ELF: Omit PT_GNU_STACK segment if -z execstack is provided.

[ruiu]

Summary

In the previous patch (r254003), I made the linker emit PT_GNU_STACK
unconditionally. But sometimes you want to have a control over the
presence of the segment. With this patch, you can omit the segment
by passing -z execstack option.

[joerg]

Thanks, this works for the cases I care about. The refinement of the test case is minor, just trying to make sure both cases are handled consistently.

test/ELF/gnustack.s

9

The intention here is that the notes are dropped, so shouldn't that be checked for the RWX case as well?

[ruiu]

test/ELF/gnustack.s

9

Good point. Done.

ELF/Writer.cpp

template <class ELFT> void Writer<ELFT>::assignAddresses() {
 
   if (HasRelro) {
     Elf_Phdr *PH = &Phdrs[++PhdrIdx];
     *PH = GnuRelroPhdr;
   }
 
   // PT_GNU_STACK is a special section to tell the loader to make the
   // pages for the stack non-executable.
+  if (!Config->ZExecStack) {
     Elf_Phdr *PH = &Phdrs[++PhdrIdx];
     PH->p_type = PT_GNU_STACK;
-  if (Config->ZExecStack)
-    PH->p_flags = PF_R | PF_W | PF_X;
-  else
     PH->p_flags = PF_R | PF_W;
+  }
 
   // Fix up PT_INTERP as we now know the address of .interp section.
   if (Interp) {
     Interp->p_type = PT_INTERP;
     copyPhdr(Interp, Out<ELFT>::Interp);
   }
 
   // Add space for section headers.
   SectionHeaderOff = RoundUpToAlignment(FileOff, ELFT::Is64Bits ? 8 : 4);
   FileSize = SectionHeaderOff + getNumSections() * sizeof(Elf_Shdr);
 
   // Update "_end" and "end" symbols so that they
   // point to the end of the data segment.
   DefinedAbsolute<ELFT>::End.st_value = VA;
 
   // Update MIPS _gp absolute symbol so that it points to the static data.
   if (Config->EMachine == EM_MIPS)
     DefinedAbsolute<ELFT>::MipsGp.st_value = getMipsGpAddr<ELFT>();
 }
 
 // Returns the number of PHDR entries.
 template <class ELFT> int Writer<ELFT>::getPhdrsNum() const {
   bool Tls = false;
-  int I = 3; // 3 for PT_PHDR, first PT_LOAD and PT_GNU_STACK
+  int I = 2; // 2 for PT_PHDR and first PT_LOAD
   if (needsInterpSection())
     ++I;
   if (isOutputDynamic())
     ++I;
+  if (!Config->ZExecStack)
+    ++I;
   uintX_t Last = PF_R;
   for (OutputSectionBase<ELFT> *Sec : OutputSections) {
     if (!needsPhdr<ELFT>(Sec))
       continue;
     if (Sec->getFlags() & SHF_TLS)
       Tls = true;
     uintX_t Flags = toPhdrFlags(Sec->getFlags());
     if (Last != Flags) {

test/ELF/gnustack.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t1
 # RUN: ld.lld %t1 -z execstack -o %t
-# RUN: llvm-readobj --program-headers -s %t | FileCheck --check-prefix=CHECK_RWX %s
+# RUN: llvm-readobj --program-headers -s %t | FileCheck --check-prefix=RWX %s
 # RUN: ld.lld %t1 -o %t
-# RUN: llvm-readobj --program-headers -s %t | FileCheck --check-prefix=CHECK_RW %s
+# RUN: llvm-readobj --program-headers -s %t | FileCheck --check-prefix=RW %s
 
-# CHECK_RW:       Sections [
-# CHECK_RW-NOT:   Name: .note.GNU-stack
-# CHECK_RW:       ProgramHeaders [
-# CHECK_RW:        ProgramHeader {
-# CHECK_RW:        Type: PT_GNU_STACK
-# CHECK_RW-NEXT:   Offset: 0x0
-# CHECK_RW-NEXT:   VirtualAddress: 0x0
-# CHECK_RW-NEXT:   PhysicalAddress: 0x0
-# CHECK_RW-NEXT:   FileSize: 0
-# CHECK_RW-NEXT:   MemSize: 0
-# CHECK_RW-NEXT:   Flags [
-# CHECK_RW-NEXT:     PF_R
-# CHECK_RW-NEXT:     PF_W
-# CHECK_RW-NEXT:   ]
-# CHECK_RW-NEXT:   Alignment: 0
-# CHECK_RW-NEXT:   }
-# CHECK_RW-NEXT: ]
+# RW:       Sections [
+# RW-NOT:   Name: .note.GNU-stack
+# RW:       ProgramHeaders [
+# RW:        ProgramHeader {
+# RW:        Type: PT_GNU_STACK
+# RW-NEXT:   Offset: 0x0
+# RW-NEXT:   VirtualAddress: 0x0
+# RW-NEXT:   PhysicalAddress: 0x0
+# RW-NEXT:   FileSize: 0
+# RW-NEXT:   MemSize: 0
+# RW-NEXT:   Flags [
+# RW-NEXT:     PF_R
+# RW-NEXT:     PF_W
+# RW-NEXT:   ]
+# RW-NEXT:   Alignment: 0
+# RW-NEXT:   }
+# RW-NEXT: ]
 
-# CHECK_RWX:       Sections [
-# CHECK_RWX-NOT:   Name: .note.GNU-stack
-# CHECK_RWX:       ProgramHeaders [
-# CHECK_RWX:        ProgramHeader {
-# CHECK_RWX:        Type: PT_GNU_STACK
-# CHECK_RWX-NEXT:   Offset: 0x0
-# CHECK_RWX-NEXT:   VirtualAddress: 0x0
-# CHECK_RWX-NEXT:   PhysicalAddress: 0x0
-# CHECK_RWX-NEXT:   FileSize: 0
-# CHECK_RWX-NEXT:   MemSize: 0
-# CHECK_RWX-NEXT:   Flags [
-# CHECK_RWX-NEXT:     PF_R
-# CHECK_RWX-NEXT:     PF_W
-# CHECK_RWX-NEXT:     PF_X
-# CHECK_RWX-NEXT:   ]
-# CHECK_RWX-NEXT:   Alignment: 0
-# CHECK_RWX-NEXT:   }
-# CHECK_RWX-NEXT: ]
+# RWX-NOT: Type: PT_GNU_STACK
 
 .globl _start
 _start: