cmd/6l, cmd/internal/ld: handle R_PCREL to function in other shared library

An ELF linker handles a PC-relative reference to an STT_FUNC defined in a
shared library by building a PLT entry and referring to that, so do the
same in 6l.

Fixes #10690

Change-Id: I061a96fd4400d957e301d0ac86760ce256910e1d
Reviewed-on: https://go-review.googlesource.com/9711
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/6l/asm.go

@@ -33,6 +33,7 @@ package main
 import (
 	"cmd/internal/ld"
 	"cmd/internal/obj"
+	"debug/elf"
 	"fmt"
 	"log"
 )
@@ -381,7 +382,11 @@ func elfreloc1(r *ld.Reloc, sectoff int64) int {
 
 	case obj.R_PCREL:
 		if r.Siz == 4 {
+			if r.Xsym.Type == obj.SDYNIMPORT && r.Xsym.ElfType == elf.STT_FUNC {
+				ld.Thearch.Vput(ld.R_X86_64_PLT32 | uint64(elfsym)<<32)
+			} else {
 				ld.Thearch.Vput(ld.R_X86_64_PC32 | uint64(elfsym)<<32)
+			}
 		} else {
 			return -1
 		}

src/cmd/internal/ld/lib.go

@@ -1197,6 +1197,7 @@ func ldshlibsyms(shlib string) {
 				s.Name, shlib, lsym.File)
 		}
 		lsym.Type = obj.SDYNIMPORT
+		lsym.ElfType = elf.ST_TYPE(s.Info)
 		lsym.File = libpath
 		if strings.HasPrefix(lsym.Name, "type.") {
 			data := make([]byte, s.Size)

src/cmd/internal/ld/link.go

@@ -32,6 +32,7 @@ package ld
 
 import (
 	"cmd/internal/obj"
+	"debug/elf"
 	"encoding/binary"
 )
 
@@ -52,6 +53,10 @@ type LSym struct {
 	Leaf       uint8
 	Localentry uint8
 	Onlist     uint8
+	// ElfType is set for symbols read from shared libraries by ldshlibsyms. It
+	// is not set for symbols defined by the packages being linked or by symbols
+	// read by ldelf (and so is left as elf.STT_NOTYPE).
+	ElfType     elf.SymType
 	Dynid       int32
 	Plt         int32
 	Got         int32

src/cmd/internal/ld/symtab.go

@@ -106,10 +106,9 @@ func putelfsym(x *LSym, s string, t int, addr int64, size int64, ver int, go_ *L
 		type_ = STT_OBJECT
 
 	case 'U':
-		type_ = STT_NOTYPE
-		if x == Ctxt.Tlsg {
-			type_ = STT_TLS
-		}
+		// ElfType is only set for symbols read from Go shared libraries, but
+		// for other symbols it is left as STT_NOTYPE which is fine.
+		type_ = int(x.ElfType)
 
 	case 't':
 		type_ = STT_TLS