cmd/link: mark rel.ro segment as PT_GNU_RELRO

Details: http://www.airs.com/blog/archives/189

Part of adding PIE internal linking on linux/amd64.

Change-Id: I8843a97f22f6f120346cccd694c7fff32f09f60b
Reviewed-on: https://go-review.googlesource.com/28541Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -236,6 +236,7 @@ const (
 	PT_LOPROC            = 0x70000000
 	PT_HIPROC            = 0x7fffffff
 	PT_GNU_STACK         = 0x6474e551
+	PT_GNU_RELRO         = 0x6474e552
 	PT_PAX_FLAGS         = 0x65041580
 	PF_X                 = 0x1
 	PF_W                 = 0x2
@@ -1599,6 +1600,17 @@ func elfphload(ctxt *Link, seg *Segment) *ElfPhdr {
 	return ph
 }
 
+func elfphrelro(ctxt *Link, seg *Segment) {
+	ph := newElfPhdr(ctxt)
+	ph.type_ = PT_GNU_RELRO
+	ph.vaddr = seg.Vaddr
+	ph.paddr = seg.Vaddr
+	ph.memsz = seg.Length
+	ph.off = seg.Fileoff
+	ph.filesz = seg.Filelen
+	ph.align = uint64(*FlagRound)
+}
+
 func elfshname(ctxt *Link, name string) *ElfShdr {
 	var off int
 	var sh *ElfShdr
@@ -2291,6 +2303,7 @@ func Asmbelf(ctxt *Link, symo int64) {
 	}
 	if Segrelrodata.Sect != nil {
 		elfphload(ctxt, &Segrelrodata)
+		elfphrelro(ctxt, &Segrelrodata)
 	}
 	elfphload(ctxt, &Segdata)