cmd/link: handle Mach-O X86_64_RELOC_SUBTRACTOR in internal linking

With recent LLVM toolchain, on macOS/AMD64, the race detector syso
file built from it contains X86_64_RELOC_SUBTRACTOR relocations,
which the Go linker currently doesn't handle in internal linking
mode. To ensure internal linking mode continue to work with the
race detector syso, this CL adds support of X86_64_RELOC_SUBTRACTOR
relocations.

X86_64_RELOC_SUBTRACTOR is actually a pair of relocations that
resolves to the difference between two symbol addresses (each
relocation specifies a symbol). For the cases we care (the race
syso), the symbol being subtracted out is always in the current
section, so we can just convert it to a PC-relative relocation,
with the addend adjusted. If later we need the more general form,
we can introduce a new mechanism (say, objabi.R_DIFF) that works
as a pair of relocations like the Mach-O one.

As we expect the pair of relocations be consecutive, don't reorder
(sort) relocation records when loading Mach-O objects.

Change-Id: I757456b07270fb4b2a41fd0fef67a2b39dd6b238
Reviewed-on: https://go-review.googlesource.com/c/go/+/660715
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/cmd/link/internal/amd64/asm.go

@@ -185,6 +185,38 @@ func adddynrel(target *ld.Target, ldr *loader.Loader, syms *ld.ArchSyms, s loade
 		}
 		return true
 
+	case objabi.MachoRelocOffset + ld.MACHO_X86_64_RELOC_SUBTRACTOR*2 + 0:
+		// X86_64_RELOC_SUBTRACTOR must be followed by X86_64_RELOC_UNSIGNED.
+		// The pair of relocations resolves to the difference between two
+		// symbol addresses (each relocation specifies a symbol).
+		// See Darwin's header file include/mach-o/x86_64/reloc.h.
+		// ".quad _foo - _bar" is expressed as
+		// r_type=X86_64_RELOC_SUBTRACTOR, r_length=3, r_extern=1, r_pcrel=0, r_symbolnum=_bar
+		// r_type=X86_64_RELOC_UNSIGNED, r_length=3, r_extern=1, r_pcrel=0, r_symbolnum=_foo
+		//
+		// For the cases we care (the race syso), the symbol being subtracted
+		// out is always in the current section, so we can just convert it to
+		// a PC-relative relocation, with the addend adjusted.
+		// If later we need the more general form, we can introduce objabi.R_DIFF
+		// that works like this Mach-O relocation.
+		su := ldr.MakeSymbolUpdater(s)
+		outer, off := ld.FoldSubSymbolOffset(ldr, targ)
+		if outer != s {
+			ldr.Errorf(s, "unsupported X86_64_RELOC_SUBTRACTOR reloc: target %s, outer %s",
+				ldr.SymName(targ), ldr.SymName(outer))
+			break
+		}
+		relocs := su.Relocs()
+		if rIdx+1 >= relocs.Count() || relocs.At(rIdx+1).Type() != objabi.MachoRelocOffset+ld.MACHO_X86_64_RELOC_UNSIGNED*2+0 || relocs.At(rIdx+1).Off() != r.Off() {
+			ldr.Errorf(s, "unexpected X86_64_RELOC_SUBTRACTOR reloc, must be followed by X86_64_RELOC_UNSIGNED at the same offset: %d %v %d", rIdx, relocs.At(rIdx+1).Type(), relocs.At(rIdx+1).Off())
+		}
+		// The second relocation has the target symbol we want
+		su.SetRelocType(rIdx+1, objabi.R_PCREL)
+		su.SetRelocAdd(rIdx+1, r.Add()+int64(r.Off())-off)
+		// Remove the other relocation
+		su.SetRelocSiz(rIdx, 0)
+		return true
+
 	case objabi.MachoRelocOffset + ld.MACHO_X86_64_RELOC_BRANCH*2 + 1:
 		if targType == sym.SDYNIMPORT {
 			addpltsym(target, ldr, syms, targ)

src/cmd/link/internal/loader/symbolbuilder.go

@@ -139,6 +139,11 @@ func (sb *SymbolBuilder) SetRelocAdd(i int, a int64) {
 	sb.relocs[i].SetAdd(a)
 }
 
+// SetRelocAdd sets the size of the 'i'-th relocation on this sym to 'sz'
+func (sb *SymbolBuilder) SetRelocSiz(i int, sz uint8) {
+	sb.relocs[i].SetSiz(sz)
+}
+
 // Add n relocations, return a handle to the relocations.
 func (sb *SymbolBuilder) AddRelocs(n int) Relocs {
 	sb.relocs = append(sb.relocs, make([]goobj.Reloc, n)...)

src/cmd/link/internal/loadmacho/ldmacho.go

@@ -790,8 +790,6 @@ func Load(l *loader.Loader, arch *sys.Arch, localSymVersion int, f *bio.Reader,
 
 			rAdd = 0 // clear rAdd for next iteration
 		}
-
-		sb.SortRelocs()
 	}
 
 	return textp, nil