[ObjectYAML][MachO] Encode export trie address as ULEB128, not as SLEB128

[drodriguez]

Summary

The dumpExportEntry was dumping everything using signed LEB128, but
the format seems to use unsigned LEB128. This can be cross-checked with
the implementation in MachOObjectFile.cpp, the implementation in LLD's
ExportTrie.cpp, and the implementation in macho2yaml.cpp, which all use
ULEB128 functions..

The difference is only apparent when encoding some values with specific
bit patterns (bit active in the 7th, 14th, ... bits of the binary). The
encoding was not always creating problems in the resulting binaries
because if the extra byte was part of the padding, the result of
decoding it as ULEB128 is the same as decoding as SLEB128, however, the
code of MachOObjectFile.cpp (used by llvm-objdump) checks the buffer
decoding position against the reported length, which triggered an error.

Modified a test that used an address with this pattern (0x3FA0, the 14th
bit is active), to show that a round trip still produces the same
results, and added a check using llvm-objdump to use their extra checks
to verify this implementation.

[pete]

Thanks for doing this! From a quick look at ld64, there are no sleb's anywhere in the Export Trie logic so LGTM.

llvm/lib/ObjectYAML/MachOEmitter.cpp

if (!Opcode.Symbol.empty()) {
       OS.write(Opcode.Symbol.data(), Opcode.Symbol.size());
       OS.write('\0');
     }
   }
 }
 
 void MachOWriter::dumpExportEntry(raw_ostream &OS,
                                   MachOYAML::ExportEntry &Entry) {
-  encodeSLEB128(Entry.TerminalSize, OS);
+  encodeULEB128(Entry.TerminalSize, OS);
   if (Entry.TerminalSize > 0) {
-    encodeSLEB128(Entry.Flags, OS);
+    encodeULEB128(Entry.Flags, OS);
     if (Entry.Flags & MachO::EXPORT_SYMBOL_FLAGS_REEXPORT) {
-      encodeSLEB128(Entry.Other, OS);
+      encodeULEB128(Entry.Other, OS);
       OS << Entry.ImportName;
       OS.write('\0');
     } else {
-      encodeSLEB128(Entry.Address, OS);
+      encodeULEB128(Entry.Address, OS);
       if (Entry.Flags & MachO::EXPORT_SYMBOL_FLAGS_STUB_AND_RESOLVER)
-        encodeSLEB128(Entry.Other, OS);
+        encodeULEB128(Entry.Other, OS);
     }
   }
   OS.write(static_cast<uint8_t>(Entry.Children.size()));
   for (auto EE : Entry.Children) {
     OS << EE.Name;
     OS.write('\0');
-    encodeSLEB128(EE.NodeOffset, OS);
+    encodeULEB128(EE.NodeOffset, OS);
   }
   for (auto EE : Entry.Children)
     dumpExportEntry(OS, EE);
 }
 
 void MachOWriter::writeExportTrie(raw_ostream &OS) {
   dumpExportEntry(OS, Obj.LinkEdit.ExportTrie);
 }

llvm/test/ObjectYAML/MachO/export_trie.yaml

-# RUN: yaml2obj %s | obj2yaml | FileCheck %s
+# RUN: yaml2obj %s -o=%t
+# RUN: obj2yaml %t | FileCheck %s
+# RUN: llvm-objdump --macho --exports-trie %t | FileCheck %s --check-prefix=OBJDUMP-VERIFY
 
 --- !mach-o
 FileHeader:      
   magic:           0xFEEDFACF
   cputype:         0x01000007
   cpusubtype:      0x80000003
   filetype:        0x00000002
   ncmds:           16
Children:
             Flags:           0x0000000000000000
             Address:         0x0000000000000000
             Other:           0x0000000000000000
             ImportName:      ''
           - TerminalSize:    3
             NodeOffset:      37
             Name:            main
             Flags:           0x0000000000000000
-            Address:         0x0000000000001160
+            Address:         0x0000000000003FA0
             Other:           0x0000000000000000
             ImportName:      ''
 ...
 
 #CHECK: ExportTrie:      
 #CHECK:     TerminalSize:    0
 #CHECK:     NodeOffset:      0
 #CHECK:     Name:            ''
 #CHECK:     Children:        
 #CHECK:       - TerminalSize:    0
 #CHECK:         NodeOffset:      5
 #CHECK:         Name:            _
 #CHECK:         Children:        
 #CHECK:           - TerminalSize:    2
 #CHECK:             NodeOffset:      33
 #CHECK:             Name:            _mh_execute_header
 #CHECK:             Address:         0x0
 #CHECK:           - TerminalSize:    3
 #CHECK:             NodeOffset:      37
 #CHECK:             Name:            main
-#CHECK:             Address:         0x1160
+#CHECK:             Address:         0x3FA0
+
+# OBJDUMP-VERIFY: Exports trie:
+# OBJDUMP-VERIFY: 0x100000000  __mh_execute_header
+# OBJDUMP-VERIFY: 0x100003FA0  _main