cmd/link: don't overalign ELF reloc sections

Internal linking on an ELF system creates two reloc sections, which
must be adjacent. The default is to base section alignment on the
section size, but doing that for ELF reloc sections can introduce a
gap. Set the reloc section alignment explicitly to avoid that.

Fixes #18044.

Change-Id: I8ccc131e60937d30c5f715a34c7803258833fc2f
Reviewed-on: https://go-review.googlesource.com/33872
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/go_test.go

@@ -3718,3 +3718,12 @@ func TestLinkXImportPathEscape(t *testing.T) {
 		tg.t.Fatal(`incorrect output: expected "linkXworked\n"`)
 	}
 }
+
+// Issue 18044.
+func TestLdBindNow(t *testing.T) {
+	tg := testgo(t)
+	defer tg.cleanup()
+	tg.parallel()
+	tg.setenv("LD_BIND_NOW", "1")
+	tg.run("help")
+}

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

@@ -1976,6 +1976,13 @@ func dodataSect(ctxt *Link, symn obj.SymKind, syms []*Symbol) (result []*Symbol,
 			copy(syms[first+2:], syms[first+1:second])
 			syms[first+0] = rel
 			syms[first+1] = plt
+
+			// Make sure alignment doesn't introduce a gap.
+			// Setting the alignment explicitly prevents
+			// symalign from basing it on the size and
+			// getting it wrong.
+			rel.Align = int32(SysArch.RegSize)
+			plt.Align = int32(SysArch.RegSize)
 		}
 	}