[ELF] Merge relaIplt into relaDyn

`relaIplt` was added so that IRELATIVE relocations are placed at the end
of .rela.dyn (since https://reviews.llvm.org/D65651) or .rela.plt
(--pack-dyn-relocs=android[+relr]). Unfortunately, handling `relaIplt`
requires special cases all over the code base. We can extend
partitionRels/computeRels to partition both RELATIVE and IRELATIVE
relocations, rendering `relaIplt` unneeded.

The change allows IRELATIVE relocations in the DT_ANDROID_REL[A] table
(untested?!), which may be processed before other types of relocations.
This seems acceptable for Bionic's DEFINE_IFUNC_FOR use cases.

In addition, this change simplies changing .rel[a].dyn to a compact
relocation format (CREL).

SHF_INFO_LINK is removed from .rel[a].dyn with IRELATIVE relocations.
(See https://reviews.llvm.org/D89828).

Author: MaskRay

lld/ELF/Relocations.cpp

@@ -1620,12 +1620,8 @@ static bool handleNonPreemptibleIfunc(Symbol &sym, uint16_t flags) {
   //   relatively straightforward. We create a PLT entry in Iplt, which is
   //   usually at the end of .plt, which makes an indirect call using a
   //   matching GOT entry in igotPlt, which is usually at the end of .got.plt.
-  //   The GOT entry is relocated using an IRELATIVE relocation in relaIplt,
-  //   which is usually at the end of .rela.plt. Unlike most relocations in
-  //   .rela.plt, which may be evaluated lazily without -z now, dynamic
-  //   loaders evaluate IRELATIVE relocs eagerly, which means that for
-  //   IRELATIVE relocs only, GOT-generating relocations can point directly to
-  //   .got.plt without requiring a separate GOT entry.
+  //   The GOT entry is relocated using an IRELATIVE relocation in relaDyn,
+  //   which is usually at the end of .rela.dyn.
   //
   // - Despite the fact that an ifunc does not have a fixed value, compilers
   //   that are not passed -fPIC will assume that they do, and will emit
@@ -1665,7 +1661,7 @@ static bool handleNonPreemptibleIfunc(Symbol &sym, uint16_t flags) {
   // section/value fixed.
   auto *directSym = makeDefined(cast<Defined>(sym));
   directSym->allocateAux();
-  addPltEntry(*in.iplt, *in.igotPlt, *in.relaIplt, target->iRelativeRel,
+  addPltEntry(*in.iplt, *in.igotPlt, *mainPart->relaDyn, target->iRelativeRel,
               *directSym);
   sym.allocateAux();
   symAux.back().pltIdx = symAux[directSym->auxIdx].pltIdx;

lld/ELF/SyntheticSections.cpp

@@ -1267,15 +1267,12 @@ DynamicSection<ELFT>::DynamicSection()
 // The output section .rela.dyn may include these synthetic sections:
 //
 // - part.relaDyn
-// - in.relaIplt: this is included if in.relaIplt is named .rela.dyn
 // - in.relaPlt: this is included if a linker script places .rela.plt inside
 //   .rela.dyn
 //
 // DT_RELASZ is the total size of the included sections.
 static uint64_t addRelaSz(const RelocationBaseSection &relaDyn) {
   size_t size = relaDyn.getSize();
-  if (in.relaIplt->getParent() == relaDyn.getParent())
-    size += in.relaIplt->getSize();
   if (in.relaPlt->getParent() == relaDyn.getParent())
     size += in.relaPlt->getSize();
   return size;
@@ -1372,9 +1369,7 @@ DynamicSection<ELFT>::computeContents() {
   if (!config->shared && !config->relocatable && !config->zRodynamic)
     addInt(DT_DEBUG, 0);
 
