[llvm-readobj] - Fix a crash scenario in GNUStyle<ELFT>::printHashSymbols().

[grimar]

Summary

We might crash when the dynamic symbols table is empty and --hash-symbols
is requested. Both .hash and .gnu.hash logic is affected.

The patch fixes this issue.

[jhenderson]

llvm/test/tools/llvm-readobj/ELF/hash-symbols.test

517

to produce -> return

(I think it reads cleaner that way. It would be just "produce" anyway).

llvm/tools/llvm-readobj/ELFDumper.cpp

4080

I'm not sure if this does this, but I think we need to distinguish between the dynsym being actually empty (i.e. sh_size == 0 specified by the .dynsym section header) and we just don't know it's empty (i.e. there's no section header at all, but there is a DT_SYMTAB tag). See rGb259ce99 for context. If this code already does that, what in the test ensures we haven't broken this situation? I'm having a bit of a struggle remembering how that change worked, so this might not be relevant at all.

[grimar]

• Addressed review comments.

llvm/tools/llvm-readobj/ELFDumper.cpp

4080

dynamic_symbols returns an empty range for both cases when either there is no DynSymRegion at all or when it is found, but empty:

Elf_Sym_Range dynamic_symbols() const {
  if (!DynSymRegion)
    return Elf_Sym_Range();
  return DynSymRegion->getAsArrayRef<Elf_Sym>();
}

To distinguish between these cases we can additionally check the DynSymRegion when reporting a warning. I've did it and added test cases.

[jhenderson]

LGTM, thanks!

