Relax non-null requirement for some rtl_uString_* functions

...that take a pointer and a length, and where it should be OK that the pointer
is null if the length is zero.  Those rtl_uString_* functions are targets of
OUString member functions that take std::[u16]string_view arguments, and
19926ed35ebb623fc896942b1f232b83edf1fc1e "loplugin:stringview: Flag empty string
converted to string view" (which changed some call sites to pass in default-
constructed std::[u16]string_view, for which data() returns null) revealed that
those rtl_uString_* functions were not prepared for such input.

(The guardings of memcpy are necessary because memcpy still requires its pointer
arguments to be non-null, even if the corresponding length is zero.)

The new sal/qa/rtl/strings/test_strings_defaultstringview.cxx systematically
tests all O[U]String[Buffer] member functions taking std::[u16]string_view
arguments.  It revealed one further issue in
IMPL_RTL_STRNAME(compare_WithLength), where UBSan reported a
nullptr-with-nonzero-offset

> sal/rtl/strtmpl.cxx:149:9: runtime error: applying non-zero offset 18446744073709551614 to null pointer

Also, rtl_uString_newReplaceFirstUtf16LUtf16L was found to lack a check for its
`from` argument to be non-null.

Change-Id: I6a7a712570f7d1e8d52097208c8a43a5a24797af
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/106295
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

2 files changed, 24 insertions, 13 deletions

diff --git a/sal/rtl/strtmpl.cxx b/sal/rtl/strtmpl.cxx
index 8ff170767b18..082dcd7d3913 100644
--- a/sal/rtl/strtmpl.cxx
+++ b/sal/rtl/strtmpl.cxx
@@ -145,9 +145,13 @@ sal_Int32 SAL_CALL IMPL_RTL_STRNAME( compare_WithLength )( const IMPL_RTL_STRCOD
         sal_Int32 nRet = nStr1Len - nStr2Len;
         int nCount = (nRet <= 0) ? nStr1Len : nStr2Len;
 
-        --pStr1;
-        --pStr2;
-        while( (--nCount >= 0) && (*++pStr1 == *++pStr2) ) ;
+        while( --nCount >= 0 ) {
+            if (*pStr1 != *pStr2) {
+                break;
+            }
+            ++pStr1;
+            ++pStr2;
+        }
 
         if( nCount >= 0 )
             nRet = static_cast<sal_Int32>(IMPL_RTL_USTRCODE( *pStr1 ))
diff --git a/sal/rtl/ustring.cxx b/sal/rtl/ustring.cxx
index 7b6687f74bd5..f84e3e1bd2d4 100644
--- a/sal/rtl/ustring.cxx
+++ b/sal/rtl/ustring.cxx
@@ -636,7 +636,7 @@ void rtl_uString_newConcatUtf16L(
 {
     assert(newString != nullptr);
     assert(left != nullptr);
-    assert(right != nullptr);
+    assert(right != nullptr || rightLength == 0);
     assert(rightLength >= 0);
     if (left->length > std::numeric_limits<sal_Int32>::max() - rightLength) {
 #if !defined(__COVERITY__)
@@ -650,9 +650,11 @@ void rtl_uString_newConcatUtf16L(
     sal_Int32 n = left->length + rightLength;
     rtl_uString_assign(newString, left);
     rtl_uString_ensureCapacity(newString, n);
-    memcpy(
-        (*newString)->buffer + (*newString)->length, right,
-        rightLength * sizeof (sal_Unicode));
+    if (rightLength != 0) {
+        memcpy(
+            (*newString)->buffer + (*newString)->length, right,
+            rightLength * sizeof (sal_Unicode));
+    }
     (*newString)->buffer[n] = 0;
     (*newString)->length = n;
 }
@@ -1316,7 +1318,7 @@ void rtl_uString_newReplaceFirstAsciiLUtf16L(
     assert(index != nullptr);
     assert(*index >= 0 && *index <= str->length);
     assert(fromLength >= 0);
-    assert(to != nullptr);
+    assert(to != nullptr || toLength == 0);
     assert(toLength >= 0);
     sal_Int32 i = rtl_ustr_indexOfAscii_WithLength(
         str->buffer + *index, str->length - *index, from, fromLength);
@@ -1338,8 +1340,10 @@ void rtl_uString_newReplaceFirstAsciiLUtf16L(
                 assert(i >= 0 && i < str->length);
                 memcpy(
                     (*newStr)->buffer, str->buffer, i * sizeof (sal_Unicode));
-                memcpy(
-                    (*newStr)->buffer + i, to, toLength * sizeof (sal_Unicode));
+                if (toLength != 0) {
+                    memcpy(
+                        (*newStr)->buffer + i, to, toLength * sizeof (sal_Unicode));
+                }
                 memcpy(
                     (*newStr)->buffer + i + toLength,
                     str->buffer + i + fromLength,
@@ -1405,8 +1409,9 @@ void rtl_uString_newReplaceFirstUtf16LUtf16L(
     assert(str != nullptr);
     assert(index != nullptr);
     assert(*index >= 0 && *index <= str->length);
+    assert(from != nullptr || fromLength == 0);
     assert(fromLength >= 0);
-    assert(to != nullptr);
+    assert(to != nullptr || toLength == 0);
     assert(toLength >= 0);
     sal_Int32 i = rtl_ustr_indexOfStr_WithLength(
         str->buffer + *index, str->length - *index, from, fromLength);
@@ -1428,8 +1433,10 @@ void rtl_uString_newReplaceFirstUtf16LUtf16L(
                 assert(i >= 0 && i < str->length);
                 memcpy(
                     (*newStr)->buffer, str->buffer, i * sizeof (sal_Unicode));
-                memcpy(
-                    (*newStr)->buffer + i, to, toLength * sizeof (sal_Unicode));
+                if (toLength != 0) {
+                    memcpy(
+                        (*newStr)->buffer + i, to, toLength * sizeof (sal_Unicode));
+                }
                 memcpy(
                     (*newStr)->buffer + i + toLength,
                     str->buffer + i + fromLength,