[llvm-objcopy] Add --strip-unneeded option

This option removes symbols that are not needed by relocations. Differential Revision: https://reviews.llvm.org/D46896 llvm-svn: 332915
2018-05-21 22:50:32 +00:00 · 2018-05-21 22:50:32 +00:00 · 040df77ed6
parent 99f7011829
commit 040df77ed6
5 changed files with 175 additions and 4 deletions
--- a/llvm/test/tools/llvm-objcopy/strip-unneeded.test
+++ b/llvm/test/tools/llvm-objcopy/strip-unneeded.test
@ -0,0 +1,136 @@
+# RUN: yaml2obj %s > %t
+# RUN: llvm-objcopy --strip-unneeded %t %t2
+# RUN: llvm-readobj -symbols %t2 | FileCheck %s
+
+!ELF
+FileHeader:
+  Class:           ELFCLASS64
+  Data:            ELFDATA2LSB
+  Type:            ET_REL
+  Machine:         EM_X86_64
+Sections:
+  - Name:            .text
+    Type:            SHT_PROGBITS
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    Address:         0x1000
+    AddressAlign:    0x0000000000000010
+    Size:            64
+  - Name:            .group
+    Type:            SHT_GROUP
+    Link:            .symtab
+    AddressAlign:    0x0000000000000004
+    Info:            barfoo
+    Members:
+      - SectionOrType:   GRP_COMDAT
+      - SectionOrType:   .text
+  - Name:            .rel.text
+    Type:            SHT_REL
+    Link:            .symtab
+    Info:            .text
+    Relocations:
+      - Offset: 0x1000
+        Symbol: foo
+        Type:   R_X86_64_PC32
+Symbols:
+  Local:
+    - Name:     foo
+      Type:     STT_FUNC
+      Section:  .text
+      Value:    0x1000
+      Size:     8
+    - Name:     bar
+      Type:     STT_FUNC
+      Size:     8
+      Section:  .text
+      Value:    0x1008
+    - Name:     barfoo
+      Type:     STT_FUNC
+      Size:     8
+      Section:  .text
+      Value:    0x1010
+    - Name:     fileSymbol
+      Type:     STT_FILE
+    - Name:     sectionSymbol
+      Type:     STT_SECTION
+  Weak:
+    - Name:     baz
+      Type:     STT_FUNC
+      Size:     8
+      Section:  .text
+      Value:    0x1018
+    - Name:     foobaz
+      Type:     STT_FUNC
+  Global:
+    - Name:     foobar
+      Type:     STT_FUNC
+    - Name:     barbaz
+      Type:     STT_FUNC
+      Size:     8
+      Section:  .text
+      Value:    0x1020
+
+#CHECK: Symbols [
+#CHECK-NEXT:  Symbol {
+#CHECK-NEXT:    Name:
+#CHECK-NEXT:    Value: 0x0
+#CHECK-NEXT:    Size: 0
+#CHECK-NEXT:    Binding: Local
+#CHECK-NEXT:    Type: None
+#CHECK-NEXT:    Other: 0
+#CHECK-NEXT:    Section: Undefined
+#CHECK-NEXT:  }
+#CHECK-NEXT:  Symbol {
+#CHECK-NEXT:    Name: foo
+#CHECK-NEXT:    Value: 0x1000
+#CHECK-NEXT:    Size: 8
+#CHECK-NEXT:    Binding: Local
+#CHECK-NEXT:    Type: Function
+#CHECK-NEXT:    Other: 0
+#CHECK-NEXT:    Section: .text
+#CHECK-NEXT:  }
+#CHECK-NEXT:  Symbol {
+#CHECK-NEXT:    Name: barfoo
+#CHECK-NEXT:    Value: 0x1010
+#CHECK-NEXT:    Size: 8
+#CHECK-NEXT:    Binding: Local
+#CHECK-NEXT:    Type: Function
+#CHECK-NEXT:    Other: 0
+#CHECK-NEXT:    Section: .text
+#CHECK-NEXT:  }
+#CHECK-NEXT:  Symbol {
+#CHECK-NEXT:    Name: fileSymbol
+#CHECK-NEXT:    Value: 0x0
+#CHECK-NEXT:    Size: 0
+#CHECK-NEXT:    Binding: Local
+#CHECK-NEXT:    Type: File
+#CHECK-NEXT:    Other: 0
+#CHECK-NEXT:    Section: Undefined
+#CHECK-NEXT:  }
+#CHECK-NEXT:  Symbol {
+#CHECK-NEXT:    Name: sectionSymbol
+#CHECK-NEXT:    Value: 0x0
+#CHECK-NEXT:    Size: 0
+#CHECK-NEXT:    Binding: Local
+#CHECK-NEXT:    Type: Section
+#CHECK-NEXT:    Other: 0
+#CHECK-NEXT:    Section: Undefined
+#CHECK-NEXT:  }
+#CHECK-NEXT:  Symbol {
+#CHECK-NEXT:    Name: barbaz
+#CHECK-NEXT:    Value: 0x1020
+#CHECK-NEXT:    Size: 8
+#CHECK-NEXT:    Binding: Global
+#CHECK-NEXT:    Type: Function
+#CHECK-NEXT:    Other: 0
+#CHECK-NEXT:    Section: .text
+#CHECK-NEXT:  }
+#CHECK-NEXT:  Symbol {
+#CHECK-NEXT:    Name: baz
+#CHECK-NEXT:    Value: 0x1018
+#CHECK-NEXT:    Size: 8
+#CHECK-NEXT:    Binding: Weak
+#CHECK-NEXT:    Type: Function
+#CHECK-NEXT:    Other: 0
+#CHECK-NEXT:    Section: .text
+#CHECK-NEXT:  }
+#CHECK-NEXT:]
--- a/llvm/tools/llvm-objcopy/ObjcopyOpts.td
+++ b/llvm/tools/llvm-objcopy/ObjcopyOpts.td
@ -90,3 +90,5 @@ def K : JoinedOrSeparate<["-"], "K">,
        Alias<keep_symbol>;
 def only_keep_debug : Flag<["-", "--"], "only-keep-debug">,
                          HelpText<"Currently ignored. Only for compaitability with GNU objcopy.">;
+def strip_unneeded : Flag<["-", "--"], "strip-unneeded">,
+                      HelpText<"Remove all symbols not needed by relocations">;
--- a/llvm/tools/llvm-objcopy/Object.cpp
+++ b/llvm/tools/llvm-objcopy/Object.cpp
@ -185,6 +185,7 @@ void SymbolTableSection::addSymbol(StringRef Name, uint8_t Bind, uint8_t Type,
  Sym.Visibility = Visibility;
  Sym.Size = Sz;
  Sym.Index = Symbols.size();
+  Sym.ReferenceCount = 0;
  Symbols.emplace_back(llvm::make_unique<Symbol>(Sym));
  Size += this->EntrySize;
 }
@ -255,6 +256,11 @@ const Symbol *SymbolTableSection::getSymbolByIndex(uint32_t Index) const {
  return Symbols[Index].get();
 }

