[llvm-objcopy][MachO] Implement --strip-all

[jhenderson]

llvm/test/tools/llvm-objcopy/MachO/strip-all.test

2

You need a comment at the start of this test explaining what the test does.

13–14

Also llvm-strip without "--strip-all" should be the same as "llvm-objcopy--strip-all". Test that too.

44

No new line at EOF.

[rupprecht]

llvm/tools/llvm-objcopy/MachO/MachOObjcopy.cpp

49

Can you skip the const_cast here and add a mutable getSymbolByIndex overload so you can remove the const modifiers elsewhere?

53–57

For ELF files, --strip-all seems to strip all symbols regardless of whether they're referenced by relocations, is that not true for MachO files?

102

Marking symbols should be behind whatever flags require it, so we don't needlessly traverse symbols

[seiya]

Addressed review comments.

llvm/tools/llvm-objcopy/MachO/MachOObjcopy.cpp

53–57

We can't remove all symbols. Symbols referenced in the indirect symbol table are necessary for dynamic linking (i.e., there're no separate sections just like .dynsym and .dynstr).

[jhenderson]

Test case looks fine to me now. Not looked at the code though.

[alexander-shaposhnikov]

LG

[alexander-shaposhnikov]

@seyia, sorry about the delays with code review, does this particular diff have any unreviewed dependencies ?

[seiya]

In D66281#1656482, @alexshap wrote:

@seyia, sorry about the delays with code review, does this particular diff have any unreviewed dependencies ?

Thank you for the review! Currently, this patch depends on D66280 and D65541. I'll re-upload the latter one after refactoring CopyConfig to allow Mach-O specific command-line parsing.

[seiya]

Rebased. No changes intended.

llvm/docs/CommandGuide/llvm-objcopy.rst

  Remove all symbols, debug sections and relocations from the output. This option
  is equivalent to GNU :program:`objcopy`'s ``--strip-all`` switch.
 
 .. option:: --strip-all, -S
 
  For ELF objects, remove from the output all symbols and non-alloc sections not
  within segments, except for .gnu.warning sections and the section name table.
 
- For COFF objects, remove all symbols, debug sections, and relocations from the
- output.
+ For COFF and Mach-O objects, remove all symbols, debug sections, and
+ relocations from the output.
 
 .. option:: --strip-debug, -g
 
  Remove all debug sections from the output.
 
 .. option:: --strip-symbol <symbol>, -N
 
  Remove all symbols named ``<symbol>`` from the output. Can be specified

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+      n_sect:          3
+      n_desc:          1
+      n_value:         1564714240
+    - n_strx:          1
+      n_type:          0x2E
+      n_sect:          1
+      n_desc:          0
+      n_value:         4294971280
+    - n_strx:          164
+      n_type:          0x24
+      n_sect:          1
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+      n_value:         4294971280
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+    - n_strx:          1
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+      n_sect:          1
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+      n_value:         36
+    - n_strx:          170
+      n_type:          0x26
+      n_sect:          3
+      n_desc:          0
+      n_value:         4294971392
+    - n_strx:          175
+      n_type:          0x20
+      n_sect:          0
+      n_desc:          0
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+    - n_strx:          1
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+      n_sect:          1
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+      n_value:         0
+    - n_strx:          180
+      n_type:          0x0E
+      n_sect:          3
+      n_desc:          0
+      n_value:         4294971392
+    - n_strx:          2
+      n_type:          0x0F
+      n_sect:          1
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+      n_value:         4294967296
+    - n_strx:          22
+      n_type:          0x0F
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+      n_type:          0x0F
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+      n_value:         4294971280
+    - n_strx:          33
+      n_type:          0x01
+      n_sect:          0
+      n_desc:          256
+      n_value:         0
+  StringTable:
+    - ' '
+    - __mh_execute_header
+    - _foo
+    - _main
+    - dyld_stub_binder
+    - '/Users/anonymous/llvm-project/llvm/build/'
+    - strip.c
+    - '/var/folders/g7/l0p5czpd2jz6w0gswg6cwgjc0000gn/T/strip-95c4bf.o'
+    - _main
+    - _bar
+    - _foo
+    - _bar
+    - ''
+    - ''
+    - ''
+    - ''
+    - ''
+    - ''
+    - ''
+...

