[elfabi] Add support for reading dynamic symbols from binaries

This patch adds initial support for reading dynamic symbols from ELF binaries. Currently, STT_NOTYPE, STT_OBJECT, STT_FUNC, and STT_TLS are explicitly supported. Other symbol types are mapped to ELFSymbolType::Unknown to improve signal/noise ratio.

Symbols must meet two criteria to be read into in an ELFStub:

 - The symbol's binding must be STB_GLOBAL or STB_WEAK.
 - The symbol's visibility must be STV_DEFAULT or STV_PROTECTED.

This filters out symbols that aren't of interest during compile-time linking against a shared object.

This change uses DT_HASH and DT_GNU_HASH to determine the size of .dynsym. Using hash tables to determine the number of symbols in .dynsym allows llvm-elfabi to work on binaries without relying on section headers.

Differential Revision: https://reviews.llvm.org/D56031

llvm-svn: 352121

1 files changed, 177 insertions, 2 deletions

diff --git a/llvm/tools/llvm-elfabi/ELFObjHandler.cpp b/llvm/tools/llvm-elfabi/ELFObjHandler.cpp
index 187a5910665..8f3b76ccc89 100644
--- a/llvm/tools/llvm-elfabi/ELFObjHandler.cpp
+++ b/llvm/tools/llvm-elfabi/ELFObjHandler.cpp
@@ -31,6 +31,11 @@ struct DynamicEntries {
   uint64_t StrSize = 0;
   Optional<uint64_t> SONameOffset;
   std::vector<uint64_t> NeededLibNames;
+  // Symbol table:
+  uint64_t DynSymAddr = 0;
+  // Hash tables:
+  Optional<uint64_t> ElfHash;
+  Optional<uint64_t> GnuHash;
 };
 
 /// This function behaves similarly to StringRef::substr(), but attempts to
@@ -81,6 +86,7 @@ static Error populateDynamic(DynamicEntries &Dyn,
   // Search .dynamic for relevant entries.
   bool FoundDynStr = false;
   bool FoundDynStrSz = false;
+  bool FoundDynSym = false;
   for (auto &Entry : DynTable) {
     switch (Entry.d_tag) {
     case DT_SONAME:
@@ -97,6 +103,15 @@ static Error populateDynamic(DynamicEntries &Dyn,
     case DT_NEEDED:
       Dyn.NeededLibNames.push_back(Entry.d_un.d_val);
       break;
+    case DT_SYMTAB:
+      Dyn.DynSymAddr = Entry.d_un.d_ptr;
+      FoundDynSym = true;
+      break;
+    case DT_HASH:
+      Dyn.ElfHash = Entry.d_un.d_ptr;
+      break;
+    case DT_GNU_HASH:
+      Dyn.GnuHash = Entry.d_un.d_ptr;
     }
   }
 
@@ -108,6 +123,10 @@ static Error populateDynamic(DynamicEntries &Dyn,
     return createError(
         "Couldn't determine dynamic string table size (no DT_STRSZ entry)");
   }
+  if (!FoundDynSym) {
+    return createError(
+        "Couldn't locate dynamic symbol table (no DT_SYMTAB entry)");
+  }
   if (Dyn.SONameOffset.hasValue() && *Dyn.SONameOffset >= Dyn.StrSize) {
     return createStringError(
         object_error::parse_failed,
@@ -126,6 +145,142 @@ static Error populateDynamic(DynamicEntries &Dyn,
   return Error::success();
 }
 