-  if (part.relaDyn->isNeeded() ||
-      (in.relaIplt->isNeeded() &&
-       part.relaDyn->getParent() == in.relaIplt->getParent())) {
+  if (part.relaDyn->isNeeded()) {
     addInSec(part.relaDyn->dynamicTag, *part.relaDyn);
     entries.emplace_back(part.relaDyn->sizeDynamicTag,
                          addRelaSz(*part.relaDyn));
@@ -1657,10 +1652,6 @@ void RelocationBaseSection::finalizeContents() {
     getParent()->flags |= ELF::SHF_INFO_LINK;
     getParent()->info = in.gotPlt->getParent()->sectionIndex;
   }
-  if (in.relaIplt.get() == this && in.igotPlt->getParent()) {
-    getParent()->flags |= ELF::SHF_INFO_LINK;
-    getParent()->info = in.igotPlt->getParent()->sectionIndex;
-  }
 }
 
 void DynamicReloc::computeRaw(SymbolTableBaseSection *symtab) {
@@ -1674,6 +1665,11 @@ void RelocationBaseSection::computeRels() {
   SymbolTableBaseSection *symTab = getPartition().dynSymTab.get();
   parallelForEach(relocs,
                   [symTab](DynamicReloc &rel) { rel.computeRaw(symTab); });
+
+  auto irelative = std::partition(
+      relocs.begin() + numRelativeRelocs, relocs.end(),
+      [t = target->iRelativeRel](auto &r) { return r.type != t; });
+
   // Sort by (!IsRelative,SymIndex,r_offset). DT_REL[A]COUNT requires us to
   // place R_*_RELATIVE first. SymIndex is to improve locality, while r_offset
   // is to make results easier to read.
@@ -1682,7 +1678,7 @@ void RelocationBaseSection::computeRels() {
     parallelSort(relocs.begin(), nonRelative,
                  [&](auto &a, auto &b) { return a.r_offset < b.r_offset; });
     // Non-relative relocations are few, so don't bother with parallelSort.
-    llvm::sort(nonRelative, relocs.end(), [&](auto &a, auto &b) {
+    llvm::sort(nonRelative, irelative, [&](auto &a, auto &b) {
       return std::tie(a.r_sym, a.r_offset) < std::tie(b.r_sym, b.r_offset);
     });
   }
@@ -3843,7 +3839,6 @@ void InStruct::reset() {
   ppc32Got2.reset();
   ibtPlt.reset();
   relaPlt.reset();
-  relaIplt.reset();
   shStrTab.reset();
   strTab.reset();
   symTab.reset();

lld/ELF/SyntheticSections.h

@@ -1358,7 +1358,6 @@ struct InStruct {
   std::unique_ptr<PPC32Got2Section> ppc32Got2;
   std::unique_ptr<IBTPltSection> ibtPlt;
   std::unique_ptr<RelocationBaseSection> relaPlt;
-  std::unique_ptr<RelocationBaseSection> relaIplt;
   std::unique_ptr<StringTableSection> shStrTab;
   std::unique_ptr<StringTableSection> strTab;
   std::unique_ptr<SymbolTableBaseSection> symTab;

lld/ELF/Writer.cpp

@@ -449,8 +449,8 @@ template <class ELFT> void elf::createSyntheticSections() {
 
       add(*part.dynamic);
       add(*part.dynStrTab);
-      add(*part.relaDyn);
     }
+    add(*part.relaDyn);
 
     if (config->relrPackDynRelocs) {
       part.relrDyn = std::make_unique<RelrSection<ELFT>>(threadCount);
@@ -550,17 +550,6 @@ template <class ELFT> void elf::createSyntheticSections() {
       /*threadCount=*/1);
   add(*in.relaPlt);
 
