[LLD][ELF] Fix nullptr dereference when creating an error message.

[seaneveson]

Summary

There is already a check for File being null, but it is then dereferenced anyway.

Our downstream port caused this to crash.

[ruiu]

Thank you for your patch!

Could you add a test?

ELF/InputSection.cpp

279–281

You want to return "(internal):" + Name + "+0x" + utohexstr(Off) +")" here if File is null, so that you can remove the null checks below.

[seaneveson]

Changed to early return as requested.

I haven't been able to find a way to test this, as the crash only occurs for errors in internal sections and I can't find a good way to force one. Do you have any ideas?

Thanks for the review.

[ruiu]

Can you test this if you pass a broken object file to the linker?

[seaneveson]

• Added a test
• Fixed a missing brace in the error message string

ELF/InputSection.cpp

 //   path/to/foo.o:(function bar)
 //
 // or
 //
 //   path/to/foo.o:(function bar) in archive path/to/bar.a
 template <class ELFT> std::string InputSectionBase::getObjMsg(uint64_t Off) {
   // Synthetic sections don't have input files.
   elf::ObjectFile<ELFT> *File = getFile<ELFT>();
-  std::string Filename = File ? File->getName() : "(internal)";
+  if (!File)
+    return ("(internal):(" + Name + "+0x" + utohexstr(Off) + ")").str();
+  std::string Filename = File->getName();
 
   std::string Archive;
   if (!File->ArchiveName.empty())
     Archive = (" in archive " + File->ArchiveName).str();
 
   // Find a symbol that encloses a given location.
   for (SymbolBody *B : getFile<ELFT>()->getSymbols())
     if (auto *D = dyn_cast<DefinedRegular>(B))

test/ELF/duplicated-synthetic-sym.s

+// RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+// RUN: cd %S
+// RUN: not ld.lld %t.o --format=binary duplicated-synthetic-sym.s -o %t.elf 2>&1 | FileCheck %s
+
+// CHECK: duplicate symbol: _binary_duplicated_synthetic_sym_s_start
+// CHECK: defined at (internal):(.data+0x0)
+
+    .globl  _binary_duplicated_synthetic_sym_s_start
+_binary_duplicated_synthetic_sym_s_start:
+    .long   0