[ELF] Simplify the condition to create .interp

[MaskRay]

Summary

(1) {gcc,clang} -fuse-ld=bfd -pie -fPIE -nostdlib a.c => .interp created
(2) {gcc,clang} -fuse-ld=lld -pie -fPIE -nostdlib a.c => .interp not created
(3) {gcc,clang} -fuse-ld=lld -pie -fPIE -nostdlib a.c a.so => .interp created

The inconsistency of (2) is due to the condition !Config->SharedFiles.empty().
To make lld behave more like ld.bfd, we could change the condition to:

Config->HasDynSymTab && !Config->DynamicLinker.empty() && Script->needsInterpSection();

However, that would bring another inconsistency as can be observed with:

(4) {gcc,clang} -fuse-ld=bfd -no-pie -nostdlib a.c => .interp not created

So instead, use !Config->DynamicLinker.empty() && Script->needsInterpSection(),
which is both simple and consistent in these cases.

The inconsistency likely originated from ld.bfd and gold's choice to
have a default --dynamic-linker. Their condition to create .interp is
ANDed with (not -shared). Since lld doesn't have a default
--dynamic-linker, and compiler drivers (gcc/clang) don't pass
--dynamic-linker with -shared, we do not need the condition
Config->Shared. For direct ld users, they are not suppoped to specify
--dynamic-linker for a -shared link.

[MaskRay]

Reword

[ruiu]

LGTM

I'm a little curious about the motivation of this change. Did this incompatibility cause a practical issue, or are you just fixing a disparity?

[MaskRay]

I'm a little curious about the motivation of this change. Did this incompatibility cause a practical issue, or are you just fixing a disparity?

I noticed this before but didn't think hard about it.

I rediscovered it while playing with -static-pie. ld.bfd has a --no-dynamic-linker (added in 2015) to suppress its default --dynamic-linker. It would not have been useful if ld.bfd didn't have the default --dynamic-linker in the first place... I guess our !SharedFiles.empty() was probably a mistake of copying gold's behavior (it should check !Config->Shared instead)

      if ((!parameters->options().shared()
	   || parameters->options().dynamic_linker() != NULL)
	  && this->interp_segment_ == NULL)

Our current rule precludes a dynamically linked executable with no DT_NEEDED. Such executables may be created by -nostdlib. I would not bother supporting this use case if the change required more code.

lld/trunk/ELF/Writer.cpp

StringRef elf::getOutputSectionName(const InputSectionBase *S) {
   // By default, it should go to .bss section.
   if (S->Name == "COMMON")
     return ".bss";
 
   return S->Name;
 }
 
 static bool needsInterpSection() {
-  return !SharedFiles.empty() && !Config->DynamicLinker.empty() &&
-         Script->needsInterpSection();
+  return !Config->DynamicLinker.empty() && Script->needsInterpSection();
 }
 
 template <class ELFT> void elf::writeResult() { Writer<ELFT>().run(); }
 
 template <class ELFT> void Writer<ELFT>::removeEmptyPTLoad() {
   llvm::erase_if(Phdrs, [&](const PhdrEntry *P) {
     if (P->p_type != PT_LOAD)
       return false;

lld/trunk/test/ELF/dynamic-linker.s

 # REQUIRES: x86
-# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %p/Inputs/shared.s -o %t1.o
-# RUN: ld.lld -shared %t1.o -o %t.so
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 
-# RUN: ld.lld --dynamic-linker foo %t.o %t.so -o %t
-# RUN: llvm-readelf -program-headers %t | FileCheck %s
+# RUN: ld.lld --dynamic-linker foo %t.o -o %t
+# RUN: llvm-readelf --program-headers %t | FileCheck %s
 
-# RUN: ld.lld --dynamic-linker=foo %t.o %t.so -o %t
-# RUN: llvm-readelf -program-headers %t | FileCheck %s
+# RUN: ld.lld --dynamic-linker=foo %t.o -o %t
+# RUN: llvm-readelf --program-headers %t | FileCheck %s
 
 # CHECK: [Requesting program interpreter: foo]
 
-# RUN: ld.lld %t.o %t.so -o %t
+# RUN: ld.lld %t.o -o %t
 # RUN: llvm-readelf -program-headers %t | FileCheck --check-prefix=NO %s
 
-# RUN: ld.lld --dynamic-linker foo --no-dynamic-linker %t.o %t.so -o %t
-# RUN: llvm-readelf -program-headers %t | FileCheck --check-prefix=NO %s
+# RUN: ld.lld --dynamic-linker foo --no-dynamic-linker %t.o -o %t
+# RUN: llvm-readelf --program-headers %t | FileCheck --check-prefix=NO %s
 
 # NO-NOT: PT_INTERP
 
 .globl _start
 _start:
   nop