cmd/internal/obj/ppc64: don't modify runtime.elf_* symbols

The runtime.elf_* symbols are assembly functions which are used
to support the gcc/llvm -Os option when used with cgo.

When compiling Go for shared code, we attempt to strip out the
TOC regenation code added by the go assembler for these symbols.

This causes the symbol to no longer appear as an assembly
function which causes problems later on when handling other
implicit symbols.

Avoid adding a TOC regeneration prologue to these functions
to avoid this issue.

Fixes golang#66265

Change-Id: Icbf8e4438d177082a57bb228e39b232e7a0d7ada
Reviewed-on: https://go-review.googlesource.com/c/go/+/571835
Reviewed-by: Than McIntosh <thanm@google.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/testdata/script/test_ppc64_linker_funcs.txt

@@ -14,6 +14,10 @@ go build -ldflags='-linkmode=internal'
 exec ./abitest
 stdout success
 
+go build -buildmode=pie -o abitest.pie -ldflags='-linkmode=internal'
+exec ./abitest.pie
+stdout success
+
 -- go.mod --
 module abitest
 

src/cmd/internal/obj/ppc64/obj9.go

@@ -38,6 +38,7 @@ import (
 	"internal/buildcfg"
 	"log"
 	"math/bits"
+	"strings"
 )
 
 // Test if this value can encoded as a mask for
@@ -73,6 +74,22 @@ func encodePPC64RLDCMask(mask int64) (mb, me int) {
 	return mb, me - 1
 }
 
+// Is this a symbol which should never have a TOC prologue generated?
+// These are special functions which should not have a TOC regeneration
+// prologue.
+func isNOTOCfunc(name string) bool {
+	switch {
+	case name == "runtime.duffzero":
+		return true
+	case name == "runtime.duffcopy":
+		return true
+	case strings.HasPrefix(name, "runtime.elf_"):
+		return true
+	default:
+		return false
+	}
+}
+
 func progedit(ctxt *obj.Link, p *obj.Prog, newprog obj.ProgAlloc) {
 	p.From.Class = 0
 	p.To.Class = 0
@@ -794,7 +811,7 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 
 			q = p
 
-			if NeedTOCpointer(c.ctxt) && c.cursym.Name != "runtime.duffzero" && c.cursym.Name != "runtime.duffcopy" {
+			if NeedTOCpointer(c.ctxt) && !isNOTOCfunc(c.cursym.Name) {
 				// When compiling Go into PIC, without PCrel support, all functions must start
 				// with instructions to load the TOC pointer into r2:
 				//

src/cmd/link/internal/ppc64/asm.go

@@ -474,24 +474,9 @@ func rewriteABIFuncReloc(ctxt *ld.Link, ldr *loader.Loader, tname string, r load
 	r.SetAdd(int64((n - minReg) * offMul))
 	firstUse = !ldr.AttrReachable(ts)
 	if firstUse {
-		ldr.SetAttrReachable(ts, true)
 		// This function only becomes reachable now. It has been dropped from
 		// the text section (it was unreachable until now), it needs included.
-		//
-		// Similarly, TOC regeneration should not happen for these functions,
-		// remove it from this save/restore function.
-		if ldr.AttrShared(ts) {
-			sb := ldr.MakeSymbolUpdater(ts)
-			sb.SetData(sb.Data()[8:])
-			sb.SetSize(sb.Size() - 8)
-			relocs := sb.Relocs()
-			// Only one PCREL reloc to .TOC. should be present.
-			if relocs.Count() != 1 {
-				log.Fatalf("Unexpected number of relocs in %s\n", ldr.SymName(ts))
-			}
-			sb.ResetRelocs()
-
-		}
+		ldr.SetAttrReachable(ts, true)
 	}
 	return ts, firstUse
 }