-  // The relaIplt immediately follows .rel[a].dyn to ensure that the IRelative
-  // relocations are processed last by the dynamic loader. We cannot place the
-  // iplt section in .rel.dyn when Android relocation packing is enabled because
-  // that would cause a section type mismatch. However, because the Android
-  // dynamic loader reads .rel.plt after .rel.dyn, we can get the desired
-  // behaviour by placing the iplt section in .rel.plt.
-  in.relaIplt = std::make_unique<RelocationSection<ELFT>>(
-      config->androidPackDynRelocs ? in.relaPlt->name : relaDynName,
-      /*sort=*/false, /*threadCount=*/1);
-  add(*in.relaIplt);
-
   if ((config->emachine == EM_386 || config->emachine == EM_X86_64) &&
       (config->andFeatures & GNU_PROPERTY_X86_FEATURE_1_IBT)) {
     in.ibtPlt = std::make_unique<IBTPltSection>();
@@ -1071,20 +1060,18 @@ void PhdrEntry::add(OutputSection *sec) {
     sec->ptLoad = this;
 }
 
-// The beginning and the ending of .rel[a].plt section are marked
-// with __rel[a]_iplt_{start,end} symbols if it is a statically linked
-// executable. The runtime needs these symbols in order to resolve
-// all IRELATIVE relocs on startup. For dynamic executables, we don't
-// need these symbols, since IRELATIVE relocs are resolved through GOT
-// and PLT. For details, see http://www.airs.com/blog/archives/403.
+// A statically linked position-dependent executable should only contain
+// IRELATIVE relocations and no other dynamic relocations. Encapsulation symbols
+// __rel[a]_iplt_{start,end} will be defined for .rel[a].dyn, to be
+// processed by the libc runtime. Other executables or DSOs use dynamic tags
+// instead.
 template <class ELFT> void Writer<ELFT>::addRelIpltSymbols() {
   if (config->isPic)
     return;
 
-  // By default, __rela_iplt_{start,end} belong to a dummy section 0
-  // because .rela.plt might be empty and thus removed from output.
-  // We'll override Out::elfHeader with In.relaIplt later when we are
-  // sure that .rela.plt exists in output.
+  // __rela_iplt_{start,end} are initially defined relative to dummy section 0.
+  // We'll override Out::elfHeader with relaDyn later when we are sure that
+  // .rela.dyn will be present in the output.
   ElfSym::relaIpltStart = addOptionalRegular(
       config->isRela ? "__rela_iplt_start" : "__rel_iplt_start",
       Out::elfHeader, 0, STV_HIDDEN);
@@ -1110,11 +1097,11 @@ template <class ELFT> void Writer<ELFT>::setReservedSymbolSections() {
     ElfSym::globalOffsetTable->section = sec;
   }
 
-  // .rela_iplt_{start,end} mark the start and the end of in.relaIplt.
-  if (ElfSym::relaIpltStart && in.relaIplt->isNeeded()) {
-    ElfSym::relaIpltStart->section = in.relaIplt.get();
-    ElfSym::relaIpltEnd->section = in.relaIplt.get();
-    ElfSym::relaIpltEnd->value = in.relaIplt->getSize();
+  // .rela_iplt_{start,end} mark the start and the end of .rel[a].dyn.
+  if (ElfSym::relaIpltStart && mainPart->relaDyn->isNeeded()) {
+    ElfSym::relaIpltStart->section = mainPart->relaDyn.get();
+    ElfSym::relaIpltEnd->section = mainPart->relaDyn.get();
+    ElfSym::relaIpltEnd->value = mainPart->relaDyn->getSize();
   }
 
   PhdrEntry *last = nullptr;
@@ -1470,14 +1457,6 @@ static void sortSection(OutputSection &osec,
   if (name == ".init" || name == ".fini")
     return;
 
