tdf#108039: check for nullptr in rtl_uString and OUString

rtl_[u]String_newConcat now checks allocation result to return
early and avoid SIGSEGV. Other functions are not modified, to
keep old behavior relying on allocation success and crashing
early on OOM to avoid added overhead in performance-critical
places.

OUString operator+= now checks rtl_uString_newConcat result and
throws std::bad_alloc on failure, to specifically address BASIC
problem. It keeps strong exception guarantee of leaving this'
state unaltered.

Concatenation in BASIC now checks for bad string allocation
(previously SIGSEGV was generated).

Unit test included.

Change-Id: I1513311d3d58eac43b2d2ec9a230e22dff0b4245
Reviewed-on: https://gerrit.libreoffice.org/37965Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Jenkins <ci@libreoffice.org>

basic/qa/basic_coverage/test_string_overflow_safe.vb

+Option Explicit
+
+Function doUnitTest As Integer
+    ' Trying to create too long string should generate proper BASIC overflow error.
+    ' Longest possible string is 2147483638 wchar_t (2G - 10).
+    ' This tries to create string with 2G wchar_t. If it does not overflow, test fails.
+    ' If overflow is not safe, it segfaults.
+    On Error GoTo errorHandler
+    Dim s As String, i As Integer
+    s = "0"
+    For i=1 To 31
+    s = s & s
+    Next i
+    doUnitTest = 0
+    Exit Function
+errorHandler:
+    If ( Err <> 6 ) Then
+        doUnitTest = 0
+    Else
+        doUnitTest = 1
+    Endif
+End Function

basic/source/sbx/sbxvalue.cxx

@@ -875,14 +875,19 @@ bool SbxValue::Compute( SbxOperator eOp, const SbxValue& rOp )
                 // #30576: To begin with test, if the conversion worked
                 if( aL.pOUString != nullptr && aR.pOUString != nullptr )
                 {
-                    *aL.pOUString += *aR.pOUString;
+                    // tdf#108039: catch possible bad_alloc
+                    try {
+                        *aL.pOUString += *aR.pOUString;
+                    }
+                    catch (const std::bad_alloc&) {
+                        SetError(ERRCODE_SBX_OVERFLOW);
+                    }
                 }
                 // Not even Left OK?
                 else if( aL.pOUString == nullptr )
                 {
                     aL.pOUString = new OUString();
                 }
-                Put( aL );
             }
             else
                 SetError( ERRCODE_SBX_CONVERSION );

include/rtl/ustring.hxx

@@ -519,13 +519,21 @@ public:
       Append a string to this string.
 
       @param    str         a OUString.
+
+      @exception std::bad_alloc is thrown if an out-of-memory condition occurs
     */
     OUString & operator+=( const OUString & str )
 #if defined LIBO_INTERNAL_ONLY && HAVE_CXX11_REF_QUALIFIER
         &
 #endif
     {
-        rtl_uString_newConcat( &pData, pData, str.pData );
+        rtl_uString* pNewData = NULL;
+        rtl_uString_newConcat( &pNewData, pData, str.pData );
+        if (pNewData == NULL) {
+            throw std::bad_alloc();
+        }
+        rtl_uString_assign(&pData, pNewData);
+        rtl_uString_release(pNewData);
         return *this;
     }
 #if defined LIBO_INTERNAL_ONLY && HAVE_CXX11_REF_QUALIFIER

sal/rtl/strtmpl.cxx

@@ -1507,11 +1507,13 @@ void SAL_CALL IMPL_RTL_STRINGNAME( newConcat )( IMPL_RTL_STRINGDATA** ppThis,
     {
         IMPL_RTL_STRINGDATA* pTempStr = IMPL_RTL_STRINGNAME( ImplAlloc )( pLeft->length + pRight->length );
         OSL_ASSERT(pTempStr != nullptr);
-        rtl_str_ImplCopy( pTempStr->buffer, pLeft->buffer, pLeft->length );
-        rtl_str_ImplCopy( pTempStr->buffer+pLeft->length, pRight->buffer, pRight->length );
         *ppThis = pTempStr;
+        if (*ppThis != nullptr) {
+            rtl_str_ImplCopy( pTempStr->buffer, pLeft->buffer, pLeft->length );
+            rtl_str_ImplCopy( pTempStr->buffer+pLeft->length, pRight->buffer, pRight->length );
 
-        RTL_LOG_STRING_NEW( *ppThis );
+            RTL_LOG_STRING_NEW( *ppThis );
+        }
     }
 
     /* must be done last, if left or right == *ppThis */