[MachO] Fix symbol merging during symtab parsing.

[JDevlieghere]

Summary

The symtab parser in ObjectFileMachO has logic to coalesce debug (STAB) and non-debug symbols, based on the address and the symbol name for static (STSYM) and global symbols (GSYM) respectively. It makes the assumption that the debug variant is always encountered first. Rather than creating a second entry in the symbol table for the non-debug symbol, the latter gets merged into the existing debug symbol.

This breaks when the linker emits the non-debug symbol first. We'd end up with two entries in the symbol table, each containing part of the information LLDB relies on. Indeed, commenting out the merging logic breaks the test suite spectacularly.

This patch solves that problem by always parsing the debug symbols first. This guarantees that the assumption for merging holds.

PS: I'm not particularly proud of the way this turned out, but after numerous attempts this is the best solution I could come up with. The symtab parsing logic is pretty complex in that it touches a lot of things. I've experienced first hand that it's very easy to break things. I believe this approach strikes a balance between fixing the issue while limiting the risk of regressions.

[labath]

I don't know much about this stuff, but it seems relatively reasonable to me. Tagging @kwk and @jankratochvil because they were involved in some elf symbol merging discussions recently, and so they may find this interesting.

lldb/lit/ObjectFile/Inputs/SymbolTable.yaml

1 ↗

(On Diff #223363)

Somewhat confusingly, the "object file" tests currently live in lit/Modules/$OBJ_FORMAT. In the current arrangement, I believe this should go there too, but if you want to rename the whole Modules folder into ObjectFile, I would be fine with that.

lldb/lit/ObjectFile/TestSymbolTable.test

1 ↗

(On Diff #223363)

Is that necessary? I believe our MachO capabilities should work fine everywhere. At least, it doesn't seem to be necessary for all (four) of our existing MachO obj file tests.

lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp

3695

Personally, I'd just move this check out of the lambda and into the code which invokes it.

[JDevlieghere]

Address code review feedback.

[JDevlieghere]

lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp

3695

Given that you need is_debug all throughout the parsing code I'm not convinced that'd be an improvement. Either you'd have to compute it twice or you'd have to pass it in which case you still have the extra argument.

[aprantl]

lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp

3736

because of the early exit above — isn't this either always true or always false?

4477

reserve symtab_load_command.nsyms elements?

[aprantl]

lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp

3736

Never mind, I thought the RHS above was the enum, but it's actually the parameter.

[JDevlieghere]

Code review feedback Adrian.

lldb/lit/ObjectFile/MachO/symtab.yaml

+# Tests that the symbol table properly merges the two entries for
+# global_constant, even when the debug symbol comes last.
+# RUN: mkdir -p %t
+# RUN: yaml2obj %s -o %t/a.out
+# RUN: %lldb %t/a.out -o "target modules dump symtab a.out" | FileCheck %s
+# CHECK: global_constant
+# CHECK-NOT: global_constant
+--- !mach-o
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+      n_type:          0x20
+      n_sect:          0
+      n_desc:          0
+      n_value:         0
+    - n_strx:          984
+      n_type:          0x20
+      n_sect:          0
+      n_desc:          0
+      n_value:         0
+    - n_strx:          996
+      n_type:          0x20
+      n_sect:          0
+      n_desc:          0
+      n_value:         0
+    - n_strx:          1023
+      n_type:          0x20
+      n_sect:          0
+      n_desc:          0
+      n_value:         0
+    - n_strx:          1
+      n_type:          0x64
+      n_sect:          1
+      n_desc:          0
+      n_value:         0
+    - n_strx:          2
+      n_type:          0x0F
+      n_sect:          1
+      n_desc:          16
+      n_value:         4294967296
+    - n_strx:          22
+      n_type:          0x0F
+      n_sect:          1
+      n_desc:          0
+      n_value:         4294970848
+    - n_strx:          28
+      n_type:          0x01
+      n_sect:          0
+      n_desc:          768
+      n_value:         0
+    - n_strx:          47
+      n_type:          0x01
+      n_sect:          0
+      n_desc:          768
+      n_value:         0
+    - n_strx:          64
+      n_type:          0x01
+      n_sect:          0
+      n_desc:          768
+      n_value:         0
+    - n_strx:          101
+      n_type:          0x01
+      n_sect:          0
+      n_desc:          512
+      n_value:         0
+  StringTable:
+    - ' '
+    - __mh_execute_header
+    - _main
+    - _swift_beginAccess
+    - _swift_endAccess
+    - _swift_getTypeByMangledNameInContext
+    - dyld_stub_binder
+    - '_$s1a3useyyxlF'
+    - '_$s1a1fyySi_SiztF'
+    - ___swift_instantiateConcreteTypeFromMangledName
+    - _symbolic Si_Sit
+    - ___swift_reflection_version
+    - __dyld_private
+    - '_$sSi_SitMD'
+    - '_$s1a15global_constantSivp'
+    - '_$s1a15global_variableSivp'
+    - '/Volumes/Jonas/internal-stable/build/Ninja-RelWithDebInfoAssert+stdlib-RelWithDebInfo/lldb-macosx-x86_64/lldb-test-build.noindex/lang/swift/variables/let/TestSwiftLetConstants.test_let_constants_dwarf/a.swiftmodule'
+    - '/Volumes/Jonas/internal-stable/lldb/packages/Python/lldbsuite/test/lang/swift/variables/let/'
+    - main.swift
+    - '/Volumes/Jonas/internal-stable/build/Ninja-RelWithDebInfoAssert+stdlib-RelWithDebInfo/lldb-macosx-x86_64/lldb-test-build.noindex/lang/swift/variables/let/TestSwiftLetConstants.test_let_constants_dwarf/main.o'
+    - _main
+    - '_$s1a3useyyxlF'
+    - '_$s1a1fyySi_SiztF'
+    - ___swift_instantiateConcreteTypeFromMangledName
+    - _symbolic Si_Sit
+    - ___swift_reflection_version
+    - '_$sSi_SitMD'
+    - '_$s1a15global_constantSivp'
+    - '_$s1a15global_variableSivp'
+    - ''
+    - ''
+    - ''
+    - ''
+    - ''
+    - ''
+...

