[ELF] - Implemented -z wxneeded.

[grimar]

Summary

-z wxneeded creates a PHDR PT_OPENBSD_WXNEEDED.

PT_OPENBSD_WXNEEDED
The array element specifies that a process executing this file may need to be able to map or protect memory regions as simultaneously executable and writable. If the system is unable or unwilling to permit that for this executable then it may fail immediately. This segment type is meaningful only for executable files and is ignored in other objects.

http://man.openbsd.org/OpenBSD-current/man5/elf.5

[ruiu]

Could you give me a pointer to the OpenBSD-local patch to GNU ld which implements this -z wxneeded feature? I'd like to look at that patch to see if this patch does the same thing.

ELF/Config.h

129 ↗

(On Diff #74238)

The variable naming rule here is to camel-case words with "-" as a word separator. Because "wxneeded" contains no hyphens, it should be handled as one word. Thus, ZWxneeded.

ELF/LinkerScript.cpp

1717 ↗

(On Diff #74238)

Sort.

[grimar]

• Addressed review comments.

[grimar]

In D25472#567483, @ruiu wrote:

Could you give me a pointer to the OpenBSD-local patch to GNU ld which implements this -z wxneeded feature? I'd like to look at that patch to see if this patch does the same thing.

https://github.com/openbsd/src/commit/2a5a8fc7e30928c2cff57cfe5fb491c90d8478ad

ELF/Config.h

129 ↗

(On Diff #74238)

Done.

ELF/LinkerScript.cpp

1717 ↗

(On Diff #74238)

This switch is sorted by constant value I think.

[ruiu]

LGTM with this fix.

ELF/Writer.cpp

1112 ↗

(On Diff #74358)

Just like other PT_ values in this function, this needs comment. We essentially should write more comments what it is and why we are doing this, because it is easy to forget and cannot always be inferred from what we are doing.

// PT_OPENBSD_WXNEEDED is a OpenBSD-specific header to mark the executable
// is expected to perform W^X violations, such as calling mprotect(2) or mmap(2)
// with PROT_WRITE | PROT_EXEC, which is prohibited by default on OpenBSD.

lld/trunk/ELF/Config.h

struct Configuration {
   bool Verbose;
   bool WarnCommon;
   bool ZCombreloc;
   bool ZExecstack;
   bool ZNodelete;
   bool ZNow;
   bool ZOrigin;
   bool ZRelro;
+  bool ZWxneeded;
   DiscardPolicy Discard;
   SortSectionPolicy SortSection;
   StripPolicy Strip = StripPolicy::None;
   UnresolvedPolicy UnresolvedSymbols;
   BuildIdKind BuildId = BuildIdKind::None;
   ELFKind EKind = ELFNoneKind;
   uint16_t DefaultSymbolVersion = llvm::ELF::VER_NDX_GLOBAL;
   uint16_t EMachine = llvm::ELF::EM_NONE;

lld/trunk/ELF/Driver.cpp

if (Config->ThinLtoJobs == 0)
     error("--thinlto-jobs: number of threads must be > 0");
 
   Config->ZCombreloc = !hasZOption(Args, "nocombreloc");
   Config->ZExecstack = hasZOption(Args, "execstack");
   Config->ZNodelete = hasZOption(Args, "nodelete");
   Config->ZNow = hasZOption(Args, "now");
   Config->ZOrigin = hasZOption(Args, "origin");
   Config->ZRelro = !hasZOption(Args, "norelro");
+  Config->ZWxneeded = hasZOption(Args, "wxneeded");
 
   if (!Config->Relocatable)
     Config->Strip = getStripOption(Args);
 
   Config->ZStackSize = getZOptionValue(Args, "stack-size", -1);
 
