[ELF] Support input section description .gnu.version* in /DISCARD/

[MaskRay]

Summary

Linux powerpc discards *(.gnu.version*) (arch/powerpc/kernel/vmlinux.lds.S)
to suppress --orphan-handling=warn warnings in the -pie output .tmp_vmlinux1

The support is simple. Just add isLive() to:

1. Fix an assertion in SectionBase::getPartition() called by VersionTableSection::isNeeded().
2. Suppress DT_VERSYM, DT_VERDEF, DT_VERNEED and DT_VERNEEDNUM, if the relevant section is discarded.

[grimar]

LGTM

[MaskRay]

Suppress DT_VER* if the relevant section is discarded.

Coincidentally, someone asked https://sourceware.org/ml/binutils/2019-12/msg00329.html

With this patch, that use case can be satisified with:

/DISCARD/ : { *(.gnu.version*) }

[grimar]

lld/ELF/SyntheticSections.cpp

1440

Seems you can just use if (part.verSym->isNeeded()) here and if (part.verNeed->isNeeded()) below?

if (part.verSym->isNeeded())
  addInSec(DT_VERSYM, part.verSym);
if (part.verDef) {
  addInSec(DT_VERDEF, part.verDef);
  addInt(DT_VERDEFNUM, getVerDefNum());
}
if (part.verNeed->isNeeded()) {
 ...
}

lld/test/ELF/linkerscript/discard-gnu-version.s

25

You need to specify -d here and below I think to dump the dynamic table.

[MaskRay]

Simplify DT_VERSYM
Fix DT_VERDEF

[grimar]

lld/ELF/SyntheticSections.cpp

1446

Can't you remove hasVerNeed variable and use verNeed->isNeeded() here too?

[grimar]

lld/ELF/SyntheticSections.cpp

1446

You perhaps need to add isLive() condition to bool VersionNeedSection<ELFT>::isNeeded() for that.

[MaskRay]

move isLive() to VersionNeedSection<ELFT>::isNeeded()

lld/ELF/SyntheticSections.cpp

1446

Yes.. I was a bit hesitant whether we should move isLive() to VersionNeedSection<ELFT>::isNeeded(). Note that we don't check isLive() in SyntheticSection::isNeeded(), the base member function.

It seems moving isLive() makes the code look better. So I will do that.

[grimar]

LGTM

lld/ELF/SyntheticSections.cpp

if (isMain) {
     if (Symbol *b = symtab->find(config->init))
       if (b->isDefined())
         addSym(DT_INIT, b);
     if (Symbol *b = symtab->find(config->fini))
       if (b->isDefined())
         addSym(DT_FINI, b);
   }
 
-  bool hasVerNeed = SharedFile::vernauxNum != 0;
-  if (hasVerNeed || part.verDef)
+  if (part.verSym && part.verSym->isNeeded())
     addInSec(DT_VERSYM, part.verSym);
-  if (part.verDef) {
+  if (part.verDef && part.verDef->isLive()) {
     addInSec(DT_VERDEF, part.verDef);
     addInt(DT_VERDEFNUM, getVerDefNum());
   }
-  if (hasVerNeed) {
+  if (part.verNeed && part.verNeed->isNeeded()) {
     addInSec(DT_VERNEED, part.verNeed);
     unsigned needNum = 0;
     for (SharedFile *f : sharedFiles)
       if (!f->vernauxs.empty())
         ++needNum;
     addInt(DT_VERNEEDNUM, needNum);
   }
 
void VersionTableSection::writeTo(uint8_t *buf) {
   buf += 2;
   for (const SymbolTableEntry &s : getPartition().dynSymTab->getSymbols()) {
     write16(buf, s.sym->versionId);
     buf += 2;
   }
 }
 
 bool VersionTableSection::isNeeded() const {
-  return getPartition().verDef || getPartition().verNeed->isNeeded();
+  return isLive() &&
+         (getPartition().verDef || getPartition().verNeed->isNeeded());
 }
 
 void addVerneed(Symbol *ss) {
   auto &file = cast<SharedFile>(*ss->file);
   if (ss->verdefIndex == VER_NDX_GLOBAL) {
     ss->versionId = VER_NDX_GLOBAL;
     return;
   }

