for _, note := range notes {
 		if note.name != "GO\x00\x00" {
 			continue
@@ -399,6 +415,12 @@ func TestNotes(t *testing.T) {
 			}
 			testABIHashNote(t, f, note)
 			abiHashNoteFound = true
+		case 3: // ELF_NOTE_GODEPS_TAG
+			if depsNoteFound {
+				t.Error("multiple abi hash notes")
+			}
+			testDepsNote(t, f, note)
+			depsNoteFound = true
 		}
 	}
 	if !pkgListNoteFound {
@@ -407,6 +429,19 @@ func TestNotes(t *testing.T) {
 	if !abiHashNoteFound {
 		t.Error("abi hash note not found")
 	}
+	if !depsNoteFound {
+		t.Error("deps note not found")
+	}
+}
+
+// Build a GOPATH package (dep) into a shared library that links against the goroot
+// runtime, another package (dep2) that links against the first, and and an
+// executable that links against dep2.
+func TestTwoGOPathShlibs(t *testing.T) {
+	goCmd(t, "install", "-buildmode=shared", "-linkshared", "dep")
+	goCmd(t, "install", "-buildmode=shared", "-linkshared", "dep2")
+	goCmd(t, "install", "-linkshared", "exe2")
+	run(t, "executable linked to GOPATH library", "./bin/exe2")
 }
 
 // Testing rebuilding of shared libraries when they are stale is a bit more

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

+package dep2
+
+import "dep"
+
+var W int = 1
+
+func G() int {
+	return dep.F() + 1
+}

misc/cgo/testshared/src/exe2/exe2.go

+package main
+
+import "dep2"
+
+func main() {
+	dep2.W = dep2.G() + 1
+}

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

@@ -9,7 +9,9 @@ import (
 	"crypto/sha1"
 	"encoding/binary"
 	"fmt"
+	"path/filepath"
 	"sort"
+	"strings"
 )
 
 /*
@@ -1205,6 +1207,7 @@ func elfwritebuildinfo() int {
 const (
 	ELF_NOTE_GOPKGLIST_TAG = 1
 	ELF_NOTE_GOABIHASH_TAG = 2
+	ELF_NOTE_GODEPS_TAG    = 3
 )
 
 var ELF_NOTE_GO_NAME = []byte("GO\x00\x00")
@@ -1697,6 +1700,7 @@ func doelf() {
 		if Buildmode == BuildmodeShared {
 			Addstring(shstrtab, ".note.go.abihash")
 			Addstring(shstrtab, ".note.go.pkg-list")
+			Addstring(shstrtab, ".note.go.deps")
 		}
 	}
 
@@ -1904,6 +1908,11 @@ func doelf() {
 		}
 		addgonote(".note.go.abihash", ELF_NOTE_GOABIHASH_TAG, h.Sum([]byte{}))
 		addgonote(".note.go.pkg-list", ELF_NOTE_GOPKGLIST_TAG, []byte(pkglistfornote))
+		var deplist []string
+		for _, shlib := range Ctxt.Shlibs {
+			deplist = append(deplist, filepath.Base(shlib.Path))
+		}
+		addgonote(".note.go.deps", ELF_NOTE_GODEPS_TAG, []byte(strings.Join(deplist, "\n")))
 	}
 }
 
@@ -1976,6 +1985,8 @@ func Asmbelf(symo int64) {
 			sh = elfshname(".note.go.abihash")
 			sh.type_ = SHT_NOTE
 			sh.flags = SHF_ALLOC
+			sh = elfshname(".note.go.deps")
+			sh.type_ = SHT_NOTE
 		}
 		goto elfobj
 	}

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

@@ -653,15 +653,14 @@ func deadcode() {
 	}
 
 	if Buildmode == BuildmodeShared {
-		// Mark all symbols as reachable when building a
-		// shared library.
+		// Mark all symbols defined in this library as reachable when
+		// building a shared library.
 		for s := Ctxt.Allsym; s != nil; s = s.Allsym {
-			if s.Type != 0 {
+			if s.Type != 0 && s.Type != obj.SDYNIMPORT {
 				mark(s)
 			}
 		}
-		mark(Linkrlookup(Ctxt, "main.main", 0))
-		mark(Linkrlookup(Ctxt, "main.init", 0))
+		markflood()
 	} else {
 		mark(Linklookup(Ctxt, INITENTRY, 0))
 		if Linkshared && Buildmode == BuildmodeExe {