+/// This function finds the number of dynamic symbols using a GNU hash table.
+///
+/// @param Table The GNU hash table for .dynsym.
+template <class ELFT>
+static uint64_t getDynSymtabSize(const typename ELFT::GnuHash &Table) {
+  using Elf_Word = typename ELFT::Word;
+  if (Table.nbuckets == 0)
+    return Table.symndx + 1;
+  uint64_t LastSymIdx = 0;
+  uint64_t BucketVal = 0;
+  // Find the index of the first symbol in the last chain.
+  for (Elf_Word Val : Table.buckets()) {
+    BucketVal = std::max(BucketVal, (uint64_t)Val);
+  }
+  LastSymIdx += BucketVal;
+  const Elf_Word *It =
+      reinterpret_cast<const Elf_Word *>(Table.values(BucketVal).end());
+  // Locate the end of the chain to find the last symbol index.
+  while ((*It & 1) == 0) {
+    LastSymIdx++;
+    It++;
+  }
+  return LastSymIdx + 1;
+}
+
+/// This function determines the number of dynamic symbols.
+/// Without access to section headers, the number of symbols must be determined
+/// by parsing dynamic hash tables.
+///
+/// @param Dyn Entries with the locations of hash tables.
+/// @param ElfFile The ElfFile that the section contents reside in.
+template <class ELFT>
+static Expected<uint64_t> getNumSyms(DynamicEntries &Dyn,
+                                     const ELFFile<ELFT> &ElfFile) {
+  using Elf_Hash = typename ELFT::Hash;
+  using Elf_GnuHash = typename ELFT::GnuHash;
+  // Search GNU hash table to try to find the upper bound of dynsym.
+  if (Dyn.GnuHash.hasValue()) {
+    Expected<const uint8_t *> TablePtr = ElfFile.toMappedAddr(*Dyn.GnuHash);
+    if (!TablePtr)
+      return TablePtr.takeError();
+    const Elf_GnuHash *Table =
+        reinterpret_cast<const Elf_GnuHash *>(TablePtr.get());
+    return getDynSymtabSize<ELFT>(*Table);
+  }
+  // Search SYSV hash table to try to find the upper bound of dynsym.
+  if (Dyn.ElfHash.hasValue()) {
+    Expected<const uint8_t *> TablePtr = ElfFile.toMappedAddr(*Dyn.ElfHash);
+    if (!TablePtr)
+      return TablePtr.takeError();
+    const Elf_Hash *Table = reinterpret_cast<const Elf_Hash *>(TablePtr.get());
+    return Table->nchain;
+  }
+  return 0;
+}
+
+/// This function extracts symbol type from a symbol's st_info member and
+/// maps it to an ELFSymbolType enum.
+/// Currently, STT_NOTYPE, STT_OBJECT, STT_FUNC, and STT_TLS are supported.
+/// Other symbol types are mapped to ELFSymbolType::Unknown.
+///
+/// @param Info Binary symbol st_info to extract symbol type from.
+static ELFSymbolType convertInfoToType(uint8_t Info) {
+  Info = Info & 0xf;
+  switch (Info) {
+    case ELF::STT_NOTYPE:
+      return ELFSymbolType::NoType;
+    case ELF::STT_OBJECT:
+      return ELFSymbolType::Object;
+    case ELF::STT_FUNC:
+      return ELFSymbolType::Func;
+    case ELF::STT_TLS:
+      return ELFSymbolType::TLS;
+    default:
+      return ELFSymbolType::Unknown;
+  }
+}
+
+/// This function creates an ELFSymbol and populates all members using
+/// information from a binary ELFT::Sym.
+///
+/// @param SymName The desired name of the ELFSymbol.
+/// @param RawSym ELFT::Sym to extract symbol information from.
+template <class ELFT>
+static ELFSymbol createELFSym(StringRef SymName,
+                              const typename ELFT::Sym &RawSym) {
+  ELFSymbol TargetSym(SymName);
+  uint8_t Binding = RawSym.getBinding();
+  if (Binding == STB_WEAK)
+    TargetSym.Weak = true;
+  else
+    TargetSym.Weak = false;
+
+  TargetSym.Undefined = RawSym.isUndefined();
+  TargetSym.Type = convertInfoToType(RawSym.st_info);
+
+  if (TargetSym.Type == ELFSymbolType::Func) {
+    TargetSym.Size = 0;
+  } else {
+    TargetSym.Size = RawSym.st_size;
+  }
+  return TargetSym;
+}
+
+/// This function populates an ELFStub with symbols using information read
+/// from an ELF binary.
+///
+/// @param TargetStub ELFStub to add symbols to.
+/// @param DynSym Range of dynamic symbols to add to TargetStub.
+/// @param DynStr StringRef to the dynamic string table.
+template <class ELFT>
+static Error populateSymbols(ELFStub &TargetStub,
+                            const typename ELFT::SymRange DynSym,
+                            StringRef DynStr) {
+  // Skips the first symbol since it's the NULL symbol.
+  for (auto RawSym : DynSym.drop_front(1)) {
+    // If a symbol does not have global or weak binding, ignore it.
+    uint8_t Binding = RawSym.getBinding();
+    if (!(Binding == STB_GLOBAL || Binding == STB_WEAK))
+      continue;
+    // If a symbol doesn't have default or protected visibility, ignore it.
+    uint8_t Visibility = RawSym.getVisibility();
+    if (!(Visibility == STV_DEFAULT || Visibility == STV_PROTECTED))
+      continue;
+    // Create an ELFSymbol and populate it with information from the symbol
+    // table entry.
+    Expected<StringRef> SymName = terminatedSubstr(DynStr, RawSym.st_name);
+    if (!SymName)
+      return SymName.takeError();
+    ELFSymbol Sym = createELFSym<ELFT>(*SymName, RawSym);
+    TargetStub.Symbols.insert(std::move(Sym));
+    // TODO: Populate symbol warning.
+  }
+  return Error::success();
+}
+
 /// Returns a new ELFStub with all members populated from an ELFObjectFile.
 /// @param ElfObj Source ELFObjectFile.
 template <class ELFT>
@@ -133,6 +288,8 @@ static Expected<std::unique_ptr<ELFStub>>
 buildStub(const ELFObjectFile<ELFT> &ElfObj) {
   using Elf_Dyn_Range = typename ELFT::DynRange;
   using Elf_Phdr_Range = typename ELFT::PhdrRange;
+  using Elf_Sym_Range = typename ELFT::SymRange;
+  using Elf_Sym = typename ELFT::Sym;
   std::unique_ptr<ELFStub> DestStub = make_unique<ELFStub>();
   const ELFFile<ELFT> *ElfFile = ElfObj.getELFFile();
   // Fetch .dynamic table.
@@ -152,7 +309,7 @@ buildStub(const ELFObjectFile<ELFT> &ElfObj) {
   if (Error Err = populateDynamic<ELFT>(DynEnt, *DynTable))
     return std::move(Err);
 