llvm/test/tools/llvm-readobj/ELF/hash-symbols.test

DynamicSymbols:
   - Name:    ddd
     Binding: STB_GLOBAL
 ProgramHeaders:
   - Type:  PT_LOAD
     Flags: [ PF_R, PF_X ]
     Sections:
       - Section: .gnu.hash
       - Section: .dynamic
+
+## Check the behavior when the dynamic symbol table is empty or not found.
+
+## Case A.1: Check we report a warning when the dynamic symbol table is empty and we attempt to print hash symbols
+##         from the .hash table. The number of symbols in the dynamic symbol table can be calculated from its size
+##         or derived from the Chain vector of the .hash table. Make both ways to return a zero to do the check.
+# RUN: yaml2obj --docnum=9 %s -o %t9.1.so
+# RUN: llvm-readelf --hash-symbols %t9.1.so 2>&1 | FileCheck %s -DFILE=%t9.1.so --check-prefix=DYNSYM-EMPTY-HASH
+
+# DYNSYM-EMPTY-HASH:      Symbol table of .hash for image:
+# DYNSYM-EMPTY-HASH-NEXT:   Num Buc:    Value  Size   Type   Bind Vis      Ndx Name
+# DYNSYM-EMPTY-HASH-NEXT: warning: '[[FILE]]': unable to print symbols for the .hash table: the dynamic symbol table is empty
+# DYNSYM-EMPTY-HASH-NOT:  {{.}}
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS32
+  Data:    ELFDATA2LSB
+  Type:    ET_DYN
+  Machine: EM_386
+Sections:
+  - Name:   .hash
+    Type:   SHT_HASH
+    Flags:  [ SHF_ALLOC ]
+    Bucket: [ 0 ]
+    Chain:  [ ]
+  - Name:  .dynamic
+    Type:  SHT_DYNAMIC
+    Flags: [ SHF_ALLOC ]
+    Entries:
+      - Tag:   DT_HASH
+        Value: 0x0
+      - Tag:   DT_STRTAB
+## PT_LOAD p_offset == .hash offset == 0x54.
+## 0x54 + 0x2c == 0x80 == .dynstr offset.
+        Value: 0x2c
+      - Tag:   DT_STRSZ
+        Value: 0x1
+      - Tag:   DT_NULL
+        Value: 0x0
+  - Name:  .dynstr
+    Type:  SHT_STRTAB
+    Flags: [ SHF_ALLOC ]
+  - Name:  .dynsym
+    Type:  [[DYNSYMTYPE=SHT_DYNSYM]]
+    Flags: [ SHF_ALLOC ]
+    Size:  0
+ProgramHeaders:
+  - Type:  PT_LOAD
+    Flags: [ PF_R, PF_X ]
+    Sections:
+      - Section: .hash
+      - Section: .dynamic
+      - Section: .dynstr
+
+## Case A.2: similar to A.1, but now check that we report a warning when the dynamic symbol table was not found.
+##           To do that, set the type of the .dynsym to SHT_PROGBITS to hide it.
+# RUN: yaml2obj --docnum=9 -DDYNSYMTYPE=SHT_PROGBITS %s -o %t9.2.so
+# RUN: llvm-readelf --hash-symbols %t9.2.so 2>&1 | FileCheck %s -DFILE=%t9.2.so --check-prefix=DYNSYM-NOTFOUND-HASH
+
+# DYNSYM-NOTFOUND-HASH:      Symbol table of .hash for image:
+# DYNSYM-NOTFOUND-HASH-NEXT:   Num Buc:    Value  Size   Type   Bind Vis      Ndx Name
+# DYNSYM-NOTFOUND-HASH-NEXT: warning: '[[FILE]]': unable to print symbols for the .hash table: the dynamic symbol table was not found
+# DYNSYM-NOTFOUND-HASH-NOT:  {{.}}
+
+## Case B.1: Check we report a warning when the dynamic symbol table is empty and we attempt to print
+##           hash symbols from the .gnu.hash table.
+# RUN: yaml2obj --docnum=10 %s -o %t10.1.so
+# RUN: llvm-readelf --hash-symbols %t10.1.so 2>&1 | FileCheck %s -DFILE=%t10.1.so --check-prefix=DYNSYM-EMPTY-GNUHASH
+
+# DYNSYM-EMPTY-GNUHASH:      Symbol table of .gnu.hash for image:
+# DYNSYM-EMPTY-GNUHASH-NEXT:  Num Buc:    Value  Size   Type   Bind Vis      Ndx Name
+# DYNSYM-EMPTY-GNUHASH-NEXT: warning: '[[FILE]]': unable to print symbols for the .gnu.hash table: the dynamic symbol table is empty
+# DYNSYM-EMPTY-GNUHASH-NOT:  {{.}}
+
+## Case B.2: similar to B.1, but now check that we report a warning when the dynamic symbol table was not found.
+##           To do that, set the type of the .dynsym to SHT_PROGBITS to hide it.
+# RUN: yaml2obj --docnum=10 -DDYNSYMTYPE=SHT_PROGBITS %s -o %t10.2.so
+# RUN: llvm-readelf --hash-symbols %t10.2.so 2>&1 | FileCheck %s -DFILE=%t10.2.so --check-prefix=DYNSYM-NOTFOUND-GNUHASH
+
+# DYNSYM-NOTFOUND-GNUHASH:      Symbol table of .gnu.hash for image:
+# DYNSYM-NOTFOUND-GNUHASH-NEXT:  Num Buc:    Value  Size   Type   Bind Vis      Ndx Name
+# DYNSYM-NOTFOUND-GNUHASH-NEXT: warning: '[[FILE]]': unable to print symbols for the .gnu.hash table: the dynamic symbol table was not found
+# DYNSYM-NOTFOUND-GNUHASH-NOT:  {{.}}
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS32
+  Data:    ELFDATA2LSB
+  Type:    ET_DYN
+  Machine: EM_386
+Sections:
+  - Name:  .gnu.hash
+    Type:  SHT_GNU_HASH
+    Flags: [ SHF_ALLOC ]
+    Header:
+      SymNdx: 0x0
+      Shift2: 0x0
+    BloomFilter: [ 0x0 ]
+    HashBuckets: [ 0x1 ]
+    HashValues:  [ 0x0 ]
+  - Name:  .dynamic
+    Type:  SHT_DYNAMIC
+    Flags: [ SHF_ALLOC ]
+    Entries:
+      - Tag:   DT_GNU_HASH
+        Value: 0x0
+      - Tag:   DT_STRTAB
+## PT_LOAD p_offset == .hash offset == 0x54.
+## 0x54 + 0x3c == 0x80 == .dynstr offset.
+        Value: 0x3c
+      - Tag:   DT_STRSZ
+        Value: 0x1
+      - Tag:   DT_NULL
+        Value: 0x0
+  - Name: .dynstr
+    Type: SHT_STRTAB
+  - Name:  .dynsym
+    Type:  [[DYNSYMTYPE=SHT_DYNSYM]]
+    Flags: [ SHF_ALLOC ]
+    Size:  0
+ProgramHeaders:
+  - Type:  PT_LOAD
+    Flags: [ PF_R, PF_X ]
+    Sections:
+      - Section: .gnu.hash
+      - Section: .dynamic
+      - Section: .dynstr

