lld: add -z nodump support

[emaste]

Summary

-z nodump sets the DF_1_NODUMP flag, marking the object as not dumpable by the runtime linker's dldump().

FreeBSD does not support dldump() and ignores the DF_1_NODUMP flag; I am not sure about support in any other operating systems outside of Solaris derivatives. That said, some third-party software sets the flag in its build, and it is trivial to support in lld.

Found via FreeBSD PR 230603, kstars failed to link with lld.

NOTE: In contrast to -z interpose I am not sure how useful this flag really is. If this change is not desired we'll make requests upstream to drop the flag in the link invocations.

[grimar]

NOTE: In contrast to -z interpose I am not sure how useful this flag really is. If this change is not desired we'll make requests upstream to drop the flag in the link invocations.

I'll leave it to Rui to decide then.

[markj]

dldump() was supposedly added to Solaris to support emacs: http://lists.gnu.org/archive/html/emacs-devel/2016-01/msg01022.html

It's not implemented by any other OS as far as I can see. I can't imagine that -z nodump sees much legitimate use these days.

[emaste]

Thanks for finding that Mark - based on that IMO this feature is not worth supporting.

[ruiu]

If no one really wants this, maybe you should abandon it.

[emaste]

Agreed - when I submitted it I thought it might be uncommon but worth supporting, but the evidence Mark found suggests it is a single-purpose feature that is not relevant to the platforms lld supports.

ELF/Config.h

struct Configuration {
   bool ZExecstack;
   bool ZGlobal;
   bool ZHazardplt;
   bool ZInitfirst;
   bool ZInterpose;
   bool ZKeepTextSectionPrefix;
   bool ZNodelete;
   bool ZNodlopen;
+  bool ZNodump;
   bool ZNow;
   bool ZOrigin;
   bool ZRelro;
   bool ZRodynamic;
   bool ZText;
   bool ZRetpolineplt;
   bool ZWxneeded;
   DiscardPolicy Discard;

ELF/Driver.cpp

 }
 
 static bool isKnown(StringRef S) {
   return S == "combreloc" || S == "copyreloc" || S == "defs" ||
          S == "execstack" || S == "global" || S == "hazardplt" ||
          S == "initfirst" || S == "interpose" ||
          S == "keep-text-section-prefix" || S == "lazy" || S == "muldefs" ||
          S == "nocombreloc" || S == "nocopyreloc" || S == "nodelete" ||
-         S == "nodlopen" || S == "noexecstack" ||
+         S == "nodlopen" || S == "nodump" || S == "noexecstack" ||
          S == "nokeep-text-section-prefix" || S == "norelro" || S == "notext" ||
          S == "now" || S == "origin" || S == "relro" || S == "retpolineplt" ||
          S == "rodynamic" || S == "text" || S == "wxneeded" ||
          S.startswith("max-page-size=") || S.startswith("stack-size=");
 }
 
 // Report an error for an unknown -z option.
 static void checkZOptions(opt::InputArgList &Args) {
void LinkerDriver::readConfigs(opt::InputArgList &Args) {
   Config->ZGlobal = hasZOption(Args, "global");
   Config->ZHazardplt = hasZOption(Args, "hazardplt");
   Config->ZInitfirst = hasZOption(Args, "initfirst");
   Config->ZInterpose = hasZOption(Args, "interpose");
   Config->ZKeepTextSectionPrefix = getZFlag(
       Args, "keep-text-section-prefix", "nokeep-text-section-prefix", false);
   Config->ZNodelete = hasZOption(Args, "nodelete");
   Config->ZNodlopen = hasZOption(Args, "nodlopen");
+  Config->ZNodump = hasZOption(Args, "nodump");
   Config->ZNow = getZFlag(Args, "now", "lazy", false);
   Config->ZOrigin = hasZOption(Args, "origin");
   Config->ZRelro = getZFlag(Args, "relro", "norelro", true);
   Config->ZRetpolineplt = hasZOption(Args, "retpolineplt");
   Config->ZRodynamic = hasZOption(Args, "rodynamic");
   Config->ZStackSize = args::getZOptionValue(Args, OPT_z, "stack-size", 0);
   Config->ZText = getZFlag(Args, "text", "notext", true);
   Config->ZWxneeded = hasZOption(Args, "wxneeded");

ELF/SyntheticSections.cpp

template <class ELFT> void DynamicSection<ELFT>::finalizeContents() {
   if (Config->ZInitfirst)
     DtFlags1 |= DF_1_INITFIRST;
   if (Config->ZInterpose)
     DtFlags1 |= DF_1_INTERPOSE;
   if (Config->ZNodelete)
     DtFlags1 |= DF_1_NODELETE;
   if (Config->ZNodlopen)
     DtFlags1 |= DF_1_NOOPEN;
+  if (Config->ZNodump)
+    DtFlags1 |= DF_1_NODUMP;
   if (Config->ZNow) {
     DtFlags |= DF_BIND_NOW;
     DtFlags1 |= DF_1_NOW;
   }
   if (Config->ZOrigin) {
     DtFlags |= DF_ORIGIN;
     DtFlags1 |= DF_1_ORIGIN;
   }

docs/ld.lld.1

 Set the
 .Dv DF_1_NODELETE
 flag to indicate that the object cannot be unloaded from a process.
 .It Cm nodlopen
 Set the
 .Dv DF_1_NOOPEN
 flag to indcate that the object may not be opened by
 .Xr dlopen 3 .
+.It Cm nodump
+Set the
+.Dv DF_1_NODUMP
+flag to indicate that the object may not be dumped by the runtime linker.
 .It Cm norelro
 Do not indicate that portions of the object shold be mapped read-only
 after initial relocation processing.
 The object will omit the
 .Dv PT_GNU_RELRO
 segment.
 .It Cm notext
 Allow relocations against read-only segments.

test/ELF/dt_flags.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 # RUN: ld.lld -shared %t -o %t.so
 
 # RUN: ld.lld -z global -z initfirst -z interpose -z now -z nodelete \
-# RUN:   -z nodlopen -z origin -Bsymbolic %t %t.so -o %t1
+# RUN:   -z nodlopen -z nodump -z origin -Bsymbolic %t %t.so -o %t1
 # RUN: llvm-readobj -dynamic-table %t1 | FileCheck -check-prefix=FLAGS %s
 
 # RUN: ld.lld %t %t.so -o %t2
 # RUN: llvm-readobj -dynamic-table %t2 | FileCheck %s
 
 # RUN: ld.lld -z lazy %t %t.so -o %t2
 # RUN: llvm-readobj -dynamic-table %t2 | FileCheck %s
 
 # FLAGS: DynamicSection [
 # FLAGS:   0x000000000000001E FLAGS ORIGIN SYMBOLIC BIND_NOW
-# FLAGS:   0x000000006FFFFFFB FLAGS_1 NOW GLOBAL NODELETE INITFIRST NOOPEN ORIGIN INTERPOSE
+# FLAGS:   0x000000006FFFFFFB FLAGS_1 NOW GLOBAL NODELETE INITFIRST NOOPEN ORIGIN INTERPOSE NODUMP
 # FLAGS: ]
 
 # CHECK: DynamicSection [
 # CHECK-NOT:   FLAGS
 # CHECK-NOT:   FLAGS_1
 # CHECK: ]
 
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