[JITLink][MachO] Fix handling of non-extern UNSIGNED pair of SUBTRACTOR relocs.

When processing a MachO SUBTRACTOR/UNSIGNED pair, if the UNSIGNED target
is non-extern then check the r_symbolnum field of the relocation to find
the targeted section and use the section's address to find 'ToSymbol'.

Previously 'ToSymbol' was found by loading the initial value stored at
the fixup location and treating this as an address to search for. This
is incorrect, however: the initial value includes the addend and will
point to the wrong block if the addend is less than zero or greater than
the block size.

rdar://65756694

Author: lhames

llvm/lib/ExecutionEngine/JITLink/MachO_arm64.cpp

@@ -148,10 +148,11 @@ class MachOLinkGraphBuilder_arm64 : public MachOLinkGraphBuilder {
       else
         return ToSymbolOrErr.takeError();
     } else {
-      if (auto ToSymbolOrErr = findSymbolByAddress(FixupValue))
-        ToSymbol = &*ToSymbolOrErr;
-      else
-        return ToSymbolOrErr.takeError();
+      auto ToSymbolSec = findSectionByIndex(UnsignedRI.r_symbolnum - 1);
+      if (!ToSymbolSec)
+        return ToSymbolSec.takeError();
+      ToSymbol = getSymbolByAddress(ToSymbolSec->Address);
+      assert(ToSymbol && "No symbol for section");
       FixupValue -= ToSymbol->getAddress();
     }
 

llvm/lib/ExecutionEngine/JITLink/MachO_x86_64.cpp

@@ -150,10 +150,11 @@ class MachOLinkGraphBuilder_x86_64 : public MachOLinkGraphBuilder {
       else
         return ToSymbolOrErr.takeError();
     } else {
-      if (auto ToSymbolOrErr = findSymbolByAddress(FixupValue))
-        ToSymbol = &*ToSymbolOrErr;
-      else
-        return ToSymbolOrErr.takeError();
+      auto ToSymbolSec = findSectionByIndex(UnsignedRI.r_symbolnum - 1);
+      if (!ToSymbolSec)
+        return ToSymbolSec.takeError();
+      ToSymbol = getSymbolByAddress(ToSymbolSec->Address);
+      assert(ToSymbol && "No symbol for section");
       FixupValue -= ToSymbol->getAddress();
     }
 

llvm/test/ExecutionEngine/JITLink/X86/MachO_x86-64_relocations.s

@@ -193,19 +193,19 @@ anon_func_addr_quad:
 
 # X86_64_RELOC_SUBTRACTOR Quad/Long in named storage with anonymous minuend
 #
-# jitlink-check: *{8}anon_minuend_quad1 = section_addr(macho_reloc.o, __data) - anon_minuend_quad1 + 2
+# jitlink-check: *{8}anon_minuend_quad1 = section_addr(macho_reloc.o, __data) - anon_minuend_quad1 - 2
 # Only the form "B: .quad LA - B + C" is tested. The form "B: .quad B - LA + C" is
 # invalid because the subtrahend can not be local.
         .globl  anon_minuend_quad1
         .p2align  3
 anon_minuend_quad1:
-        .quad Lanon_data - anon_minuend_quad1 + 2
+        .quad Lanon_data - anon_minuend_quad1 - 2
 
-# jitlink-check: *{4}anon_minuend_long1 = (section_addr(macho_reloc.o, __data) - anon_minuend_long1 + 2)[31:0]
+# jitlink-check: *{4}anon_minuend_long1 = (section_addr(macho_reloc.o, __data) - anon_minuend_long1 - 2)[31:0]
         .globl  anon_minuend_long1
         .p2align  2
 anon_minuend_long1:
-        .long Lanon_data - anon_minuend_long1 + 2
+        .long Lanon_data - anon_minuend_long1 - 2
 
 # Check X86_64_RELOC_SUBTRACTOR Quad/Long in named storage with minuend and subtrahend.
 # Both forms "A: .quad A - B + C" and "A: .quad B - A + C" are tested.