[ELF] Linkerscript: discard .interp section if we have custom PHDRS layout and PT_INTERP is not listed.

[evgeny777]

Summary

Current version may allocate new PT_LOAD in this case, which wastes space in ELF image and breaks our product linking.

[ruiu]

LGTM

ELF/LinkerScript.cpp

489

Discard sounds like this function discards (or remove) an existing section from an output section list, so it's a bit confusing. I'd name needsInterpSection.

[evgeny777]

ELF/LinkerScript.cpp

489

I can't decide in whether we need .interp section or not. I can only tell if we do not need it. So I renamed to ignoreInterpSection

ELF/LinkerScript.h

template <class ELFT> class LinkerScript {
   typedef typename ELFT::uint uintX_t;
 
 public:
   LinkerScript();
   ~LinkerScript();
   void createSections(OutputSectionFactory<ELFT> &Factory);
 
   std::vector<PhdrEntry<ELFT>> createPhdrs();
+  bool discardInterpSection();
 
   ArrayRef<uint8_t> getFiller(StringRef Name);
   bool shouldKeep(InputSectionBase<ELFT> *S);
   void assignAddresses();
   int compareSections(StringRef A, StringRef B);
   bool hasPhdrsCommands();
   uintX_t getOutputSectionSize(StringRef Name);
   uintX_t getHeaderSize();

ELF/LinkerScript.cpp

if (!PhdrIds.empty()) {
         Flags = NewFlags;
       }
       Load->add(Sec);
     }
   }
   return Ret;
 }
 
+template <class ELFT> bool LinkerScript<ELFT>::discardInterpSection() {
+  // Discard .interp section in case we have PHDRS specification
+  // and PT_INTERP isn't listed.
+  return !Opt.PhdrsCommands.empty() &&
+         llvm::find_if(Opt.PhdrsCommands, [](const PhdrsCommand &Cmd) {
+           return Cmd.Type == PT_INTERP;
+         }) == Opt.PhdrsCommands.end();
+}
+
 template <class ELFT>
 ArrayRef<uint8_t> LinkerScript<ELFT>::getFiller(StringRef Name) {
   for (const std::unique_ptr<BaseCommand> &Base : Opt.Commands)
     if (auto *Cmd = dyn_cast<OutputSectionCommand>(Base.get()))
       if (Cmd->Name == Name)
         return Cmd->Filler;
   return {};
 }

ELF/Writer.cpp

template <class ELFT> void elf::reportDiscarded(InputSectionBase<ELFT> *IS) {
   if (!Config->PrintGcSections || !IS || IS->Live)
     return;
   errs() << "removing unused section from '" << IS->getSectionName()
          << "' in file '" << IS->getFile()->getName() << "'\n";
 }
 
 template <class ELFT> static bool needsInterpSection() {
   return !Symtab<ELFT>::X->getSharedFiles().empty() &&
-         !Config->DynamicLinker.empty();
+         !Config->DynamicLinker.empty() &&
+         !Script<ELFT>::X->discardInterpSection();
 }
 
 template <class ELFT> void elf::writeResult() {
   typedef typename ELFT::uint uintX_t;
   typedef typename ELFT::Ehdr Elf_Ehdr;
 
   // Create singleton output sections.
   OutputSection<ELFT> Bss(".bss", SHT_NOBITS, SHF_ALLOC | SHF_WRITE);

test/ELF/linkerscript/linkerscript-discard-interp.s

+// RUN: llvm-mc -filetype=obj -triple=i686-unknown-linux %s -o %t.o
+// RUN: llvm-mc -filetype=obj -triple=i686-unknown-linux %p/../Inputs/shared.s -o %t2.o
+// RUN: ld.lld -shared %t2.o -o %t2.so
+// RUN: echo "PHDRS { text PT_LOAD FILEHDR PHDRS; } \
+// RUN:       SECTIONS { .text : { *(.text) } : text }" >> %t.script
+// RUN: ld.lld -dynamic-linker /lib64/ld-linux-x86-64.so.2 -rpath foo -rpath bar --script %t.script --export-dynamic %t.o %t2.so -o %t
+// RUN: llvm-readobj -s %t | FileCheck %s
+
+// CHECK-NOT:        Name: .interp
+
+.global _start
+_start: