[Object] object::ELFObjectFile::dynamic_symbol_begin(): skip symbol index 0

Summary:
Note: This revision is very similar to D62296.

In D75756, we need `getDynamicSymbolIterators()` to skip first NULL symbol in `.dynsym`. And I believe it might be worth pointing this out in a separate patch to gather you experts' opinions.

I have checked that current code base will not be affected by this change.

```
dynamic_symbol_begin()
|- dynamic_symbol_end(): Ok
`- getDynamicSymbolIterators()
   |- addDynamicElfSymbols(): llvm/tools/llvm-objdump/llvm-objdump.cpp, Line 934
   |                          Ok, NULL symbol will be omitted by Line 945-947
   |                          StringRef Name = unwrapOrError(Symbol.getName(), Obj->getName());
   |                          if (Name.empty()) continue;
   |- dumpSymbolNameFromObject(): llvm/tools/llvm-nm/llvm-nm.cpp, Line 1192
   |                          There's no test for dumping dynamic debugging symbol. This patch helps improve llvm-nm behavior. (we should add test for this later)
   `- computeSymbolSizes(): llvm/lib/Object/SymbolSize.cpp, Line 52
      |- OProfileJITEventListener::notifyObjectLoaded(): llvm/lib/ExecutionEngine/OProfileJIT/OProfileJITEventListener.cpp, Line 92
      |                                                  Ok, NULL symbol will be omitted by Line 94-95
      |                                                  if (!Sym.getType() || *Sym.getType() != SF_Function) continue;
      |- IntelJITEventListener::notifyObjectLoaded(): llvm/lib/ExecutionEngine/IntelJITEvents/IntelJITEventListener.cpp, Line 98
      |                                               Ok, NULL symbol will be omitted by Line 124-126 (same as previous one)
      |- PerfJITEventListener::notifyObjectLoaded(): llvm/lib/ExecutionEngine/PerfJITEvents/PerfJITEventListener.cpp, Line 244
      |                                              Ok, NULL symbol will be omitted by Line 254-256, (same as previous one)
      |- SymbolizableObjectFile::create(): llvm/lib/DebugInfo/Symbolize/SymbolizableObjectFile.cpp, Line 73
      |                                    Ok, NULL symbol will be omitted by Line 75
      |                                    res->addSymbol()
      |                                    In addSymbol(), Line 167-168
      |                                    if (!Sec || (Obj && Obj->section_end() == *Sec)) return std::error_code();
      |- dumpCXXData(): llvm/tools/llvm-cxxdump/llvm-cxxdump.cpp, Line 189
      |                 Ok, NULL symbol will be omitted by Line 199-202
      |                 object::section_iterator SecI = *SecIOrErr;
      |                 // Skip external symbols.
      |                 if (SecI == Obj->section_end())
      |                   continue;
      `- printLineInfoForInput(): llvm/tools/llvm-rtdyld/llvm-rtdyld.cpp, Line 418
                                  Ok, NULL symbol will be omitted by Line 430-477
                                  if (Type == object::SymbolRef::ST_Function) {
                                    ...
                                  }
```

Reviewers: grimar, jhenderson, MaskRay

Reviewed By: jhenderson, MaskRay

Subscribers: rupprecht, arphaman, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76081

include/llvm/Object/ELFObjectFile.h

@@ -1027,8 +1027,12 @@ basic_symbol_iterator ELFObjectFile<ELFT>::symbol_end() const {
 
 template <class ELFT>
 elf_symbol_iterator ELFObjectFile<ELFT>::dynamic_symbol_begin() const {
-  DataRefImpl Sym = toDRI(DotDynSymSec, 0);
-  return symbol_iterator(SymbolRef(Sym, this));
+  if (!DotDynSymSec || DotDynSymSec->sh_size < sizeof(Elf_Sym))
+    // Ignore errors here where the dynsym is empty or sh_size less than the
+    // size of one symbol. These should be handled elsewhere.
+    return symbol_iterator(SymbolRef(toDRI(DotDynSymSec, 0), this));
+  // Skip 0-index NULL symbol.
+  return symbol_iterator(SymbolRef(toDRI(DotDynSymSec, 1), this));
 }
 
 template <class ELFT>

test/tools/llvm-nm/dynamic.test

@@ -0,0 +1,62 @@
+## This is a test for --dynamic/-D option.
+
+## Test llvm-nm dumping ELF file with valid .dynsym section.
+# RUN: yaml2obj --docnum=1 %s -o %t1.o
+# RUN: llvm-nm --dynamic %t1.o | \
+# RUN:   FileCheck %s --match-full-lines --strict-whitespace --check-prefix DYNSYM
+# RUN: llvm-nm -D %t1.o | \
+# RUN:   FileCheck %s --match-full-lines --strict-whitespace --check-prefix DYNSYM
+
+#       DYNSYM:                 U globalsym
+#  DYNSYM-NEXT:                 U localsym1
+#  DYNSYM-NEXT:0000000000000000 n localsym2
+# DYNSYM-EMPTY:
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_DYN
+  Machine: EM_X86_64
+Sections:
+  - Name: section
+    Type: SHT_PROGBITS
+DynamicSymbols:
+  - Name:    localsym1
+    Type:    STT_OBJECT
+  - Name:    localsym2
+    Section: section
+  - Name:    globalsym
+    Type:    STT_OBJECT
+    Binding: STB_GLOBAL
+
+## Test llvm-nm dumping ELF file without a .dynsym section.
+# RUN: yaml2obj --docnum=2 %s -o %t2.o
+# RUN: llvm-nm --dynamic %t2.o 2>&1 | \
+# RUN:   FileCheck %s --match-full-lines --strict-whitespace -DFILE=%t2.o --check-prefix NO-SYMS
+
+#       NO-SYMS:[[FILE]]: no symbols
+# NO-SYMS-EMPTY:
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_DYN
+  Machine: EM_X86_64
+
+## Test llvm-nm dumping ELF file with an empty .dynsym section.
+# RUN: yaml2obj --docnum=3 %s -o %t3.o
+# RUN: llvm-nm --dynamic %t3.o 2>&1 | \
+# RUN:   FileCheck %s --match-full-lines --strict-whitespace -DFILE=%t3.o --check-prefix NO-SYMS
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS64
+  Data:    ELFDATA2LSB
+  Type:    ET_DYN
+  Machine: EM_X86_64
+Sections:
+  - Name: .dynsym
+    Type: SHT_DYNSYM
+    Size: 0