[Object] Fix reading objects created with -fembed-bitcode-marker

Currently, this fails with many tools, e.g.

$ clang -fembed-bitcode-marker -c -o test.o test.c
$ nm test.o
nm: test.o The file was not recognized as a valid object file

-fembed-bitcode-marker creates a LLVM,bitcode section consisting of a single
byte. When reading the object file, IRObjectFile::findBitcodeInObject succeeds,
causing SymbolicFile::createSymbolicFile to try to read the "bitcode" rather
than using the outer Mach-O data - when then fails.

Fix this by making findBitcodeInObject return an error if the section size <= 1.

Patched by: Nicholas Allegra

Differential Revision: https://reviews.llvm.org/D44373

llvm-svn: 356718

lib/Object/IRObjectFile.cpp

@@ -78,6 +78,8 @@ IRObjectFile::findBitcodeInObject(const ObjectFile &Obj) {
       StringRef SecContents;
       if (std::error_code EC = Sec.getContents(SecContents))
         return errorCodeToError(EC);
+      if (SecContents.size() <= 1)
+        return errorCodeToError(object_error::bitcode_section_not_found);
       return MemoryBufferRef(SecContents, Obj.getFileName());
     }
   }

test/Object/nm-bitcode.test

@@ -0,0 +1,12 @@
+# Inputs generated with:
+# echo 'int hello() { return 5; }' > test.c
+# clang -O -fembed-bitcode -c -o macho-bitcode-x86_64.o test.c
+# clang -O -fembed-bitcode-marker -c -o macho-bitcode-marker-x86_64.o test.c
+
+RUN: llvm-nm -a %p/Inputs/macho-bitcode-x86_64.o \
+RUN:         | FileCheck %s
+
+RUN: llvm-nm -a %p/Inputs/macho-bitcode-marker-x86_64.o \
+RUN:         | FileCheck %s
+
+CHECK: T _hello