- Merge '.openbsd.randomdata.*' sections into a single '.openbsd.randomdata'
  section when linking, as we do when using ld from binutils.
- Generate __data_start symbol that marks the start of .data when __data_start
  is referenced from code being linked.
- Put .got.plt in RELRO.
- On i386, produce binaries that are compatible with our W^X implementation.
- In the linkers, collect objects in section "openbsd.mutable" and place
  them into a page-aligned region in the bss, with the right markers for
  kernel/ld.so to identify the region and skip making it immutable.
- Implement support for PT_OPENBSD_NOBTCFI in lld(1).  This can be set using
  the -z nobtcfi option.

Index: tools/lld/ELF/Writer.cpp
--- tools/lld/ELF/Writer.cpp.orig
+++ tools/lld/ELF/Writer.cpp
@@ -145,7 +145,8 @@ StringRef elf::getOutputSectionName(const InputSection
   for (StringRef v :
        {".text.", ".rodata.", ".data.rel.ro.", ".data.", ".bss.rel.ro.",
         ".bss.", ".init_array.", ".fini_array.", ".ctors.", ".dtors.", ".tbss.",
-        ".gcc_except_table.", ".tdata.", ".ARM.exidx.", ".ARM.extab."})
+        ".gcc_except_table.", ".tdata.", ".ARM.exidx.", ".ARM.extab.",
+        ".openbsd.randomdata.", ".openbsd.mutable." })
     if (isSectionPrefix(v, s->name))
       return v.drop_back();
 
@@ -324,6 +325,7 @@ void elf::addReservedSymbols() {
   };
 
   ElfSym::bss = add("__bss_start", 0);
+  ElfSym::data = add("__data_start", 0);
   ElfSym::end1 = add("end", -1);
   ElfSym::end2 = add("_end", -1);
   ElfSym::etext1 = add("etext", -1);
@@ -883,7 +885,11 @@ static bool isRelroSection(const OutputSection *sec) {
   // However, if "-z now" is given, the lazy symbol resolution is
   // disabled, which enables us to put it into RELRO.
   if (sec == in.gotPlt->getParent())
+#ifndef __OpenBSD__
     return config->zNow;
+#else
+    return true;	/* kbind(2) means we can always put these in RELRO */
+#endif
 
   // .dynamic section contains data for the dynamic linker, and
   // there's no need to write to it at runtime, so it's better to put
@@ -1193,6 +1199,9 @@ template <class ELFT> void Writer<ELFT>::setReservedSy
   if (ElfSym::bss)
     ElfSym::bss->section = findSection(".bss");
 
+  if (ElfSym::data)
+    ElfSym::data->section = findSection(".data");
+
   // Setup MIPS _gp_disp/__gnu_local_gp symbols which should
   // be equal to the _gp symbol's value.
   if (ElfSym::mipsGp) {
@@ -2461,6 +2470,12 @@ std::vector<PhdrEntry *> Writer<ELFT>::createPhdrs(Par
     addHdr(PT_GNU_EH_FRAME, part.ehFrameHdr->getParent()->getPhdrFlags())
         ->add(part.ehFrameHdr->getParent());
 
+  // PT_OPENBSD_MUTABLE is an OpenBSD-specific feature. That makes
+  // the dynamic linker fill the segment with zero data, like bss, but
+  // it can be treated differently.
+  if (OutputSection *cmd = findSection(".openbsd.mutable", partNo))
+    addHdr(PT_OPENBSD_MUTABLE, cmd->getPhdrFlags())->add(cmd);
+
   // PT_OPENBSD_RANDOMIZE is an OpenBSD-specific feature. That makes
   // the dynamic linker fill the segment with random data.
   if (OutputSection *cmd = findSection(".openbsd.randomdata", partNo))
@@ -2484,6 +2499,11 @@ std::vector<PhdrEntry *> Writer<ELFT>::createPhdrs(Par
   if (config->zWxneeded)
     addHdr(PT_OPENBSD_WXNEEDED, PF_X);
 
+  // PT_OPENBSD_NOBTCFI is an OpenBSD-specific header to mark that the
+  // executable is expected to violate branch-target CFI checks.
+  if (config->zNoBtCfi)
+    addHdr(PT_OPENBSD_NOBTCFI, PF_X);
+
   if (OutputSection *cmd = findSection(".note.gnu.property", partNo))
     addHdr(PT_GNU_PROPERTY, PF_R)->add(cmd);
 
@@ -2576,6 +2596,31 @@ template <class ELFT> void Writer<ELFT>::fixSectionAli
         };
     }
   };
+
+#ifdef __OpenBSD__
+  // On i386, produce binaries that are compatible with our W^X implementation
+  if (config->emachine == EM_386) {
+    auto NXAlign = [](OutputSection *Cmd) {
+      if (Cmd && !Cmd->addrExpr)
+        Cmd->addrExpr = [=] {
+          return alignTo(script->getDot(), 0x20000000);
+        };
+    };
+
+    for (Partition &part : partitions) {
+      PhdrEntry *firstRW = nullptr;
+      for (PhdrEntry *P : part.phdrs) {
+        if (P->p_type == PT_LOAD && (P->p_flags & PF_W)) {
+          firstRW = P;
+          break;
+        }
+      }
+
+      if (firstRW)
+        NXAlign(firstRW->firstSec);
+    }
+  }
+#endif
 
   for (Partition &part : partitions) {
     prev = nullptr;