llvm/test/tools/llvm-objcopy/MachO/strip-all.test

+## Show that llvm-objcopy/llvm-strip removes all symbols and debug sections.
+
+# RUN: yaml2obj %p/Inputs/strip-all.yaml > %t.exec
+# RUN: yaml2obj %p/Inputs/strip-all-with-dwarf.yaml > %t.dwarf
+
+# RUN: llvm-objcopy --strip-all %t.exec %t.exec.stripped
+# RUN: llvm-readobj --sections --relocations --symbols %t.exec.stripped \
+# RUN:   | FileCheck --check-prefixes=COMMON,EXEC %s
+
+# RUN: llvm-objcopy --strip-all %t.dwarf %t.dwarf.stripped
+# RUN: llvm-readobj --sections --relocations --symbols %t.dwarf.stripped \
+# RUN:   | FileCheck --check-prefixes=COMMON,DWARF %s
+
+## The output of "llvm-strip" should be identical with that of
+## "llvm-strip --strip-all" and "llvm-objcopy --strip-all".
+# RUN: llvm-strip %t.exec -o %t2
+# RUN: llvm-strip --strip-all %t.exec -o %t3
+# cmp %t2 %t.exec.stripped
+# cmp %t3 %t.exec.stripped
+
+# RUN: llvm-strip %t.dwarf -o %t4
+# RUN: llvm-strip --strip-all %t.dwarf -o %t5
+# cmp %t4 %t.dwarf.stripped
+# cmp %t5 %t.dwarf.stripped
+
+## Make sure that debug sections are removed.
+# DWARF:       Sections [
+# DWARF-NOT:     Name: __debug_str
+# DWARF-NOT:     Name: __debug_abbrev
+# DWARF-NOT:     Name: __debug_info
+# DWARF-NOT:     Name: __debug_macinfo
+# DWARF-NOT:     Name: __apple_names
+# DWARF-NOT:     Name: __apple_objc
+# DWARF-NOT:     Name: __apple_namespac
+# DWARF-NOT:     Name: __apple_types
+# DWARF-NOT:     Name: __debug_line
+# DWARF:       ]
+
+## Make sure that all relocations and symbols are removed.
+# COMMON:      Relocations [
+# COMMON-NEXT: ]
+# COMMON:      Symbols [
+# COMMON-NEXT: ]

llvm/tools/llvm-objcopy/MachO/MachOObjcopy.cpp

 namespace macho {
 
 using namespace object;
 using SectionPred = std::function<bool(const Section &Sec)>;
 
 static void removeSections(const CopyConfig &Config, Object &Obj) {
   SectionPred RemovePred = [](const Section &) { return false; };
 
+  if (Config.StripAll) {
+    // Remove all debug sections.
+    RemovePred = [RemovePred](const Section &Sec) {
+      if (Sec.Segname == "__DWARF")
+        return true;
+
+      return RemovePred(Sec);
+    };
+  }
+
   if (!Config.OnlySection.empty()) {
     RemovePred = [&Config, RemovePred](const Section &Sec) {
       return !is_contained(Config.OnlySection, Sec.CannonicalName);
     };
   }
 
   return Obj.removeSections(RemovePred);
 }
 
+static void markSymbols(const CopyConfig &Config, Object &Obj) {
+  // Symbols referenced from the indirect symbol table must not be removed.
+  for (IndirectSymbolEntry &ISE : Obj.IndirectSymTable.Symbols)
+    if (ISE.Symbol)
+      (*ISE.Symbol)->Referenced = true;
+}
+
+static void removeSymbols(const CopyConfig &Config, Object &Obj) {
+  auto RemovePred = [Config](const std::unique_ptr<SymbolEntry> &N) {
+    if (N->Referenced)
+      return false;
+    return Config.StripAll;
+  };
+
+  Obj.SymTable.removeSymbols(RemovePred);
+}
+
 static Error validateOptions(const CopyConfig &Config) {
   // TODO: Support section renaming in GNU objcopy for compatibility (see
   // http://lists.llvm.org/pipermail/llvm-dev/2019-May/132570.html).
 
   if (!Config.OnlySection.empty()) {
     for (const NameOrRegex &NR : Config.OnlySection)
       if (Error E = NR.isMachOCannonicalName())
         return E;
if (Config.AllowBrokenLinks || !Config.BuildIdLinkDir.empty() ||
       !Config.DumpSection.empty() || !Config.KeepSection.empty() ||
       !Config.SymbolsToGlobalize.empty() || !Config.SymbolsToKeep.empty() ||
       !Config.SymbolsToLocalize.empty() || !Config.SymbolsToWeaken.empty() ||
       !Config.SymbolsToKeepGlobal.empty() || !Config.SectionsToRename.empty() ||
       !Config.SymbolsToRename.empty() ||
       !Config.UnneededSymbolsToRemove.empty() ||
       !Config.SetSectionFlags.empty() || !Config.ToRemove.empty() ||
       Config.ExtractDWO || Config.KeepFileSymbols || Config.LocalizeHidden ||
-      Config.PreserveDates || Config.StripDWO || Config.StripNonAlloc ||
-      Config.StripSections || Config.Weaken || Config.DecompressDebugSections ||
-      Config.StripDebug || Config.StripNonAlloc || Config.StripSections ||
-      Config.StripUnneeded || Config.DiscardMode != DiscardType::None ||
-      !Config.SymbolsToAdd.empty() || Config.EntryExpr) {
+      Config.PreserveDates || Config.StripAllGNU || Config.StripDWO ||
+      Config.StripNonAlloc || Config.StripSections || Config.Weaken ||
+      Config.DecompressDebugSections || Config.StripDebug ||
+      Config.StripNonAlloc || Config.StripSections || Config.StripUnneeded ||
+      Config.DiscardMode != DiscardType::None || !Config.SymbolsToAdd.empty() ||
+      Config.EntryExpr) {
     return createStringError(llvm::errc::invalid_argument,
                              "option not supported by llvm-objcopy for MachO");
   }
 
