[clang] [-Wunsafe-buffer-usage] Allow strerror for %s in printf-like calls (PR #175208)

[Hans Wennborg via cfe-commits]

https://github.com/zmodem created https://github.com/llvm/llvm-project/pull/175208

Passing strerror(errno) to printf of printf-like logging functions is a common pattern, and strerror() returns a null-terminated string.

Follow-up to #173096

>From 39dcae4284d0d83ce236f3c2be24e0b71a3da294 Mon Sep 17 00:00:00 2001
From: Hans Wennborg <hans at chromium.org>
Date: Fri, 9 Jan 2026 18:23:57 +0100
Subject: [PATCH] [-Wunsafe-buffer-usage] Allow strerror for %s in printf-like
 calls

Passing strerror(errno) to printf of printf-like logging functions is a
common pattern, and strerror() returns a null-terminated string.

Follow-up to #173096
---
 clang/lib/Analysis/UnsafeBufferUsage.cpp                     | 5 +++++
 .../test/SemaCXX/warn-unsafe-buffer-usage-libc-functions.cpp | 4 ++++
 2 files changed, 9 insertions(+)

diff --git a/clang/lib/Analysis/UnsafeBufferUsage.cpp b/clang/lib/Analysis/UnsafeBufferUsage.cpp
index 7c0eaa2e589f5..4614a586565cb 100644
--- a/clang/lib/Analysis/UnsafeBufferUsage.cpp
+++ b/clang/lib/Analysis/UnsafeBufferUsage.cpp
@@ -771,6 +771,11 @@ static bool isNullTermPointer(const Expr *Ptr, ASTContext &Ctx) {
       if (MD->getName() == "c_str" && RD->getName() == "basic_string")
         return true;
   }
+  if (auto *CE = dyn_cast<CallExpr>(Ptr->IgnoreParenImpCasts())) {
+    const FunctionDecl *F = CE->getDirectCallee();
+    if (F && F->getName() == "strerror")
+      return true;
+  }
   return false;
 }
 
diff --git a/clang/test/SemaCXX/warn-unsafe-buffer-usage-libc-functions.cpp b/clang/test/SemaCXX/warn-unsafe-buffer-usage-libc-functions.cpp
index d824463ad9618..fe9bc7c809c96 100644
--- a/clang/test/SemaCXX/warn-unsafe-buffer-usage-libc-functions.cpp
+++ b/clang/test/SemaCXX/warn-unsafe-buffer-usage-libc-functions.cpp
@@ -298,6 +298,10 @@ void test_format_attr(char * Str, std::string StdStr) {
   myprintf("hello %s", Str);  // expected-warning{{function 'myprintf' is unsafe}} \
 			         expected-note{{string argument is not guaranteed to be null-terminated}}
 
+  extern int errno;
+  extern char *strerror(int errnum);
+  myprintf("errno: %s", strerror(errno));
+
   myprintf_2("hello", 0, Str);
   myprintf_2("hello %s", 0, StdStr.c_str());
   myprintf_2("hello %s", 0, Str);  // expected-warning{{function 'myprintf_2' is unsafe}} \