-  // Convert .dynstr address to an offset.
+    // Get pointer to in-memory location of .dynstr section.
   Expected<const uint8_t *> DynStrPtr =
       ElfFile->toMappedAddr(DynEnt.StrTabAddr);
   if (!DynStrPtr)
@@ -185,7 +342,25 @@ buildStub(const ELFObjectFile<ELFT> &ElfObj) {
     DestStub->NeededLibs.push_back(*LibNameOrErr);
   }
 
-  // TODO: Populate Symbols from .dynsym table and linked string table.
+  // Populate Symbols from .dynsym table and dynamic string table.
+  Expected<uint64_t> SymCount = getNumSyms(DynEnt, *ElfFile);
+  if (!SymCount)
+    return SymCount.takeError();
+  if (*SymCount > 0) {
+    // Get pointer to in-memory location of .dynsym section.
+    Expected<const uint8_t *> DynSymPtr =
+        ElfFile->toMappedAddr(DynEnt.DynSymAddr);
+    if (!DynSymPtr)
+      return appendToError(DynSymPtr.takeError(),
+                           "when locating .dynsym section contents");
+    Elf_Sym_Range DynSyms =
+        ArrayRef<Elf_Sym>(reinterpret_cast<const Elf_Sym *>(*DynSymPtr),
+                          *SymCount);
+    Error SymReadError = populateSymbols<ELFT>(*DestStub, DynSyms, DynStr);
+    if (SymReadError)
+      return appendToError(std::move(SymReadError),
+                           "when reading dynamic symbols");
+  }
 
   return std::move(DestStub);
 }