   if (auto E = validateOptions(Config))
     return E;
 
   removeSections(Config, Obj);
+
+  // Mark symbols to determine which symbols are still needed.
+  if (Config.StripAll)
+    markSymbols(Config, Obj);
+
+  removeSymbols(Config, Obj);
+
+  if (Config.StripAll)
+    for (LoadCommand &LC : Obj.LoadCommands)
+      for (Section &Sec : LC.Sections)
+        Sec.Relocations.clear();
+
   return Error::success();
 }
 
 Error executeObjcopyOnBinary(const CopyConfig &Config,
                              object::MachOObjectFile &In, Buffer &Out) {
   MachOReader Reader(In);
   std::unique_ptr<Object> O = Reader.create();
   if (!O)

llvm/tools/llvm-objcopy/MachO/Object.h

struct LoadCommand {
   // corresponding content inside the binary.
   std::vector<Section> Sections;
 };
 
 // A symbol information. Fields which starts with "n_" are same as them in the
 // nlist.
 struct SymbolEntry {
   std::string Name;
+  bool Referenced = false;
   uint32_t Index;
   uint8_t n_type;
   uint8_t n_sect;
   uint16_t n_desc;
   uint64_t n_value;
 
   bool isExternalSymbol() const {
     return n_type & ((MachO::N_EXT | MachO::N_PEXT));
   }
 
   bool isLocalSymbol() const { return !isExternalSymbol(); }
 
   bool isUndefinedSymbol() const {
     return (n_type & MachO::N_TYPE) == MachO::N_UNDF;
   }
 };
 
 /// The location of the symbol table inside the binary is described by LC_SYMTAB
 /// load command.
 struct SymbolTable {
   std::vector<std::unique_ptr<SymbolEntry>> Symbols;
 
   const SymbolEntry *getSymbolByIndex(uint32_t Index) const;
+  SymbolEntry *getSymbolByIndex(uint32_t Index);
+  void removeSymbols(
+      function_ref<bool(const std::unique_ptr<SymbolEntry> &)> ToRemove);
 };
 
 struct IndirectSymbolEntry {
   uint32_t OriginalIndex;
   /// The Symbol referenced by this entry. It's None if the index is
   /// INDIRECT_SYMBOL_LOCAL or INDIRECT_SYMBOL_ABS.
-  Optional<const SymbolEntry *> Symbol;
+  Optional<SymbolEntry *> Symbol;
 
   IndirectSymbolEntry(uint32_t OriginalIndex,
-                      Optional<const SymbolEntry *> Symbol)
+                      Optional<SymbolEntry *> Symbol)
       : OriginalIndex(OriginalIndex), Symbol(Symbol) {}
 };
 
 struct IndirectSymbolTable {
   std::vector<IndirectSymbolEntry> Symbols;
 };
 
 /// The location of the string table inside the binary is described by LC_SYMTAB

llvm/tools/llvm-objcopy/MachO/Object.cpp

 #include "Object.h"
 #include "../llvm-objcopy.h"
 
 namespace llvm {
 namespace objcopy {
 namespace macho {
 
 const SymbolEntry *SymbolTable::getSymbolByIndex(uint32_t Index) const {
   assert(Index < Symbols.size() && "invalid symbol index");
   return Symbols[Index].get();
 }
 
+SymbolEntry *SymbolTable::getSymbolByIndex(uint32_t Index) {
+  return const_cast<SymbolEntry *>(
+    static_cast<const SymbolTable *>(this)->getSymbolByIndex(Index));
+}
+
+void SymbolTable::removeSymbols(
+    function_ref<bool(const std::unique_ptr<SymbolEntry> &)> ToRemove) {
+  Symbols.erase(
+      std::remove_if(std::begin(Symbols), std::end(Symbols), ToRemove),
+      std::end(Symbols));
+}
+
 void Object::removeSections(function_ref<bool(const Section &)> ToRemove) {
   for (LoadCommand &LC : LoadCommands)
     LC.Sections.erase(std::remove_if(std::begin(LC.Sections),
                                      std::end(LC.Sections), ToRemove),
                       std::end(LC.Sections));
 }
 
 } // end namespace macho
 } // end namespace objcopy
 } // end namespace llvm