lldb/source/Plugins/ObjectFile/Mach-O/ObjectFileMachO.cpp

UUID ObjectFileMachO::GetSharedCacheUUID(FileSpec dyld_shared_cache,
   if (log && dsc_uuid.IsValid()) {
     LLDB_LOGF(log, "Shared cache %s has UUID %s",
               dyld_shared_cache.GetPath().c_str(),
               dsc_uuid.GetAsString().c_str());
   }
   return dsc_uuid;
 }
 
-static bool ParseNList(DataExtractor &nlist_data,
-                       lldb::offset_t &nlist_data_offset,
-                       size_t nlist_byte_size, struct nlist_64 &nlist) {
+static llvm::Optional<struct nlist_64>
+ParseNList(DataExtractor &nlist_data, lldb::offset_t &nlist_data_offset,
+           size_t nlist_byte_size) {
+  struct nlist_64 nlist;
   if (!nlist_data.ValidOffsetForDataOfSize(nlist_data_offset, nlist_byte_size))
-    return false;
+    return {};
   nlist.n_strx = nlist_data.GetU32_unchecked(&nlist_data_offset);
   nlist.n_type = nlist_data.GetU8_unchecked(&nlist_data_offset);
   nlist.n_sect = nlist_data.GetU8_unchecked(&nlist_data_offset);
   nlist.n_desc = nlist_data.GetU16_unchecked(&nlist_data_offset);
   nlist.n_value = nlist_data.GetAddress_unchecked(&nlist_data_offset);
-  return true;
+  return nlist;
 }
 
+enum { DebugSymbols = true, NonDebugSymbols = false };
+
 size_t ObjectFileMachO::ParseSymtab() {
   static Timer::Category func_cat(LLVM_PRETTY_FUNCTION);
   Timer scoped_timer(func_cat, "ObjectFileMachO::ParseSymtab () module = %s",
                      m_file.GetFilename().AsCString(""));
   ModuleSP module_sp(GetModule());
   if (!module_sp)
     return 0;
 
if (nlist_data.GetByteSize() > 0) {
     } else {
       nlist_idx = 0;
     }
 
     typedef std::map<ConstString, uint16_t> UndefinedNameToDescMap;
     typedef std::map<uint32_t, ConstString> SymbolIndexToName;
     UndefinedNameToDescMap undefined_name_to_desc;
     SymbolIndexToName reexport_shlib_needs_fixup;
-    for (; nlist_idx < symtab_load_command.nsyms; ++nlist_idx) {
-      struct nlist_64 nlist;
-      if (!ParseNList(nlist_data, nlist_data_offset, nlist_byte_size, nlist))
-        break;
+
+    // Symtab parsing is a huge mess. Everything is entangled and the code
+    // requires access to a ridiculous amount of variables. LLDB depends
+    // heavily on the proper merging of symbols and to get that right we need
+    // to make sure we have parsed all the debug symbols first. Therefore we
+    // invoke the lambda twice, once to parse only the debug symbols and then
+    // once more to parse the remaining symbols.
+    auto ParseSymbolLambda = [&](struct nlist_64 &nlist, uint32_t nlist_idx,
+                                 bool debug_only) {
+      const bool is_debug = ((nlist.n_type & N_STAB) != 0);
+      if (is_debug != debug_only)
+        return true;
 