+Symbol *SymbolTableSection::getSymbolByIndex(uint32_t Index) {
+  return const_cast<Symbol *>(
+      static_cast<const SymbolTableSection *>(this)->getSymbolByIndex(Index));
+}
+
 template <class ELFT>
 void ELFSectionWriter<ELFT>::visit(const SymbolTableSection &Sec) {
  uint8_t *Buf = Out.getBufferStart();
@ -344,6 +350,12 @@ void RelocationSection::accept(SectionVisitor &Visitor) const {
  Visitor.visit(*this);
 }

+RelocationSection::~RelocationSection() {
+  for (auto &Rel : Relocations) {
+    --Rel.RelocSymbol->ReferenceCount;
+  }
+}
+
 void RelocationSection::removeSymbols(
    function_ref<bool(const Symbol &)> ToRemove) {
  for (const Relocation &Reloc : Relocations)
@ -647,7 +659,9 @@ void initRelocations(RelocationSection *Relocs, SymbolTableSection *SymbolTable,
    ToAdd.Offset = Rel.r_offset;
    getAddend(ToAdd.Addend, Rel);
    ToAdd.Type = Rel.getType(false);
-    ToAdd.RelocSymbol = SymbolTable->getSymbolByIndex(Rel.getSymbol(false));
+    Symbol *Sym = SymbolTable->getSymbolByIndex(Rel.getSymbol(false));
+    ++Sym->ReferenceCount;
+    ToAdd.RelocSymbol = Sym;
    Relocs->addRelocation(ToAdd);
  }
 }
--- a/llvm/tools/llvm-objcopy/Object.h
+++ b/llvm/tools/llvm-objcopy/Object.h
@ -344,6 +344,7 @@ struct Symbol {
  uint8_t Type;
  uint64_t Value;
  uint8_t Visibility;
+  uint32_t ReferenceCount;