-  // IRelative relocations that usually live in the .rel[a].dyn section should
-  // be processed last by the dynamic loader. To achieve that we add synthetic
-  // sections in the required order from the beginning so that the in.relaIplt
-  // section is placed last in an output section. Here we just do not apply
-  // sorting for an output section which holds the in.relaIplt section.
-  if (in.relaIplt->getParent() == &osec)
-    return;
-
   // Sort input sections by priority using the list provided by
   // --symbol-ordering-file or --shuffle-sections=. This is a least significant
   // digit radix sort. The sections may be sorted stably again by a more
@@ -2196,7 +2175,6 @@ template <class ELFT> void Writer<ELFT>::finalizeSections() {
     finalizeSynthetic(in.mipsGot.get());
     finalizeSynthetic(in.igotPlt.get());
     finalizeSynthetic(in.gotPlt.get());
-    finalizeSynthetic(in.relaIplt.get());
     finalizeSynthetic(in.relaPlt.get());
     finalizeSynthetic(in.plt.get());
     finalizeSynthetic(in.iplt.get());

lld/test/ELF/aarch64-gnu-ifunc.s

@@ -11,13 +11,12 @@
 // CHECK-NEXT:  Type: SHT_RELA
 // CHECK-NEXT:  Flags [
 // CHECK-NEXT:    SHF_ALLOC
-// CHECK-NEXT:    SHF_INFO_LINK
 // CHECK-NEXT:  ]
 // CHECK-NEXT:  Address: [[RELA:.*]]
 // CHECK-NEXT:  Offset: 0x158
 // CHECK-NEXT:  Size: 48
 // CHECK-NEXT:  Link: 0
-// CHECK-NEXT:  Info: 4
+// CHECK-NEXT:  Info: 0
 // CHECK-NEXT:  AddressAlignment: 8
 // CHECK-NEXT:  EntrySize: 24
 // CHECK-NEXT: }

lld/test/ELF/arm-gnu-ifunc.s

@@ -30,13 +30,12 @@ _start:
 // CHECK-NEXT:     Type: SHT_REL
 // CHECK-NEXT:     Flags [
 // CHECK-NEXT:       SHF_ALLOC
-// CHECK-NEXT:       SHF_INFO_LINK
 // CHECK-NEXT:     ]
 // CHECK-NEXT:     Address: 0x100F4
 // CHECK-NEXT:     Offset: 0xF4
 // CHECK-NEXT:     Size: 16
 // CHECK-NEXT:     Link:
-// CHECK-NEXT:     Info: 4
+// CHECK-NEXT:     Info: 0
 // CHECK:          Name: .iplt
 // CHECK-NEXT:     Type: SHT_PROGBITS
 // CHECK-NEXT:     Flags [

lld/test/ELF/gnu-ifunc-i386.s

@@ -11,13 +11,12 @@
 // CHECK-NEXT:  Type: SHT_REL
 // CHECK-NEXT:  Flags [
 // CHECK-NEXT:    SHF_ALLOC
-// CHECK-NEXT:    SHF_INFO_LINK
 // CHECK-NEXT:  ]
 // CHECK-NEXT:  Address: [[RELA:.*]]
 // CHECK-NEXT:  Offset: 0xD4
 // CHECK-NEXT:  Size: 16
 // CHECK-NEXT:  Link: 0
-// CHECK-NEXT:  Info: 4
+// CHECK-NEXT:  Info: 0
 // CHECK-NEXT:  AddressAlignment: 4
 // CHECK-NEXT:  EntrySize: 8
 // CHECK-NEXT: }

lld/test/ELF/systemz-ifunc-nonpreemptible.s

@@ -10,7 +10,7 @@
 # CHECK:      Section Headers:
 # CHECK-NEXT:  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
 # CHECK-NEXT:  [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
-# CHECK-NEXT:  [ 1] .rela.dyn         RELA            0000000001000158 000158 000030 18  AI  0   4  8
+# CHECK-NEXT:  [ 1] .rela.dyn         RELA            0000000001000158 000158 000030 18   A  0   0  8
 # CHECK-NEXT:  [ 2] .text             PROGBITS        0000000001001188 000188 00001c 00  AX  0   0  4
 # CHECK-NEXT:  [ 3] .iplt             PROGBITS        00000000010011b0 0001b0 000040 00  AX  0   0 16
 # CHECK-NEXT:  [ 4] .got.plt          PROGBITS        00000000010021f0 0001f0 000010 00  WA  0   0  8