[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

llvm/test/tools/llvm-objcopy/ELF/preserve-segment-contents-ehdr-phdrs.test

@@ -13,7 +13,7 @@
 # BEFORE:      Type: PT_LOAD
 # BEFORE-NEXT: Offset: 0x240
 
-# AFTER:      SectionHeaderCount: 5
+# AFTER:      SectionHeaderCount: 3
 # AFTER:      Type: PT_LOAD
 # AFTER-NEXT: Offset: 0x0
 # AFTER:      Type: PT_LOAD

llvm/test/tools/llvm-objcopy/ELF/remove-multiple-sections.test

@@ -87,34 +87,6 @@ Sections:
 # CHECK:   }
 # CHECK:   Section {
 # CHECK:     Index: 4
-# CHECK:     Name: .symtab
-# CHECK:     Type: SHT_SYMTAB (0x2)
-# CHECK:     Flags [ (0x0)
-# CHECK:     ]
-# CHECK:     Address: 0x0
-# CHECK:     Offset:
-# CHECK:     Size:
-# CHECK:     Link: 5
-# CHECK:     Info: 1
-# CHECK:     AddressAlignment: 8
-# CHECK:     EntrySize: 24
-# CHECK:   }
-# CHECK:   Section {
-# CHECK:     Index: 5
-# CHECK:     Name: .strtab
-# CHECK:     Type: SHT_STRTAB (0x3)
-# CHECK:     Flags [ (0x0)
-# CHECK:     ]
-# CHECK:     Address: 0x0
-# CHECK:     Offset:
-# CHECK:     Size:
-# CHECK:     Link: 0
-# CHECK:     Info: 0
-# CHECK:     AddressAlignment: 1
-# CHECK:     EntrySize: 0
-# CHECK:   }
-# CHECK:   Section {
-# CHECK:     Index: 6
 # CHECK:     Name: .shstrtab
 # CHECK:     Type: SHT_STRTAB (0x3)
 # CHECK:     Flags [ (0x0)

llvm/test/tools/llvm-objcopy/ELF/segment-shift-section-remove.test

@@ -41,7 +41,7 @@ ProgramHeaders:
     Sections:
       - Section: .text3
 
-#CHECK: SectionHeaderCount: 6
+#CHECK: SectionHeaderCount: 4
 
 # CHECK:           Sections [
 # CHECK-NEXT:        Section {
@@ -92,34 +92,6 @@ ProgramHeaders:
 # CHECK-NEXT:        }
 # CHECK-NEXT:        Section {
 # CHECK-NEXT:          Index: 3
-# CHECK-NEXT:          Name: .symtab
-# CHECK-NEXT:          Type: SHT_SYMTAB (0x2)
-# CHECK-NEXT:          Flags [ (0x0)
-# CHECK-NEXT:          ]
-# CHECK-NEXT:          Address: 0x0
-# CHECK-NEXT:          Offset: 0x3000
-# CHECK-NEXT:          Size: 24
-# CHECK-NEXT:          Link: 4
-# CHECK-NEXT:          Info: 1
-# CHECK-NEXT:          AddressAlignment: 8
-# CHECK-NEXT:          EntrySize: 24
-# CHECK-NEXT:        }
-# CHECK-NEXT:        Section {
-# CHECK-NEXT:          Index: 4
-# CHECK-NEXT:          Name: .strtab
-# CHECK-NEXT:          Type: SHT_STRTAB (0x3)
-# CHECK-NEXT:          Flags [ (0x0)
-# CHECK-NEXT:          ]
-# CHECK-NEXT:          Address: 0x0
-# CHECK-NEXT:          Offset: 0x3018
-# CHECK-NEXT:          Size:
-# CHECK-NEXT:          Link: 0
-# CHECK-NEXT:          Info: 0
-# CHECK-NEXT:          AddressAlignment: 1
-# CHECK-NEXT:          EntrySize: 0
-# CHECK-NEXT:        }
-# CHECK-NEXT:        Section {
-# CHECK-NEXT:          Index: 5
 # CHECK-NEXT:          Name: .shstrtab
 # CHECK-NEXT:          Type: SHT_STRTAB (0x3)
 # CHECK-NEXT:          Flags [ (0x0)

llvm/test/tools/llvm-objcopy/ELF/segment-test-remove-section.test

@@ -46,7 +46,7 @@ ProgramHeaders:
 # Make sure that when we remove a section we overwrite it with zeros
 # DATA: {{^[^[:blank:]]+}} 00 00 00 00
 
-#CHECK: SectionHeaderCount: 6
+#CHECK: SectionHeaderCount: 4
 
 # CHECK: Sections [
 # CHECK:   Section {
@@ -97,34 +97,6 @@ ProgramHeaders:
 # CHECK-NEXT:   }
 # CHECK-NEXT:   Section {
 # CHECK-NEXT:     Index: 3
-# CHECK-NEXT:     Name: .symtab
-# CHECK-NEXT:     Type: SHT_SYMTAB (0x2)
-# CHECK-NEXT:     Flags [ (0x0)
-# CHECK-NEXT:     ]
-# CHECK-NEXT:     Address: 0x0
-# CHECK-NEXT:     Offset:
-# CHECK-NEXT:     Size:
-# CHECK-NEXT:     Link: 4
-# CHECK-NEXT:     Info: 1
-# CHECK-NEXT:     AddressAlignment: 8
-# CHECK-NEXT:     EntrySize: 24
-# CHECK-NEXT:   }
-# CHECK-NEXT:   Section {
-# CHECK-NEXT:     Index: 4
-# CHECK-NEXT:     Name: .strtab
-# CHECK-NEXT:     Type: SHT_STRTAB (0x3)
-# CHECK-NEXT:     Flags [ (0x0)
-# CHECK-NEXT:     ]
-# CHECK-NEXT:     Address: 0x0
-# CHECK-NEXT:     Offset:
-# CHECK-NEXT:     Size:
-# CHECK-NEXT:     Link: 0
-# CHECK-NEXT:     Info: 0
-# CHECK-NEXT:     AddressAlignment: 1
-# CHECK-NEXT:     EntrySize: 0
-# CHECK-NEXT:   }
-# CHECK-NEXT:   Section {
-# CHECK-NEXT:     Index: 5
 # CHECK-NEXT:     Name: .shstrtab
 # CHECK-NEXT:     Type: SHT_STRTAB (0x3)
 # CHECK-NEXT:     Flags [ (0x0)

llvm/test/tools/llvm-objcopy/ELF/strip-unneeded-all-symbols.test

@@ -0,0 +1,21 @@
+## Stripping unneeded symbols from execuatble/DSO should
+## eliminate the static symbol table, because it's not used
+## by the dynamic loader. 
+
+# RUN: yaml2obj %s > %t
+# RUN: cp %t %t1
+# RUN: llvm-objcopy --strip-unneeded %t %t2
+# RUN: llvm-readobj --section-headers %t2 | FileCheck %s
+
+!ELF
+FileHeader:
+  Class:    ELFCLASS64
+  Data:     ELFDATA2LSB
+  Type:     ET_EXEC
+  Machine:  EM_X86_64
+Symbols:
+  - Name:     bar
+  - Name:     foo
+    Binding:  STB_GLOBAL
+
+# CHECK-NOT: .symtab

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.

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

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