  uint16_t getShndx() const;
 };
@ -367,6 +368,7 @@ public:
  void addSymbolNames();
  const SectionBase *getStrTab() const { return SymbolNames; }
  const Symbol *getSymbolByIndex(uint32_t Index) const;
+  Symbol *getSymbolByIndex(uint32_t Index);
  void updateSymbols(function_ref<void(Symbol &)> Callable);

  void removeSectionReferences(const SectionBase *Sec) override;
@ -381,7 +383,7 @@ public:
 };

 struct Relocation {
-  const Symbol *RelocSymbol = nullptr;
+  Symbol *RelocSymbol = nullptr;
  uint64_t Offset;
  uint64_t Addend;
  uint32_t Type;
@ -432,6 +434,7 @@ class RelocationSection
  std::vector<Relocation> Relocations;

 public:
+  ~RelocationSection();
  void addRelocation(Relocation Rel) { Relocations.push_back(Rel); }
  void accept(SectionVisitor &Visitor) const override;
  void removeSymbols(function_ref<bool(const Symbol &)> ToRemove) override;
@ -449,7 +452,7 @@ public:
 class GroupSection : public SectionBase {
  MAKE_SEC_WRITER_FRIEND
  const SymbolTableSection *SymTab = nullptr;
-  const Symbol *Sym = nullptr;
+  Symbol *Sym = nullptr;
  ELF::Elf32_Word FlagWord;
  SmallVector<SectionBase *, 3> GroupMembers;

@ -459,9 +462,16 @@ public:
  ArrayRef<uint8_t> Contents;

  explicit GroupSection(ArrayRef<uint8_t> Data) : Contents(Data) {}
+  ~GroupSection() {
+    if (Sym)
+      --Sym->ReferenceCount;
+  }

  void setSymTab(const SymbolTableSection *SymTabSec) { SymTab = SymTabSec; }
-  void setSymbol(const Symbol *S) { Sym = S; }
+  void setSymbol(Symbol *S) {
+    Sym = S;
+    ++Sym->ReferenceCount;
+  }
  void setFlagWord(ELF::Elf32_Word W) { FlagWord = W; }
  void addMember(SectionBase *Sec) { GroupMembers.push_back(Sec); }

--- a/llvm/tools/llvm-objcopy/llvm-objcopy.cpp
+++ b/llvm/tools/llvm-objcopy/llvm-objcopy.cpp
@ -156,6 +156,7 @@ struct CopyConfig {
  bool StripSections = false;
  bool StripNonAlloc = false;
  bool StripDWO = false;
+  bool StripUnneeded = false;
  bool ExtractDWO = false;
  bool LocalizeHidden = false;
  bool Weaken = false;
@ -389,6 +390,13 @@ void HandleArgs(const CopyConfig &Config, Object &Obj, const Reader &Reader,
        return true;
      }

+      // TODO: We might handle the 'null symbol' in a different way
+      // by probably handling it the same way as we handle 'null section' ?
+      if (Config.StripUnneeded && !Sym.ReferenceCount && Sym.Index != 0 &&
+          (Sym.Binding == STB_LOCAL || Sym.getShndx() == SHN_UNDEF) &&
+          Sym.Type != STT_FILE && Sym.Type != STT_SECTION)
+        return true;
+
      return false;
    });
  }
@ -479,6 +487,7 @@ CopyConfig ParseObjcopyOptions(ArrayRef<const char *> ArgsArr) {
  Config.StripDWO = InputArgs.hasArg(OBJCOPY_strip_dwo);
  Config.StripSections = InputArgs.hasArg(OBJCOPY_strip_sections);
  Config.StripNonAlloc = InputArgs.hasArg(OBJCOPY_strip_non_alloc);
+  Config.StripUnneeded = InputArgs.hasArg(OBJCOPY_strip_unneeded);
  Config.ExtractDWO = InputArgs.hasArg(OBJCOPY_extract_dwo);
  Config.LocalizeHidden = InputArgs.hasArg(OBJCOPY_localize_hidden);
  Config.Weaken = InputArgs.hasArg(OBJCOPY_weaken);