[llvm-objcopy] - Strip undefined symbols if they are no longer referenced following --only-section

This is https://bugs.llvm.org/show_bug.cgi?id=40004.

In this patch I teach llvm-objcopy to remove undefined symbols if
them are not used anymore after applying -j/--only-section option.

Differential revision: https://reviews.llvm.org/D62317

llvm-svn: 361642

test/tools/llvm-objcopy/ELF/only-section-strip-undefined.test

@@ -0,0 +1,60 @@
+## Here we want to check that llvm-objcopy removes an undefined symbol
+## if all references to it have been stripped.
+
+# RUN: yaml2obj --docnum=1 %s -o %t.o
+# RUN: llvm-objcopy -j .other.section %t.o %t2.o
+# RUN: llvm-readobj --symbols %t2.o | FileCheck %s --implicit-check-not=bar
+
+# RUN: llvm-objcopy -j .text -j .rela.text1 %t.o %t2.o
+# RUN: llvm-readobj --symbols %t2.o | FileCheck %s --check-prefix=BAR
+
+# BAR: bar
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_X86_64
+Sections:
+  - Name:        .text
+    Type:        SHT_PROGBITS
+  - Name:        .rela.text1
+    Type:        SHT_RELA
+    Relocations:
+      - Offset:  0x0000000000000001
+        Symbol:  bar
+        Type:    R_X86_64_32
+  - Name:        .rela.text2
+    Type:        SHT_RELA
+    Relocations:
+      - Offset:  0x0000000000000001
+        Symbol:  bar
+        Type:    R_X86_64_32
+  - Name:        .other.section
+    Type:        SHT_PROGBITS
+Symbols:
+  - Name:    bar
+    Binding: STB_GLOBAL
+...
+
+## Check we remove unreferenced undefined symbols, even if
+## they weren't previously referenced. This follows GNU.
+
+# RUN: yaml2obj --docnum=2 %s -o %t.o
+# RUN: llvm-objcopy -j .keep_me %t.o %t2.o
+# RUN: llvm-readobj --symbols %t2.o | FileCheck %s --implicit-check-not=bar
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_X86_64
+Sections:
+  - Name: .keep_me
+    Type: SHT_PROGBITS
+Symbols:
+  - Name:    bar
+    Binding: STB_GLOBAL
+...

tools/llvm-objcopy/ELF/ELFObjcopy.cpp

@@ -387,7 +387,8 @@ static Error updateAndRemoveSymbols(const CopyConfig &Config, Object &Obj) {
   // The purpose of this loop is to mark symbols referenced by sections
   // (like GroupSection or RelocationSection). This way, we know which
   // symbols are still 'needed' and which are not.
-  if (Config.StripUnneeded || !Config.UnneededSymbolsToRemove.empty()) {
+  if (Config.StripUnneeded || !Config.UnneededSymbolsToRemove.empty() ||
+      !Config.OnlySection.empty()) {
     for (auto &Section : Obj.sections())
       Section.markSymbols();
   }
@@ -415,6 +416,11 @@ static Error updateAndRemoveSymbols(const CopyConfig &Config, Object &Obj) {
         isUnneededSymbol(Sym))
       return true;
 
+    // We want to remove undefined symbols if all references have been stripped.
+    if (!Config.OnlySection.empty() && !Sym.Referenced &&
+        Sym.getShndx() == SHN_UNDEF)
+      return true;
+
     return false;
   };