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

@@ -983,15 +983,35 @@ func hostlink() {
 	argv = append(argv, fmt.Sprintf("%s/go.o", tmpdir))
 
 	if Linkshared {
-		for _, shlib := range Ctxt.Shlibs {
-			dir, base := filepath.Split(shlib.Path)
+		seenDirs := make(map[string]bool)
+		seenLibs := make(map[string]bool)
+		addshlib := func(path string) {
+			dir, base := filepath.Split(path)
+			if !seenDirs[dir] {
 				argv = append(argv, "-L"+dir)
 				if !rpath.set {
 					argv = append(argv, "-Wl,-rpath="+dir)
 				}
+				seenDirs[dir] = true
+			}
 			base = strings.TrimSuffix(base, ".so")
 			base = strings.TrimPrefix(base, "lib")
+			if !seenLibs[base] {
 				argv = append(argv, "-l"+base)
+				seenLibs[base] = true
+			}
+		}
+		for _, shlib := range Ctxt.Shlibs {
+			addshlib(shlib.Path)
+			for _, dep := range shlib.Deps {
+				if dep == "" {
+					continue
+				}
+				libpath := findshlib(dep)
+				if libpath != "" {
+					addshlib(libpath)
+				}
+			}
 		}
 	}
 
@@ -1214,18 +1234,20 @@ func readnote(f *elf.File, name []byte, typ int32) ([]byte, error) {
 	return nil, nil
 }
 
-func ldshlibsyms(shlib string) {
-	found := false
-	libpath := ""
+func findshlib(shlib string) string {
 	for _, libdir := range Ctxt.Libdir {
-		libpath = filepath.Join(libdir, shlib)
+		libpath := filepath.Join(libdir, shlib)
 		if _, err := os.Stat(libpath); err == nil {
-			found = true
-			break
+			return libpath
 		}
 	}
-	if !found {
 	Diag("cannot find shared library: %s", shlib)
+	return ""
+}
+
+func ldshlibsyms(shlib string) {
+	libpath := findshlib(shlib)
+	if libpath == "" {
 		return
 	}
 	for _, processedlib := range Ctxt.Shlibs {
@@ -1251,6 +1273,13 @@ func ldshlibsyms(shlib string) {
 		return
 	}
 
+	depsbytes, err := readnote(f, ELF_NOTE_GO_NAME, ELF_NOTE_GODEPS_TAG)
+	if err != nil {
+		Diag("cannot read dep list from shared library %s: %v", libpath, err)
+		return
+	}
+	deps := strings.Split(string(depsbytes), "\n")
+
 	syms, err := f.Symbols()
 	if err != nil {
 		Diag("cannot read symbols from shared library: %s", libpath)
@@ -1272,12 +1301,6 @@ func ldshlibsyms(shlib string) {
 		if elf.ST_TYPE(s.Info) == elf.STT_NOTYPE || elf.ST_TYPE(s.Info) == elf.STT_SECTION {
 			continue
 		}
-		if s.Section == elf.SHN_UNDEF {
-			continue
-		}
-		if strings.HasPrefix(s.Name, "_") {
-			continue
-		}
 		if strings.HasPrefix(s.Name, "runtime.gcbits.") {
 			gcmasks[s.Value] = readelfsymboldata(f, &s)
 		}
@@ -1285,7 +1308,7 @@ func ldshlibsyms(shlib string) {
 			continue
 		}
 		lsym := Linklookup(Ctxt, s.Name, 0)
-		if lsym.Type != 0 && lsym.Dupok == 0 {
+		if lsym.Type != 0 && lsym.Type != obj.SDYNIMPORT && lsym.Dupok == 0 {
 			Diag(
 				"Found duplicate symbol %s reading from %s, first found in %s",
 				s.Name, shlib, lsym.File)
@@ -1342,7 +1365,7 @@ func ldshlibsyms(shlib string) {
 		Ctxt.Etextp = last
 	}
 
-	Ctxt.Shlibs = append(Ctxt.Shlibs, Shlib{Path: libpath, Hash: hash})
+	Ctxt.Shlibs = append(Ctxt.Shlibs, Shlib{Path: libpath, Hash: hash, Deps: deps})
 }
 
 func mywhatsys() {

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

@@ -117,6 +117,7 @@ type Auto struct {
 type Shlib struct {
 	Path string
 	Hash []byte
+	Deps []string
 }
 
 type Link struct {