[lld][AArch64][ELF][PAC] Support .relr.auth.dyn section

This re-applies #87635 with fix of issue described in
#87635 (comment).

We need to discard `rel(a).dyn` section when output is a relocatable object
file. The section is always empty in such a case (as well as both auth and
regular relr sections), and emitting that resulted in its `sh_info` equal to 0.
Section with zero index is always a NULL section according to ELF spec,
and lld is unable to run `ObjFile<ELFT>::getRelocTarget` against
relocation section with `sh_info` equal to zero (the ELF spec does not
seem to define behavior for such a corner case). This is only an issue
with relocatable object file output - having `sh_info` equal to zero for
shared objects is OK.

Original commit description below.

Support `R_AARCH64_AUTH_RELATIVE` relocation compression as described in
https://github.com/ARM-software/abi-aa/blob/main/pauthabielf64/pauthabielf64.rst#relocation-compression.

Author: kovdan01

lld/ELF/Arch/AArch64.cpp

@@ -429,6 +429,19 @@ void AArch64::relocate(uint8_t *loc, const Relocation &rel,
   case R_AARCH64_PREL64:
     write64(loc, val);
     break;
+  case R_AARCH64_AUTH_ABS64:
+    // If val is wider than 32 bits, the relocation must have been moved from
+    // .relr.auth.dyn to .rela.dyn, and the addend write is not needed.
+    //
+    // If val fits in 32 bits, we have two potential scenarios:
+    // * True RELR: Write the 32-bit `val`.
+    // * RELA: Even if the value now fits in 32 bits, it might have been
+    //   converted from RELR during an iteration in
+    //   finalizeAddressDependentContent(). Writing the value is harmless
+    //   because dynamic linking ignores it.
+    if (isInt<32>(val))
+      write32(loc, val);
+    break;
   case R_AARCH64_ADD_ABS_LO12_NC:
     or32AArch64Imm(loc, val);
     break;

lld/ELF/Relocations.cpp

@@ -898,9 +898,9 @@ static void addRelativeReloc(InputSectionBase &isec, uint64_t offsetInSec,
     isec.addReloc({expr, type, offsetInSec, addend, &sym});
     if (shard)
       part.relrDyn->relocsVec[parallel::getThreadIndex()].push_back(
-          {&isec, offsetInSec});
+          {&isec, isec.relocs().size() - 1});
     else
-      part.relrDyn->relocs.push_back({&isec, offsetInSec});
+      part.relrDyn->relocs.push_back({&isec, isec.relocs().size() - 1});
     return;
   }
   part.relaDyn->addRelativeReloc<shard>(target->relativeRel, isec, offsetInSec,
@@ -1154,6 +1154,12 @@ void RelocationScanner::processAux(RelExpr expr, RelType type, uint64_t offset,
         // relative relocation. Use a symbolic relocation instead.
         if (sym.isPreemptible) {
           part.relaDyn->addSymbolReloc(type, *sec, offset, sym, addend, type);
+        } else if (part.relrAuthDyn && sec->addralign >= 2 && offset % 2 == 0) {
+          // When symbol values are determined in
+          // finalizeAddressDependentContent, some .relr.auth.dyn relocations
+          // may be moved to .rela.dyn.
+          sec->addReloc({expr, type, offset, addend, &sym});
+          part.relrAuthDyn->relocs.push_back({sec, sec->relocs().size() - 1});
         } else {
           part.relaDyn->addReloc({R_AARCH64_AUTH_RELATIVE, sec, offset,
                                   DynamicReloc::AddendOnlyWithTargetVA, sym,

