[AIX][BigArchive] Treat the archive is empty if the first child member offset is zero

If the archive contains free list and contains no member file, the buffer length doesn't equal to length of the header.

Reviewed By: Esme, DiggerLin, #powerpc

Differential Revision: https://reviews.llvm.org/D138986

Author: Kai Luo

llvm/include/llvm/Object/Archive.h

@@ -410,9 +410,7 @@ class BigArchive : public Archive {
   BigArchive(MemoryBufferRef Source, Error &Err);
   uint64_t getFirstChildOffset() const override { return FirstChildOffset; }
   uint64_t getLastChildOffset() const { return LastChildOffset; }
-  bool isEmpty() const override {
-    return Data.getBufferSize() == sizeof(FixLenHdr);
-  };
+  bool isEmpty() const override { return getFirstChildOffset() == 0; }
 };
 
 } // end namespace object

llvm/lib/Object/Archive.cpp

@@ -949,7 +949,15 @@ Archive::child_iterator Archive::child_begin(Error &Err,
     return child_iterator::itr(
         Child(this, FirstRegularData, FirstRegularStartOfFile), Err);
 
-  const char *Loc = Data.getBufferStart() + getFirstChildOffset();
+  uint64_t FirstChildOffset = getFirstChildOffset();
+  const char *Loc = Data.getBufferStart() + FirstChildOffset;
+  if (Loc >= Data.getBufferEnd()) {
+    Err = malformedError("First member offset " + Twine(FirstChildOffset) +
+                         " is beyond the data buffer which has size of " +
+                         Twine(Data.getBufferSize()));
+    return child_end();
+  }
+
   Child C(this, Loc, &Err);
   if (Err)
     return child_end();

llvm/test/Object/archive-big-malformed-first-member.test

@@ -0,0 +1,7 @@
+# Test reading an empty archive with first member's offset is not zero.
+# RUN: echo    "<bigaf>" > %t.a
+# RUN: echo -n "0                   0                   0                   128                 0                   0                   " >> %t.a
+# RUN: not llvm-ar tv %t.a 2>&1 | grep 'truncated or malformed archive'
+# RUN: echo    "<bigaf>" > %t.a
+# RUN: echo -n "0                   0                   0                   28                  0                   0                   " >> %t.a
+# RUN: not llvm-ar tv %t.a 2>&1 | grep 'truncated or malformed archive'

llvm/test/Object/archive-big-read-empty-with-freelist.test

@@ -1,3 +1,3 @@
 # Test reading an empty archive with free list in it.
-# RUN: not llvm-ar tv %p/Inputs/aix-empty-big-archive-with-freelist.a 2>&1 \
-# RUN:   | grep 'truncated or malformed archive'
+# RUN: llvm-ar tv %p/Inputs/aix-empty-big-archive-with-freelist.a 2>&1 \
+# RUN:   | not grep 'truncated or malformed archive'