 }
 
 template <class ELFT> size_t VersionNeedSection<ELFT>::getSize() const {
   return verneeds.size() * sizeof(Elf_Verneed) +
          SharedFile::vernauxNum * sizeof(Elf_Vernaux);
 }
 
 template <class ELFT> bool VersionNeedSection<ELFT>::isNeeded() const {
-  return SharedFile::vernauxNum != 0;
+  return isLive() && SharedFile::vernauxNum != 0;
 }
 
 void MergeSyntheticSection::addSection(MergeInputSection *ms) {
   ms->parent = this;
   sections.push_back(ms);
   assert(alignment == ms->alignment || !(ms->flags & SHF_STRINGS));
   alignment = std::max(alignment, ms->alignment);
 }

lld/test/ELF/linkerscript/discard-gnu-version.s

+# REQUIRES: x86
+# RUN: echo '.globl f; f:' | llvm-mc -filetype=obj -triple=x86_64 - -o %t1.o
+# RUN: echo 'v1 { f; };' > %t1.ver
+# RUN: ld.lld -shared --version-script %t1.ver %t1.o -o %t1.so
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o
+# RUN: echo 'v1 { foo; };' > %t.ver
+# RUN: ld.lld -shared --version-script %t.ver %t.o %t1.so -o %t.so
+# RUN: llvm-readelf -S -d %t.so | FileCheck --check-prefixes=SYM,DEF,NEED %s
+
+# SYM:  .gnu.version
+# DEF:  .gnu.version_d
+# NEED: .gnu.version_r
+
+# SYM:  (VERSYM)
+# DEF:  (VERDEF)
+# DEF:  (VERDEFNUM)
+# NEED: (VERNEED)
+# NEED: (VERNEEDNUM)
+
+## Discard all of .gnu.version*
+# RUN: echo 'SECTIONS { /DISCARD/ : { *(.gnu.version*) } }' > %t.script
+# RUN: ld.lld -shared --version-script %t.ver -T %t.script %t.o %t1.so -o %t.so
+# RUN: llvm-readelf -S -d %t.so | FileCheck /dev/null \
+# RUN:   --implicit-check-not='(VER' --implicit-check-not=.gnu.version
+
+## Discard .gnu.version
+# RUN: echo 'SECTIONS { /DISCARD/ : { *(.gnu.version) } }' > %t.noversym.script
+# RUN: ld.lld -shared --version-script %t.ver -T %t.noversym.script %t.o %t1.so -o %t.noversym.so
+# RUN: llvm-readelf -S -d %t.noversym.so | FileCheck --check-prefixes=DEF,NEED %s \
+# RUN:   --implicit-check-not='(VER' --implicit-check-not=.gnu.version
+
+## Discard .gnu.version_d
+# RUN: echo 'SECTIONS { /DISCARD/ : { *(.gnu.version_d) } }' > %t.noverdef.script
+# RUN: ld.lld -shared --version-script %t.ver -T %t.noverdef.script %t.o %t1.so -o %t.noverdef.so
+# RUN: llvm-readelf -S -d %t.noverdef.so | FileCheck --check-prefixes=SYM,NEED %s \
+# RUN:   --implicit-check-not='(VER' --implicit-check-not=.gnu.version
+
+## Discard .gnu.version_r
+# RUN: echo 'SECTIONS { /DISCARD/ : { *(.gnu.version_r) } }' > %t.noverneed.script
+# RUN: ld.lld -shared --version-script %t.ver -T %t.noverneed.script %t.o %t1.so -o %t.noverneed.so
+# RUN: llvm-readelf -S -d %t.noverneed.so | FileCheck --check-prefixes=SYM,DEF %s \
+# RUN:   --implicit-check-not='(VER' --implicit-check-not=.gnu.version
+
+.globl foo
+foo:
+  call f@plt