lld/ELF/SyntheticSections.cpp

@@ -1420,6 +1420,12 @@ DynamicSection<ELFT>::computeContents() {
     addInt(config->useAndroidRelrTags ? DT_ANDROID_RELRENT : DT_RELRENT,
            sizeof(Elf_Relr));
   }
+  if (part.relrAuthDyn && part.relrAuthDyn->getParent() &&
+      !part.relrAuthDyn->relocs.empty()) {
+    addInSec(DT_AARCH64_AUTH_RELR, *part.relrAuthDyn);
+    addInt(DT_AARCH64_AUTH_RELRSZ, part.relrAuthDyn->getParent()->size);
+    addInt(DT_AARCH64_AUTH_RELRENT, sizeof(Elf_Relr));
+  }
   if (isMain && in.relaPlt->isNeeded()) {
     addInSec(DT_JMPREL, *in.relaPlt);
     entries.emplace_back(DT_PLTRELSZ, addPltRelSz());
@@ -1731,10 +1737,13 @@ template <class ELFT> void RelocationSection<ELFT>::writeTo(uint8_t *buf) {
   }
 }
 
-RelrBaseSection::RelrBaseSection(unsigned concurrency)
-    : SyntheticSection(SHF_ALLOC,
-                       config->useAndroidRelrTags ? SHT_ANDROID_RELR : SHT_RELR,
-                       config->wordsize, ".relr.dyn"),
+RelrBaseSection::RelrBaseSection(unsigned concurrency, bool isAArch64Auth)
+    : SyntheticSection(
+          SHF_ALLOC,
+          isAArch64Auth
+              ? SHT_AARCH64_AUTH_RELR
+              : (config->useAndroidRelrTags ? SHT_ANDROID_RELR : SHT_RELR),
+          config->wordsize, isAArch64Auth ? ".relr.auth.dyn" : ".relr.dyn"),
       relocsVec(concurrency) {}
 
 void RelrBaseSection::mergeRels() {
@@ -2002,8 +2011,8 @@ bool AndroidPackedRelocationSection<ELFT>::updateAllocSize() {
 }
 
 template <class ELFT>
-RelrSection<ELFT>::RelrSection(unsigned concurrency)
-    : RelrBaseSection(concurrency) {
+RelrSection<ELFT>::RelrSection(unsigned concurrency, bool isAArch64Auth)
+    : RelrBaseSection(concurrency, isAArch64Auth) {
   this->entsize = config->wordsize;
 }
 
@@ -4774,6 +4783,9 @@ template <class ELFT> void elf::createSyntheticSections() {
     if (config->relrPackDynRelocs) {
       part.relrDyn = std::make_unique<RelrSection<ELFT>>(threadCount);
       add(*part.relrDyn);
+      part.relrAuthDyn = std::make_unique<RelrSection<ELFT>>(
+          threadCount, /*isAArch64Auth=*/true);
+      add(*part.relrAuthDyn);
     }
 
     if (!config->relocatable) {

lld/ELF/SyntheticSections.h

@@ -548,7 +548,9 @@ class RelocationBaseSection : public SyntheticSection {
   static bool classof(const SectionBase *d) {
     return SyntheticSection::classof(d) &&
            (d->type == llvm::ELF::SHT_RELA || d->type == llvm::ELF::SHT_REL ||
-            d->type == llvm::ELF::SHT_RELR);
+            d->type == llvm::ELF::SHT_RELR ||
+            (d->type == llvm::ELF::SHT_AARCH64_AUTH_RELR &&
+             config->emachine == llvm::ELF::EM_AARCH64));
   }
   int32_t dynamicTag, sizeDynamicTag;
   SmallVector<DynamicReloc, 0> relocs;
@@ -596,15 +598,17 @@ class AndroidPackedRelocationSection final : public RelocationBaseSection {
 };
 
 struct RelativeReloc {
-  uint64_t getOffset() const { return inputSec->getVA(offsetInSec); }
+  uint64_t getOffset() const {
+    return inputSec->getVA(inputSec->relocs()[relocIdx].offset);
+  }
 
   const InputSectionBase *inputSec;
-  uint64_t offsetInSec;
+  size_t relocIdx;
 };
 
 class RelrBaseSection : public SyntheticSection {
 public:
-  RelrBaseSection(unsigned concurrency);
+  RelrBaseSection(unsigned concurrency, bool isAArch64Auth = false);
   void mergeRels();
   bool isNeeded() const override {
     return !relocs.empty() ||
@@ -622,7 +626,7 @@ template <class ELFT> class RelrSection final : public RelrBaseSection {
   using Elf_Relr = typename ELFT::Relr;
 