       const char *symbol_name_non_abi_mangled = nullptr;
       const char *symbol_name = nullptr;
 
       if (have_strtab_data) {
         symbol_name = strtab_data.PeekCStr(nlist.n_strx);
 
         if (symbol_name == nullptr) {
           // No symbol should be NULL, even the symbols with no string values
           // should have an offset zero which points to an empty C-string
           Host::SystemLog(Host::eSystemLogError,
                           "error: symbol[%u] has invalid string table offset "
                           "0x%x in %s, ignoring symbol\n",
                           nlist_idx, nlist.n_strx,
                           module_sp->GetFileSpec().GetPath().c_str());
-          continue;
+          return true;
         }
         if (symbol_name[0] == '\0')
           symbol_name = nullptr;
       } else {
         const addr_t str_addr = strtab_addr + nlist.n_strx;
         Status str_error;
         if (process->ReadCStringFromMemory(str_addr, memory_symbol_name,
                                            str_error))
           symbol_name = memory_symbol_name.c_str();
       }
 
       SymbolType type = eSymbolTypeInvalid;
       SectionSP symbol_section;
       lldb::addr_t symbol_byte_size = 0;
       bool add_nlist = true;
       bool is_gsym = false;
       bool demangled_is_synthesized = false;
       bool set_value = true;
 
-      const bool is_debug = ((nlist.n_type & N_STAB) != 0);
       assert(sym_idx < num_syms);
       sym[sym_idx].SetDebug(is_debug);
 
       if (is_debug) {
         switch (nlist.n_type) {
         case N_GSYM:
           // global symbol: name,,NO_SECT,type,0
           // Sometimes the N_GSYM value contains the address.
 
           // FIXME: In the .o files, we have a GSYM and a debug symbol for all
           // the ObjC data.  They
           // have the same address, but we want to ensure that we always find
auto ParseSymbolLambda = [&](struct nlist_64 &nlist, uint32_t nlist_idx,
             }
           }
         } break;
         }
       }
 
       if (!add_nlist) {
         sym[sym_idx].Clear();
-        continue;
+        return true;
       }
 
       uint64_t symbol_value = nlist.n_value;
 
       if (symbol_name_non_abi_mangled) {
         sym[sym_idx].GetMangled().SetMangledName(
             ConstString(symbol_name_non_abi_mangled));
         sym[sym_idx].GetMangled().SetDemangledName(ConstString(symbol_name));
auto ParseSymbolLambda = [&](struct nlist_64 &nlist, uint32_t nlist_idx,
           // find a match, and the name matches, then we can merge the two into
           // just the function symbol to avoid duplicate entries in the symbol
           // table.
           std::pair<ValueToSymbolIndexMap::const_iterator,
                     ValueToSymbolIndexMap::const_iterator>
               range;
           range = N_FUN_addr_to_sym_idx.equal_range(nlist.n_value);
           if (range.first != range.second) {
-            bool found_it = false;
             for (ValueToSymbolIndexMap::const_iterator pos = range.first;
                  pos != range.second; ++pos) {
               if (sym[sym_idx].GetMangled().GetName(lldb::eLanguageTypeUnknown,
                                                     Mangled::ePreferMangled) ==
                   sym[pos->second].GetMangled().GetName(
                       lldb::eLanguageTypeUnknown, Mangled::ePreferMangled)) {
                 m_nlist_idx_to_sym_idx[nlist_idx] = pos->second;
                 // We just need the flags from the linker symbol, so put these
                 // flags into the N_FUN flags to avoid duplicate symbols in the
                 // symbol table.
                 sym[pos->second].SetExternal(sym[sym_idx].IsExternal());
                 sym[pos->second].SetFlags(nlist.n_type << 16 | nlist.n_desc);
                 if (resolver_addresses.find(nlist.n_value) !=
                     resolver_addresses.end())
                   sym[pos->second].SetType(eSymbolTypeResolver);
                 sym[sym_idx].Clear();
-                found_it = true;
-                break;
+                return true;
               }
             }
-            if (found_it)
-              continue;
           } else {
             if (resolver_addresses.find(nlist.n_value) !=
                 resolver_addresses.end())
               type = eSymbolTypeResolver;
           }
         } else if (type == eSymbolTypeData || type == eSymbolTypeObjCClass ||
                    type == eSymbolTypeObjCMetaClass ||
                    type == eSymbolTypeObjCIVar) {
           // See if we can find a N_STSYM entry for any data symbols. If we do
           // find a match, and the name matches, then we can merge the two into
           // just the Static symbol to avoid duplicate entries in the symbol
           // table.
           std::pair<ValueToSymbolIndexMap::const_iterator,
                     ValueToSymbolIndexMap::const_iterator>
               range;
           range = N_STSYM_addr_to_sym_idx.equal_range(nlist.n_value);
           if (range.first != range.second) {
-            bool found_it = false;
             for (ValueToSymbolIndexMap::const_iterator pos = range.first;
                  pos != range.second; ++pos) {
               if (sym[sym_idx].GetMangled().GetName(lldb::eLanguageTypeUnknown,
                                                     Mangled::ePreferMangled) ==
                   sym[pos->second].GetMangled().GetName(
                       lldb::eLanguageTypeUnknown, Mangled::ePreferMangled)) {
                 m_nlist_idx_to_sym_idx[nlist_idx] = pos->second;
                 // We just need the flags from the linker symbol, so put these
                 // flags into the N_STSYM flags to avoid duplicate symbols in
                 // the symbol table.
                 sym[pos->second].SetExternal(sym[sym_idx].IsExternal());
                 sym[pos->second].SetFlags(nlist.n_type << 16 | nlist.n_desc);
                 sym[sym_idx].Clear();
-                found_it = true;
-                break;
+                return true;
               }
             }
-            if (found_it)
-              continue;
           } else {
             // Combine N_GSYM stab entries with the non stab symbol.
             const char *gsym_name = sym[sym_idx]
                                         .GetMangled()
                                         .GetName(lldb::eLanguageTypeUnknown,
                                                  Mangled::ePreferMangled)
                                         .GetCString();
             if (gsym_name) {
               ConstNameToSymbolIndexMap::const_iterator pos =
                   N_GSYM_name_to_sym_idx.find(gsym_name);
               if (pos != N_GSYM_name_to_sym_idx.end()) {
                 const uint32_t GSYM_sym_idx = pos->second;
                 m_nlist_idx_to_sym_idx[nlist_idx] = GSYM_sym_idx;
                 // Copy the address, because often the N_GSYM address has an
                 // invalid address of zero when the global is a common symbol.
                 sym[GSYM_sym_idx].GetAddressRef().SetSection(symbol_section);
                 sym[GSYM_sym_idx].GetAddressRef().SetOffset(symbol_value);
                 // We just need the flags from the linker symbol, so put these
                 // flags into the N_GSYM flags to avoid duplicate symbols in
                 // the symbol table.
                 sym[GSYM_sym_idx].SetFlags(nlist.n_type << 16 | nlist.n_desc);
                 sym[sym_idx].Clear();
-                continue;
+                return true;
               }
             }
           }
         }
       }
 
