Handle the lack of a symbol table correctly.

[srhines]

Summary

These two cases will trigger a dereference on a nullptr, since the
SymbolTable can be nonexistent for a given library, in addition to just
being empty.

[srhines]

Ran git-clang-format and added rationale to the test.

[srhines]

• Fix positional output argument for llvm-strip.

[alexander-shaposhnikov]

tools/llvm-objcopy/llvm-objcopy.cpp

653

if i understand correctly, this line should be reverted back as discussed on the other diff + remove the lines 45-49 in the test.

the other change (line 418 + tests) looks fine to me

[srhines]

Revert back to not layering the positional argument change.

[srhines]

I restored the original change, since I had confused things with arcanist before. PTAL

[alexander-shaposhnikov]

LG

[alexander-shaposhnikov]

@srhines - is there anything blocking this diff ?

[srhines]

In D49534#1177242, @alexshap wrote:

@srhines - is there anything blocking this diff ?

Nope, I just missed that it was approved (and promptly forgot about it since I was chasing other problems). I will go ahead and submit it. Thanks for the review.

test/tools/llvm-objcopy/strip-all.test

 # RUN: cp %t %t7
 # RUN: llvm-strip -strip-all %t7
 # RUN: cmp %t2 %t7
 
 # RUN: cp %t %t8
 # RUN: llvm-strip --strip-all %t8
 # RUN: cmp %t2 %t8
 
+# Verify that a non-existent symbol table (after first call to llvm-strip)
+# can be handled correctly.
+# RUN: cp %t %t9
+# RUN: llvm-strip --strip-all -keep=unavailable_symbol %t9
+# RUN: llvm-strip --strip-all -keep=unavailable_symbol %t9
+# RUN: cmp %t2 %t9
+
 !ELF
 FileHeader:
   Class:           ELFCLASS64
   Data:            ELFDATA2LSB
   Type:            ET_REL
   Machine:         EM_X86_64
 Sections:
   - Name:            .bss

tools/llvm-objcopy/llvm-objcopy.cpp

RemovePred = [&Config, RemovePred, &Obj](const SectionBase &Sec) {
 
       // Allow all implicit removes.
       if (RemovePred(Sec))
         return true;
 
       // Keep special sections.
       if (Obj.SectionNames == &Sec)
         return false;
-      if (Obj.SymbolTable == &Sec || Obj.SymbolTable->getStrTab() == &Sec)
+      if (Obj.SymbolTable == &Sec ||
+          (Obj.SymbolTable && Obj.SymbolTable->getStrTab() == &Sec))
         return false;
 
       // Remove everything else.
       return true;
     };
   }
 
   if (!Config.Keep.empty()) {
     RemovePred = [Config, RemovePred](const SectionBase &Sec) {
       // Explicitly keep these sections regardless of previous removes.
       if (std::find(std::begin(Config.Keep), std::end(Config.Keep), Sec.Name) !=
           std::end(Config.Keep))
         return false;
       // Otherwise defer to RemovePred.
       return RemovePred(Sec);
     };
   }
 
   // This has to be the last predicate assignment.
   // If the option --keep-symbol has been specified
   // and at least one of those symbols is present
   // (equivalently, the updated symbol table is not empty)
   // the symbol table and the string table should not be removed.
   if ((!Config.SymbolsToKeep.empty() || Config.KeepFileSymbols) &&
-      !Obj.SymbolTable->empty()) {
+      Obj.SymbolTable && !Obj.SymbolTable->empty()) {
     RemovePred = [&Obj, RemovePred](const SectionBase &Sec) {
       if (&Sec == Obj.SymbolTable || &Sec == Obj.SymbolTable->getStrTab())
         return false;
       return RemovePred(Sec);
     };
   }
 
   Obj.removeSections(RemovePred);
if (Positional.empty())
     error("No input file specified");
 
   if (Positional.size() > 2)
     error("Support for multiple input files is not implemented yet");
 
   CopyConfig Config;
   Config.InputFilename = Positional[0];
   Config.OutputFilename =
       InputArgs.getLastArgValue(STRIP_output, Positional[0]);
 
   Config.StripDebug = InputArgs.hasArg(STRIP_strip_debug);
 
   Config.DiscardAll = InputArgs.hasArg(STRIP_discard_all);
   Config.StripUnneeded = InputArgs.hasArg(STRIP_strip_unneeded);
   Config.StripAll = InputArgs.hasArg(STRIP_strip_all);
 
   if (!Config.StripDebug && !Config.StripUnneeded && !Config.DiscardAll)