 public:
-  RelrSection(unsigned concurrency);
+  RelrSection(unsigned concurrency, bool isAArch64Auth = false);
 
   bool updateAllocSize() override;
   size_t getSize() const override { return relrRelocs.size() * this->entsize; }
@@ -1460,6 +1464,7 @@ struct Partition {
   std::unique_ptr<PackageMetadataNote> packageMetadataNote;
   std::unique_ptr<RelocationBaseSection> relaDyn;
   std::unique_ptr<RelrBaseSection> relrDyn;
+  std::unique_ptr<RelrBaseSection> relrAuthDyn;
   std::unique_ptr<VersionDefinitionSection> verDef;
   std::unique_ptr<SyntheticSection> verNeed;
   std::unique_ptr<VersionTableSection> verSym;

lld/ELF/Writer.cpp

@@ -1458,9 +1458,32 @@ template <class ELFT> void Writer<ELFT>::finalizeAddressDependentContent() {
       in.mipsGot->updateAllocSize();
 
     for (Partition &part : partitions) {
+      // The R_AARCH64_AUTH_RELATIVE has a smaller addend field as bits [63:32]
+      // encode the signing schema. We've put relocations in .relr.auth.dyn
+      // during RelocationScanner::processAux, but the target VA for some of
+      // them might be wider than 32 bits. We can only know the final VA at this
+      // point, so move relocations with large values from .relr.auth.dyn to
+      // .rela.dyn. See also AArch64::relocate.
+      if (part.relrAuthDyn) {
+        auto it = llvm::remove_if(
+            part.relrAuthDyn->relocs, [&part](const RelativeReloc &elem) {
+              const Relocation &reloc = elem.inputSec->relocs()[elem.relocIdx];
+              if (isInt<32>(reloc.sym->getVA(reloc.addend)))
+                return false;
+              part.relaDyn->addReloc({R_AARCH64_AUTH_RELATIVE, elem.inputSec,
+                                      reloc.offset,
+                                      DynamicReloc::AddendOnlyWithTargetVA,
+                                      *reloc.sym, reloc.addend, R_ABS});
+              return true;
+            });
+        changed |= (it != part.relrAuthDyn->relocs.end());
+        part.relrAuthDyn->relocs.erase(it, part.relrAuthDyn->relocs.end());
+      }
       changed |= part.relaDyn->updateAllocSize();
       if (part.relrDyn)
         changed |= part.relrDyn->updateAllocSize();
+      if (part.relrAuthDyn)
+        changed |= part.relrAuthDyn->updateAllocSize();
       if (part.memtagGlobalDescriptors)
         changed |= part.memtagGlobalDescriptors->updateAllocSize();
     }
@@ -1624,6 +1647,15 @@ static void removeUnusedSyntheticSections() {
         auto *sec = cast<SyntheticSection>(s);
         if (sec->getParent() && sec->isNeeded())
           return false;
+        // .relr.auth.dyn relocations may be moved to .rela.dyn in
+        // finalizeAddressDependentContent, making .rela.dyn no longer empty.
+        // Conservatively keep .rela.dyn. .relr.auth.dyn can be made empty, but
+        // we would fail to remove it here.
+        if (config->emachine == EM_AARCH64 && config->relrPackDynRelocs &&
+            !config->relocatable)
+          if (auto *relSec = dyn_cast<RelocationBaseSection>(sec))
+            if (relSec == mainPart->relaDyn.get())
+              return false;
         unused.insert(sec);
         return true;
       });
@@ -1936,6 +1968,10 @@ template <class ELFT> void Writer<ELFT>::finalizeSections() {
         part.relrDyn->mergeRels();
         finalizeSynthetic(part.relrDyn.get());
       }
+      if (part.relrAuthDyn) {
+        part.relrAuthDyn->mergeRels();
+        finalizeSynthetic(part.relrAuthDyn.get());
+      }
 
       finalizeSynthetic(part.dynSymTab.get());
       finalizeSynthetic(part.gnuHashTab.get());