llvm/tools/llvm-readobj/ELFDumper.cpp

void GNUStyle<ELFT>::printSymbols(const ELFO *Obj, bool PrintSymbols,
   if (PrintSymbols)
     this->dumper()->printSymbolsHelper(false);
 }
 
 template <class ELFT> void GNUStyle<ELFT>::printHashSymbols(const ELFO *Obj) {
   if (this->dumper()->getDynamicStringTable().empty())
     return;
   auto StringTable = this->dumper()->getDynamicStringTable();
-  auto DynSyms = this->dumper()->dynamic_symbols();
+  Elf_Sym_Range DynSyms = this->dumper()->dynamic_symbols();
 
   auto PrintHashTable = [&](const Elf_Hash *SysVHash) {
     if (ELFT::Is64Bits)
       OS << "  Num Buc:    Value          Size   Type   Bind Vis      Ndx Name";
     else
       OS << "  Num Buc:    Value  Size   Type   Bind Vis      Ndx Name";
     OS << "\n";
 
+    const Elf_Sym *FirstSym = DynSyms.empty() ? nullptr : &DynSyms[0];
+    if (!FirstSym) {
+      Optional<DynRegionInfo> DynSymRegion = this->dumper()->getDynSymRegion();
+      this->reportUniqueWarning(
+          createError(Twine("unable to print symbols for the .hash table: the "
+                            "dynamic symbol table ") +
+                      (DynSymRegion ? "is empty" : "was not found")));
+      return;
+    }
+
     auto Buckets = SysVHash->buckets();
     auto Chains = SysVHash->chains();
     for (uint32_t Buc = 0; Buc < SysVHash->nbucket; Buc++) {
       if (Buckets[Buc] == ELF::STN_UNDEF)
         continue;
       std::vector<bool> Visited(SysVHash->nchain);
       for (uint32_t Ch = Buckets[Buc]; Ch < SysVHash->nchain; Ch = Chains[Ch]) {
         if (Ch == ELF::STN_UNDEF)
           break;
 
         if (Visited[Ch]) {
           reportWarning(
               createError(".hash section is invalid: bucket " + Twine(Ch) +
                           ": a cycle was detected in the linked chain"),
               this->FileName);
           break;
         }
 
-        printHashedSymbol(Obj, &DynSyms[0], Ch, StringTable, Buc);
+        printHashedSymbol(Obj, FirstSym, Ch, StringTable, Buc);
         Visited[Ch] = true;
       }
     }
   };
 
   if (const Elf_Hash *SysVHash = this->dumper()->getHashTable()) {
     OS << "\n Symbol table of .hash for image:\n";
     if (Error E = checkHashTable<ELFT>(Obj, SysVHash))
else
     OS << "  Num Buc:    Value  Size   Type   Bind Vis      Ndx Name";
   OS << "\n";
 
   if (Error E = checkGNUHashTable<ELFT>(Obj, GnuHash)) {
     this->reportUniqueWarning(std::move(E));
     return;
   }
 
+  const Elf_Sym *FirstSym = DynSyms.empty() ? nullptr : &DynSyms[0];
+  if (!FirstSym) {
+    Optional<DynRegionInfo> DynSymRegion = this->dumper()->getDynSymRegion();
+    this->reportUniqueWarning(createError(
+        Twine("unable to print symbols for the .gnu.hash table: the "
+              "dynamic symbol table ") +
+        (DynSymRegion ? "is empty" : "was not found")));
+    return;
+  }
+
   auto Buckets = GnuHash->buckets();
   for (uint32_t Buc = 0; Buc < GnuHash->nbuckets; Buc++) {
     if (Buckets[Buc] == ELF::STN_UNDEF)
       continue;
     uint32_t Index = Buckets[Buc];
     uint32_t GnuHashable = Index - GnuHash->symndx;
     // Print whole chain
     while (true) {
-      printHashedSymbol(Obj, &DynSyms[0], Index++, StringTable, Buc);
+      printHashedSymbol(Obj, FirstSym, Index++, StringTable, Buc);
       // Chain ends at symbol with stopper bit
       if ((GnuHash->values(DynSyms.size())[GnuHashable++] & 1) == 1)
         break;
     }
   }
 }
 
 static inline std::string printPhdrFlags(unsigned Flag) {