       sym[sym_idx].SetID(nlist_idx);
       sym[sym_idx].SetType(type);
       if (set_value) {
         sym[sym_idx].GetAddressRef().SetSection(symbol_section);
         sym[sym_idx].GetAddressRef().SetOffset(symbol_value);
       }
       sym[sym_idx].SetFlags(nlist.n_type << 16 | nlist.n_desc);
       if (nlist.n_desc & N_WEAK_REF)
         sym[sym_idx].SetIsWeak(true);
 
       if (symbol_byte_size > 0)
         sym[sym_idx].SetByteSize(symbol_byte_size);
 
       if (demangled_is_synthesized)
         sym[sym_idx].SetDemangledNameIsSynthesized(true);
 
       ++sym_idx;
+      return true;
+    };
+
+    // First parse all the nlists but don't process them yet. See the next
+    // comment for an explanation why.
+    std::vector<struct nlist_64> nlists;
+    nlists.reserve(symtab_load_command.nsyms);
+    for (; nlist_idx < symtab_load_command.nsyms; ++nlist_idx) {
+      if (auto nlist =
+              ParseNList(nlist_data, nlist_data_offset, nlist_byte_size))
+        nlists.push_back(*nlist);
+      else
+        break;
+    }
+
+    // Now parse all the debug symbols. This is needed to merge non-debug
+    // symbols in the next step. Non-debug symbols are always coalesced into
+    // the debug symbol. Doing this in one step would mean that some symbols
+    // won't be merged.
+    nlist_idx = 0;
+    for (auto &nlist : nlists) {
+      if (!ParseSymbolLambda(nlist, nlist_idx++, DebugSymbols))
+        break;
+    }
+
+    // Finally parse all the non debug symbols.
+    nlist_idx = 0;
+    for (auto &nlist : nlists) {
+      if (!ParseSymbolLambda(nlist, nlist_idx++, NonDebugSymbols))
+        break;
     }
 
     for (const auto &pos : reexport_shlib_needs_fixup) {
       const auto undef_pos = undefined_name_to_desc.find(pos.second);
       if (undef_pos != undefined_name_to_desc.end()) {
         const uint8_t dylib_ordinal =
             llvm::MachO::GET_LIBRARY_ORDINAL(undef_pos->second);
         if (dylib_ordinal > 0 && dylib_ordinal < dylib_files.GetSize())