[llvm-objcopy] Allow strip symtab in executables and DSOs

Re-commit of the patch after addressing -Wl,--emit-relocs case.
Differential revision: https://reviews.llvm.org/D61672

llvm-svn: 366787

3 files changed, 27 insertions, 1 deletions

diff --git a/llvm/tools/llvm-objcopy/ELF/ELFObjcopy.cpp b/llvm/tools/llvm-objcopy/ELF/ELFObjcopy.cpp
index b366c6e5598..bbaac96f070 100644
--- a/llvm/tools/llvm-objcopy/ELF/ELFObjcopy.cpp
+++ b/llvm/tools/llvm-objcopy/ELF/ELFObjcopy.cpp
@@ -423,7 +423,7 @@ static Error updateAndRemoveSymbols(const CopyConfig &Config, Object &Obj) {
 
     if ((Config.StripUnneeded ||
          is_contained(Config.UnneededSymbolsToRemove, Sym.Name)) &&
-        isUnneededSymbol(Sym))
+        (!Obj.isRelocatable() || isUnneededSymbol(Sym)))
       return true;
 
     // We want to remove undefined symbols if all references have been stripped.
diff --git a/llvm/tools/llvm-objcopy/ELF/Object.cpp b/llvm/tools/llvm-objcopy/ELF/Object.cpp
index fa696380e17..2d85b3ad36f 100644
--- a/llvm/tools/llvm-objcopy/ELF/Object.cpp
+++ b/llvm/tools/llvm-objcopy/ELF/Object.cpp
@@ -1995,6 +1995,25 @@ template <class ELFT> Error ELFWriter<ELFT>::write() {
   return Buf.commit();
 }
 
+static Error removeUnneededSections(Object &Obj) {
+  // We can remove an empty symbol table from non-relocatable objects.
+  // Relocatable objects typically have relocation sections whose
+  // sh_link field points to .symtab, so we can't remove .symtab
+  // even if it is empty.
+  if (Obj.isRelocatable() || Obj.SymbolTable == nullptr ||
+      !Obj.SymbolTable->empty())
+    return Error::success();
+
+  // .strtab can be used for section names. In such a case we shouldn't
+  // remove it.
+  auto *StrTab = Obj.SymbolTable->getStrTab() == Obj.SectionNames
+                     ? nullptr
+                     : Obj.SymbolTable->getStrTab();
+  return Obj.removeSections(false, [&](const SectionBase &Sec) {
+    return &Sec == Obj.SymbolTable || &Sec == StrTab;
+  });
+}
+
 template <class ELFT> Error ELFWriter<ELFT>::finalize() {
   // It could happen that SectionNames has been removed and yet the user wants
   // a section header table output. We need to throw an error if a user tries
@@ -2004,6 +2023,8 @@ template <class ELFT> Error ELFWriter<ELFT>::finalize() {
                              "cannot write section header table because "
                              "section header string table was removed");
 
+  if (Error E = removeUnneededSections(Obj))
+    return E;
   Obj.sortSections();
 
   // We need to assign indexes before we perform layout because we need to know
diff --git a/llvm/tools/llvm-objcopy/ELF/Object.h b/llvm/tools/llvm-objcopy/ELF/Object.h
index f3df93b9662..3b4f3c7a0d7 100644
--- a/llvm/tools/llvm-objcopy/ELF/Object.h
+++ b/llvm/tools/llvm-objcopy/ELF/Object.h
@@ -1018,6 +1018,7 @@ public:
   uint32_t Flags;
 
   bool HadShdrs = true;
+  bool MustBeRelocatable = false;
   StringTableSection *SectionNames = nullptr;
   SymbolTableSection *SymbolTable = nullptr;
   SectionIndexSection *SectionIndexTable = nullptr;
@@ -1043,6 +1044,7 @@ public:
   template <class T, class... Ts> T &addSection(Ts &&... Args) {
     auto Sec = llvm::make_unique<T>(std::forward<Ts>(Args)...);
     auto Ptr = Sec.get();
+    MustBeRelocatable |= isa<RelocationSection>(*Ptr);
     Sections.emplace_back(std::move(Sec));
     Ptr->Index = Sections.size();
     return *Ptr;
@@ -1051,6 +1053,9 @@ public:
     Segments.emplace_back(llvm::make_unique<Segment>(Data));
     return *Segments.back();
   }
+  bool isRelocatable() const {
+    return (Type != ELF::ET_DYN && Type != ELF::ET_EXEC) || MustBeRelocatable;
+  }
 };
 
 } // end namespace elf