cmd/link: only dynamically export necessary symbols on darwin

Currently on darwin, when a symbol needs to be exported, we
export it both statically and dynamically. The dynamic export is
unnecessary for some symbols. Only export the necessary ones.

For special runtime C symbols (e.g. crosscall2), they used to be
exported dynamically, and we had a special case for pclntab to
not include those symbols (otherwise, when the dynamic linker
dedup them, the pclntab entries end up pointing out of the
module's address space). This CL changes it to not export those
symbols, and remove the special case.

Change-Id: I2ab40630742d48a09b86ee150aa5f1f7002b134d
Reviewed-on: https://go-review.googlesource.com/c/go/+/261497
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

diff --git a/src/cmd/link/internal/ld/macho.go b/src/cmd/link/internal/ld/macho.go
index 80a753438e9ac7d8bde2ac5f3d5199a2927246c5..2c7f6111de32cc2c820d6ae436e8dc0cc876121b 100644 (file)
--- a/src/cmd/link/internal/ld/macho.go
+++ b/src/cmd/link/internal/ld/macho.go
@@ -475,6 +475,18 @@ func (ctxt *Link) domacho() {
                sb.SetReachable(true)
                sb.AddUint8(0)
        }
+
+       // Do not export C symbols dynamically in plugins, as runtime C symbols like crosscall2
+       // are in pclntab and end up pointing at the host binary, breaking unwinding.
+       // See Issue #18190.
+       if ctxt.BuildMode == BuildModePlugin {
+               for _, name := range []string{"_cgo_topofstack", "__cgo_topofstack", "_cgo_panic", "crosscall2"} {
+                       s := ctxt.loader.Lookup(name, 0)
+                       if s != 0 {
+                               ctxt.loader.SetAttrCgoExportDynamic(s, false)
+                       }
+               }
+       }
 }
 
 func machoadddynlib(lib string, linkmode LinkMode) {
@@ -899,19 +911,12 @@ func machosymtab(ctxt *Link) {
                symtab.AddUint32(ctxt.Arch, uint32(symstr.Size()))
 
                export := machoShouldExport(ctxt, ldr, s)
-               isGoSymbol := strings.Contains(ldr.SymExtname(s), ".")
-
-               // In normal buildmodes, only add _ to C symbols, as
-               // Go symbols have dot in the name.
-               //
-               // 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 := !isGoSymbol && (ctxt.BuildMode != BuildModePlugin || onlycsymbol(ldr.SymName(s)))
-               if cexport || export || isGoSymbol {
-                       symstr.AddUint8('_')
-               }
+
+               // Prefix symbol names with "_" to match the system toolchain.
+               // (We used to only prefix C symbols, which is all required for the build.
+               // But some tools don't recognize Go symbols as symbols, so we prefix them
+               // as well.)
+               symstr.AddUint8('_')
 
                // replace "·" as ".", because DTrace cannot handle it.
                symstr.Addstring(strings.Replace(ldr.SymExtname(s), "·", ".", -1))
@@ -922,10 +927,13 @@ func machosymtab(ctxt *Link) {
                        symtab.AddUint16(ctxt.Arch, 0)                    // desc
                        symtab.AddUintXX(ctxt.Arch, 0, ctxt.Arch.PtrSize) // no value
                } else {
-                       if ldr.AttrCgoExport(s) || export {
-                               symtab.AddUint8(0x0f)
+                       if export || ldr.AttrCgoExportDynamic(s) {
+                               symtab.AddUint8(0x0f) // N_SECT | N_EXT
+                       } else if ldr.AttrCgoExportStatic(s) {
+                               // Only export statically, not dynamically. (N_PEXT is like hidden visibility)
+                               symtab.AddUint8(0x1f) // N_SECT | N_EXT | N_PEXT
                        } else {
-                               symtab.AddUint8(0x0e)
+                               symtab.AddUint8(0x0e) // N_SECT
                        }
                        o := s
                        if outer := ldr.OuterSym(o); outer != 0 {

diff --git a/src/cmd/link/internal/ld/pcln.go b/src/cmd/link/internal/ld/pcln.go
index 75e63248df675f54f0e900136e5f974627599874..facb30fe15c7b4aeb60f308fad65d9054877e579 100644 (file)
--- a/src/cmd/link/internal/ld/pcln.go
+++ b/src/cmd/link/internal/ld/pcln.go
@@ -13,7 +13,6 @@ import (
        "fmt"
        "os"
        "path/filepath"
-       "strings"
 )
 
 // pclntab holds the state needed for pclntab generation.
@@ -113,23 +112,7 @@ func makePclntab(ctxt *Link, container loader.Bitmap) (*pclntab, []*sym.Compilat
        return state, compUnits, funcs
 }
 
-// onlycsymbol looks at a symbol's name to report whether this is a
-// symbol that is referenced by C code
-func onlycsymbol(sname string) bool {
-       switch sname {
-       case "_cgo_topofstack", "__cgo_topofstack", "_cgo_panic", "crosscall2":
-               return true
-       }
-       if strings.HasPrefix(sname, "_cgoexp_") {
-               return true
-       }
-       return false
-}
-
 func emitPcln(ctxt *Link, s loader.Sym, container loader.Bitmap) bool {
-       if ctxt.BuildMode == BuildModePlugin && ctxt.HeadType == objabi.Hdarwin && onlycsymbol(ctxt.loader.SymName(s)) {
-               return false
-       }
        // We want to generate func table entries only for the "lowest
        // level" symbols, not containers of subsymbols.
        return !container.Has(s)