[lld] [ELF] Add '-z nognustack' opt to suppress emitting PT_GNU_STACK

Add a new '-z nognustack' option that suppresses emitting PT_GNU_STACK
segment.  This segment is not supported at all on NetBSD (stack is
always non-executable), and the option is meant to be used to disable
emitting it.

Differential Revision: https://reviews.llvm.org/D56554

lld/ELF/Config.h

@@ -64,6 +64,9 @@ enum class ARMVFPArgKind { Default, Base, VFP, ToolChain };
 // For -z noseparate-code, -z separate-code and -z separate-loadable-segments.
 enum class SeparateSegmentKind { None, Code, Loadable };
 
+// For -z *stack
+enum class GnuStackKind { None, Exec, NoExec };
+
 struct SymbolVersion {
   llvm::StringRef name;
   bool isExternCpp;
@@ -216,6 +219,7 @@ struct Configuration {
   bool zRetpolineplt;
   bool zWxneeded;
   DiscardPolicy discard;
+  GnuStackKind zGnustack;
   ICFLevel icf;
   OrphanHandlingPolicy orphanHandling;
   SortSectionPolicy sortSection;

lld/ELF/Driver.cpp

@@ -394,6 +394,20 @@ static SeparateSegmentKind getZSeparate(opt::InputArgList &args) {
   return SeparateSegmentKind::None;
 }
 
+static GnuStackKind getZGnuStack(opt::InputArgList &args) {
+  for (auto *arg : args.filtered_reverse(OPT_z)) {
+    if (StringRef("execstack") == arg->getValue())
+      return GnuStackKind::Exec;
+    if (StringRef("noexecstack") == arg->getValue())
+      return GnuStackKind::NoExec;
+    if (StringRef("nognustack") == arg->getValue())
+      return GnuStackKind::None;
+  }
+
+  // default
+  return GnuStackKind::NoExec;
+}
+
 static bool isKnownZFlag(StringRef s) {
   return s == "combreloc" || s == "copyreloc" || s == "defs" ||
          s == "execstack" || s == "global" || s == "hazardplt" ||
@@ -402,6 +416,7 @@ static bool isKnownZFlag(StringRef s) {
          s == "separate-code" || s == "separate-loadable-segments" ||
          s == "nocombreloc" || s == "nocopyreloc" || s == "nodefaultlib" ||
          s == "nodelete" || s == "nodlopen" || s == "noexecstack" ||
+         s == "nognustack" ||
          s == "nokeep-text-section-prefix" || s == "norelro" ||
          s == "noseparate-code" || s == "notext" || s == "now" ||
          s == "origin" || s == "relro" || s == "retpolineplt" ||
@@ -951,6 +966,7 @@ static void readConfigs(opt::InputArgList &args) {
   config->zCopyreloc = getZFlag(args, "copyreloc", "nocopyreloc", true);
   config->zExecstack = getZFlag(args, "execstack", "noexecstack", false);
   config->zGlobal = hasZOption(args, "global");
+  config->zGnustack = getZGnuStack(args);
   config->zHazardplt = hasZOption(args, "hazardplt");
   config->zIfuncNoplt = hasZOption(args, "ifunc-noplt");
   config->zInitfirst = hasZOption(args, "initfirst");

lld/ELF/Writer.cpp

@@ -2172,14 +2172,16 @@ std::vector<PhdrEntry *> Writer<ELFT>::createPhdrs(Partition &part) {
   if (OutputSection *cmd = findSection(".openbsd.randomdata", partNo))
     addHdr(PT_OPENBSD_RANDOMIZE, cmd->getPhdrFlags())->add(cmd);
 
-  // PT_GNU_STACK is a special section to tell the loader to make the
-  // pages for the stack non-executable. If you really want an executable
-  // stack, you can pass -z execstack, but that's not recommended for
-  // security reasons.
-  unsigned perm = PF_R | PF_W;
-  if (config->zExecstack)
-    perm |= PF_X;
-  addHdr(PT_GNU_STACK, perm)->p_memsz = config->zStackSize;
+  if (config->zGnustack != GnuStackKind::None) {
+    // PT_GNU_STACK is a special section to tell the loader to make the
+    // pages for the stack non-executable. If you really want an executable
+    // stack, you can pass -z execstack, but that's not recommended for
+    // security reasons.
+    unsigned perm = PF_R | PF_W;
+    if (config->zGnustack == GnuStackKind::Exec)
+      perm |= PF_X;
+    addHdr(PT_GNU_STACK, perm)->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

lld/docs/ld.lld.1

@@ -655,6 +655,11 @@ Set the
 flag to indicate that the object may not be opened by
 .Xr dlopen 3 .
 .Pp
+.It Cm nognustack
+Do not emit the
+.Dv PT_GNU_STACK
+segment.
+.Pp
 .It Cm norelro
 Do not indicate that portions of the object shold be mapped read-only
 after initial relocation processing.

lld/test/ELF/gnustack.s

@@ -10,6 +10,9 @@
 # RUN: ld.lld %t1 -o %t -z noexecstack
 # RUN: llvm-readobj --program-headers -S %t | FileCheck --check-prefix=RW %s
 
+# RUN: ld.lld %t1 -o %t -z nognustack
+# RUN: llvm-readobj --program-headers -s %t | FileCheck --check-prefix=NOGNUSTACK %s
+
 # RW:      Type: PT_GNU_STACK
 # RW-NEXT: Offset: 0x0
 # RW-NEXT: VirtualAddress: 0x0
@@ -35,5 +38,7 @@
 # RWX-NEXT: ]
 # RWX-NEXT: Alignment: 0
 
+# NOGNUSTACK-NOT: Type: PT_GNU_STACK
+
 .globl _start
 _start: