[flang-commits] [flang] d889a74 - [flang] Avoid UB in CharBlock Compare to C string (#147329)

[via flang-commits]

Author: David Spickett
Date: 2025-07-08T08:52:36+01:00
New Revision: d889a7485f77c214ee57d0a74bf164237fdfa423

URL: https://github.com/llvm/llvm-project/commit/d889a7485f77c214ee57d0a74bf164237fdfa423
DIFF: https://github.com/llvm/llvm-project/commit/d889a7485f77c214ee57d0a74bf164237fdfa423.diff

LOG: [flang] Avoid UB in CharBlock Compare to C string (#147329)

The behaviour of strncmp is undefined if either string pointer is null
(https://en.cppreference.com/w/cpp/string/byte/strncmp.html).

I've copied the logic over from Compare to another CharBlock, which had
code to avoid UB in memcmp.

The test Preprocessing/kind-suffix.F90 was failing with UBSAN enabled,
and now passes.

Added: 
    

Modified: 
    flang/include/flang/Parser/char-block.h

Removed: 
    


################################################################################
diff  --git a/flang/include/flang/Parser/char-block.h b/flang/include/flang/Parser/char-block.h
index 38f4f7b82e1ea..b3b1f04034d3c 100644
--- a/flang/include/flang/Parser/char-block.h
+++ b/flang/include/flang/Parser/char-block.h
@@ -150,7 +150,12 @@ class CharBlock {
 
   int Compare(const char *that) const {
     std::size_t bytes{size()};
-    if (int cmp{std::strncmp(begin(), that, bytes)}) {
+    // strncmp is undefined if either pointer is null.
+    if (!bytes) {
+      return that == nullptr ? 0 : -1;
+    } else if (!that) {
+      return 1;
+    } else if (int cmp{std::strncmp(begin(), that, bytes)}) {
       return cmp;
     }
     return that[bytes] == '\0' ? 0 : -1;