   // Config->Pic is true if we are generating position-independent code.
   Config->Pic = Config->Pie || Config->Shared;

lld/trunk/ELF/LinkerScript.cpp

unsigned Ret = StringSwitch<unsigned>(Tok)
                      .Case("PT_INTERP", PT_INTERP)
                      .Case("PT_NOTE", PT_NOTE)
                      .Case("PT_SHLIB", PT_SHLIB)
                      .Case("PT_PHDR", PT_PHDR)
                      .Case("PT_TLS", PT_TLS)
                      .Case("PT_GNU_EH_FRAME", PT_GNU_EH_FRAME)
                      .Case("PT_GNU_STACK", PT_GNU_STACK)
                      .Case("PT_GNU_RELRO", PT_GNU_RELRO)
+                     .Case("PT_OPENBSD_WXNEEDED", PT_OPENBSD_WXNEEDED)
                      .Default(-1);
 
   if (Ret == (unsigned)-1) {
     setError("invalid program header type: " + Tok);
     return PT_NULL;
   }
   return Ret;
 }

lld/trunk/ELF/Writer.cpp

std::vector<PhdrEntry<ELFT>> Writer<ELFT>::createPhdrs() {
   // PT_GNU_STACK is a special section to tell the loader to make the
   // pages for the stack non-executable.
   if (!Config->ZExecstack) {
     Phdr &Hdr = *AddHdr(PT_GNU_STACK, PF_R | PF_W);
     if (Config->ZStackSize != uint64_t(-1))
       Hdr.H.p_memsz = Config->ZStackSize;
   }
 
+  // PT_OPENBSD_WXNEEDED is a OpenBSD-specific header to mark the executable
+  // is expected to perform W^X violations, such as calling mprotect(2) or
+  // mmap(2) with PROT_WRITE | PROT_EXEC, which is prohibited by default on
+  // OpenBSD.
+  if (Config->ZWxneeded)
+    AddHdr(PT_OPENBSD_WXNEEDED, PF_X);
+
   if (Note.First)
     Ret.push_back(std::move(Note));
   return Ret;
 }
 
 // The first section of each PT_LOAD and the first section after PT_GNU_RELRO
 // have to be page aligned so that the dynamic linker can set the permissions.
 template <class ELFT> void Writer<ELFT>::fixSectionAlignments() {

lld/trunk/test/ELF/linkerscript/openbsd-wxneeded.s

+# RUN: llvm-mc -filetype=obj -triple=i686-unknown-linux %s -o %t.o
+# RUN: echo "PHDRS { text PT_LOAD FILEHDR PHDRS; wxneeded PT_OPENBSD_WXNEEDED; }" > %t.script
+# RUN: ld.lld -z wxneeded --script %t.script %t.o -o %t
+# RUN: llvm-readobj --program-headers %t | FileCheck %s
+
+# CHECK:      ProgramHeader {
+# CHECK:        Type:  (0x65A3DBE7)
+# CHECK-NEXT:   Offset: 0x0
+# CHECK-NEXT:   VirtualAddress: 0x0
+# CHECK-NEXT:   PhysicalAddress: 0x0
+# CHECK-NEXT:   FileSize: 0
+# CHECK-NEXT:   MemSize: 0
+# CHECK-NEXT:   Flags [
+# CHECK-NEXT:     PF_R
+# CHECK-NEXT:   ]
+# CHECK-NEXT:   Alignment: 0
+# CHECK-NEXT: }

lld/trunk/test/ELF/openbsd-wxneeded.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t
+# RUN: ld.lld -z wxneeded %t -o %t.out
+# RUN: llvm-readobj --program-headers %t.out | FileCheck %s
+
+# CHECK:      ProgramHeader {
+# CHECK:        Type:  (0x65A3DBE7)
+# CHECK-NEXT:   Offset: 0x0
+# CHECK-NEXT:   VirtualAddress: 0x0
+# CHECK-NEXT:   PhysicalAddress: 0x0
+# CHECK-NEXT:   FileSize: 0
+# CHECK-NEXT:   MemSize: 0
+# CHECK-NEXT:   Flags [
+# CHECK-NEXT:     PF_X
+# CHECK-NEXT:   ]
+# CHECK-NEXT:   Alignment: 0
+# CHECK-NEXT: }