[PATCH] D62317: [llvm-objcopy] - Strip undefined symbols if they are no longer referenced following --only-section

[George Rimar via Phabricator via llvm-commits]

grimar created this revision.
grimar added reviewers: jhenderson, rupprecht, jakehehrlich.
Herald added subscribers: MaskRay, arichardson, emaste.
Herald added a reviewer: espindola.
Herald added a reviewer: alexshap.

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.


https://reviews.llvm.org/D62317

Files:
  test/tools/llvm-objcopy/ELF/only-sesction-strip-undefined.test
  tools/llvm-objcopy/ELF/ELFObjcopy.cpp


Index: tools/llvm-objcopy/ELF/ELFObjcopy.cpp
===================================================================
--- tools/llvm-objcopy/ELF/ELFObjcopy.cpp
+++ tools/llvm-objcopy/ELF/ELFObjcopy.cpp
@@ -387,7 +387,8 @@
   // 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 +416,12 @@
         isUnneededSymbol(Sym))
       return true;
 
+    // We want to remove the undefined symbols if the source of reference,
+    // like relocation section was stripped.
+    if (!Config.OnlySection.empty() && !Sym.Referenced &&
+        Sym.getShndx() == SHN_UNDEF)
+      return true;
+
     return false;
   };
 
Index: test/tools/llvm-objcopy/ELF/only-sesction-strip-undefined.test
===================================================================
--- /dev/null
+++ test/tools/llvm-objcopy/ELF/only-sesction-strip-undefined.test
@@ -0,0 +1,42 @@
+## Here we want to check that llvm-objcopy removes the undefined symbol
+## if the source of reference was stripped.
+
+# RUN: yaml2obj %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.text %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
+    Flags:           [ SHF_ALLOC, SHF_EXECINSTR ]
+    AddressAlign:    0x0000000000000001
+    Content:         BA00000000
+  - Name:            .rela.text
+    Type:            SHT_RELA
+    Flags:           [ SHF_INFO_LINK ]
+    Link:            .symtab
+    AddressAlign:    0x0000000000000008
+    EntSize:         0x0000000000000018
+    Info:            .text
+    Relocations:
+      - Offset:      0x0000000000000001
+        Symbol:      bar
+        Type:        R_X86_64_32
+  - Name:            .other.section
+    Type:            SHT_PROGBITS
+    AddressAlign:    0x0000000000000001
+    Content:         '90'
+Symbols:
+  - Name:            bar
+    Binding:         STB_GLOBAL
+...


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