[clang] [Clang][Sema] Add fortify warnings for strcat (PR #168965)

[via cfe-commits]

https://github.com/venk-ks updated https://github.com/llvm/llvm-project/pull/168965

>From e87a76909a05e83a653281f6142b4d01667adad0 Mon Sep 17 00:00:00 2001
From: Venkatesh Srinivasan <venk at google.com>
Date: Thu, 20 Nov 2025 22:12:18 +0000
Subject: [PATCH] [Clang][Sema] Add fortify warnings for strcat

---
 clang/lib/Sema/SemaChecking.cpp       | 3 +++
 clang/test/Sema/warn-fortify-source.c | 8 ++++++++
 2 files changed, 11 insertions(+)

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 3e1edc4548034..f4e58de91286b 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -1263,6 +1263,8 @@ void Sema::checkFortifiedBuiltinMemoryFunction(FunctionDecl *FD,
   switch (BuiltinID) {
   default:
     return;
+  case Builtin::BI__builtin_strcat:
+  case Builtin::BIstrcat:
   case Builtin::BI__builtin_stpcpy:
   case Builtin::BIstpcpy:
   case Builtin::BI__builtin_strcpy:
@@ -1273,6 +1275,7 @@ void Sema::checkFortifiedBuiltinMemoryFunction(FunctionDecl *FD,
     break;
   }
 
+  case Builtin::BI__builtin___strcat_chk:
   case Builtin::BI__builtin___stpcpy_chk:
   case Builtin::BI__builtin___strcpy_chk: {
     DiagID = diag::warn_fortify_strlen_overflow;
diff --git a/clang/test/Sema/warn-fortify-source.c b/clang/test/Sema/warn-fortify-source.c
index 216878c0836d8..750bd5361ade9 100644
--- a/clang/test/Sema/warn-fortify-source.c
+++ b/clang/test/Sema/warn-fortify-source.c
@@ -76,6 +76,14 @@ void call_strcpy_nowarn(void) {
   __builtin_strcpy(dst, src);
 }
 
+void call_strcat(void) {
+  const char *const src = "abcd";
+  char dst1[5];
+  char dst2[4];
+  __builtin_strcat(dst1, src);
+  __builtin_strcat(dst2, src); // expected-warning {{'strcat' will always overflow; destination buffer has size 4, but the source string has length 5 (including NUL byte)}}
+}
+
 void call_stpcpy(void) {
   const char *const src = "abcd";
   char dst1[5];