cmd/link: do not export plugin C symbols

Explicitly filter any C-only cgo functions out of pclntable,
which allows them to be duplicated with the host binary.

Updates #18190.

Change-Id: I50d8706777a6133b3e95f696bc0bc586b84faa9e
Reviewed-on: https://go-review.googlesource.com/34199Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/testplugin/src/plugin2/plugin2.go

@@ -4,12 +4,21 @@
 
 package main
 
-// // No C code required.
+//#include <errno.h>
+//#include <string.h>
 import "C"
 
-import "common"
+// #include
+// void cfunc() {} // uses cgo_topofstack
+
+import (
+	"common"
+	"strings"
+)
 
 func init() {
+	_ = strings.NewReplacer() // trigger stack unwind, Issue #18190.
+	C.strerror(C.EIO)         // uses cgo_topofstack
 	common.X = 2
 }
 

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

@@ -695,7 +695,16 @@ func machoShouldExport(ctxt *Link, s *Symbol) bool {
 	if Buildmode == BuildmodePlugin && strings.HasPrefix(s.Extname, *flagPluginPath) {
 		return true
 	}
-	return s.Type != obj.STEXT
+	if strings.HasPrefix(s.Name, "type.") && !strings.HasPrefix(s.Name, "type..") {
+		// reduce runtime typemap pressure, but do not
+		// export alg functions (type..*), as these
+		// appear in pclntable.
+		return true
+	}
+	if strings.HasPrefix(s.Name, "go.link.pkghash") {
+		return true
+	}
+	return s.Type >= obj.SELFSECT // only writable sections
 }
 
 func machosymtab(ctxt *Link) {
@@ -710,7 +719,13 @@ func machosymtab(ctxt *Link) {
 
 		// In normal buildmodes, only add _ to C symbols, as
 		// Go symbols have dot in the name.
-		if !strings.Contains(s.Extname, ".") || export {
+		//
+		// Do not export C symbols in plugins, as runtime C
+		// symbols like crosscall2 are in pclntab and end up
+		// pointing at the host binary, breaking unwinding.
+		// See Issue #18190.
+		cexport := !strings.Contains(s.Extname, ".") && (Buildmode != BuildmodePlugin || onlycsymbol(s))
+		if cexport || export {
 			Adduint8(ctxt, symstr, '_')
 		}
 

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

@@ -154,10 +154,26 @@ func renumberfiles(ctxt *Link, files []*Symbol, d *Pcdata) {
 	*d = out
 }
 
+// onlycsymbol reports whether this is a cgo symbol provided by the
+// runtime and only used from C code.
+func onlycsymbol(s *Symbol) bool {
+	switch s.Name {
+	case "_cgo_topofstack", "_cgo_panic", "crosscall2":
+		return true
+	}
+	return false
+}
+
 func container(s *Symbol) int {
+	if s == nil {
+		return 0
+	}
+	if Buildmode == BuildmodePlugin && onlycsymbol(s) {
+		return 1
+	}
 	// We want to generate func table entries only for the "lowest level" symbols,
 	// not containers of subsymbols.
-	if s != nil && s.Type&obj.SCONTAINER != 0 {
+	if s.Type&obj.SCONTAINER != 0 {
 		return 1
 	}
 	return 0

src/runtime/plugin.go

@@ -51,6 +51,9 @@ func plugin_lastmoduleinit() (path string, syms map[string]interface{}, mismatch
 	modulesinit()
 	typelinksinit()
 
+	pluginftabverify(md)
+	moduledataverify1(md)
+
 	lock(&ifaceLock)
 	for _, i := range md.itablinks {
 		additab(i, true, false)
@@ -82,6 +85,35 @@ func plugin_lastmoduleinit() (path string, syms map[string]interface{}, mismatch
 	return md.pluginpath, syms, ""
 }
 
+func pluginftabverify(md *moduledata) {
+	badtable := false
+	for i := 0; i < len(md.ftab); i++ {
+		entry := md.ftab[i].entry
+		if md.minpc <= entry && entry <= md.maxpc {
+			continue
+		}
+
+		f := (*_func)(unsafe.Pointer(&md.pclntable[md.ftab[i].funcoff]))
+		name := funcname(f)
+
+		// A common bug is f.entry has a relocation to a duplicate
+		// function symbol, meaning if we search for its PC we get
+		// a valid entry with a name that is useful for debugging.
+		name2 := "none"
+		entry2 := uintptr(0)
+		f2 := findfunc(entry)
+		if f2 != nil {
+			name2 = funcname(f2)
+			entry2 = f2.entry
+		}
+		badtable = true
+		println("ftab entry outside pc range: ", hex(entry), "/", hex(entry2), ": ", name, "/", name2)
+	}
+	if badtable {
+		throw("runtime: plugin has bad symbol table")
+	}
+}
+
 // inRange reports whether v0 or v1 are in the range [r0, r1].
 func inRange(r0, r1, v0, v1 uintptr) bool {
 	return (v0 >= r0 && v0 <= r1) || (v1 >= r0 && v1 <= r1)