[clang] [clang] __builtin_os_log_format has incorrect PrintfFormat Attribute argument (PR #178320)

[Ziqing Luo via cfe-commits]

https://github.com/ziqingluo-90 updated https://github.com/llvm/llvm-project/pull/178320

>From d3ad7ac147634500c15c92ef9cf9b1bb0356da1f Mon Sep 17 00:00:00 2001
From: Ziqing Luo <ziqing_luo at apple.com>
Date: Tue, 27 Jan 2026 15:41:48 -0800
Subject: [PATCH 1/4] [clang] __builtin_os_log_format has incorrect
 PrintfFormat Attribute argument

The incorrect format attribute argument causes false positive
-Wunsafe-buffer-usage-in-format-attr-call warnings.

The format string is the 2nd argument of __builtin_os_log_format, thus
has index 1 in 0-based indexing.

rdar://169043228
---
 clang/include/clang/Basic/Builtins.td | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/clang/include/clang/Basic/Builtins.td b/clang/include/clang/Basic/Builtins.td
index dcf07d659cb15..1bfee45feb84f 100644
--- a/clang/include/clang/Basic/Builtins.td
+++ b/clang/include/clang/Basic/Builtins.td
@@ -4970,8 +4970,7 @@ def OSLogFormatBufferSize : Builtin {
 
 def OSLogFormat : Builtin {
   let Spellings = ["__builtin_os_log_format"];
-  // FIXME: The printf attribute looks suspiciously like it should be argument #1.
-  let Attributes = [PrintfFormat<0>, NoThrow, CustomTypeChecking];
+  let Attributes = [PrintfFormat<1>, NoThrow, CustomTypeChecking];
   let Prototype = "void*(void*, char const*, ...)";
 }
 

>From 8012d6c9718a928e13c18d6a9c4d35792290b1e0 Mon Sep 17 00:00:00 2001
From: Ziqing Luo <ziqing_luo at apple.com>
Date: Wed, 28 Jan 2026 16:52:19 -0800
Subject: [PATCH 2/4] add a test

---
 ...safe-buffer-usage-format-attr-builtins.cpp | 33 +++++++++++++++++++
 1 file changed, 33 insertions(+)
 create mode 100644 clang/test/SemaCXX/warn-unsafe-buffer-usage-format-attr-builtins.cpp

diff --git a/clang/test/SemaCXX/warn-unsafe-buffer-usage-format-attr-builtins.cpp b/clang/test/SemaCXX/warn-unsafe-buffer-usage-format-attr-builtins.cpp
new file mode 100644
index 0000000000000..efa10096775ae
--- /dev/null
+++ b/clang/test/SemaCXX/warn-unsafe-buffer-usage-format-attr-builtins.cpp
@@ -0,0 +1,33 @@
+// RUN: %clang_cc1 -std=c++20 -Wno-all -Wformat \
+// RUN:            -verify=expected-format %s
+// RUN: %clang_cc1 -std=c++20 -Wno-all -Wunsafe-buffer-usage -Wunsafe-buffer-usage-in-format-attr-call \
+// RUN:            -verify %s
+
+namespace std {
+  template<typename T>
+  struct basic_string {
+    T* p;
+    T *c_str();
+    T *data();
+    unsigned size_bytes();
+    unsigned size();
+  };
+
+  typedef basic_string<char> string;
+} // namespace std
+
+// PR#178320 corrects the format attribute arguments for
+// '__builtin_os_log_format' so that
+// '-Wunsafe-buffer-usage-in-format-attr-call' behaves correctly on
+// them.  For '-Wformat', the check for '__builtin_os_log_format' is
+// hand-craft without using the attribute. So they are still fine.
+
+void test_format_attr(char * Str, std::string StdStr) {
+  __builtin_os_log_format(nullptr, "hello", Str); // expected-format-warning{{data argument not used by format string}}
+  __builtin_os_log_format(nullptr, "hello %s", StdStr.c_str());
+  __builtin_os_log_format(nullptr, "hello %s", Str);  // expected-warning{{formatting function '__builtin_os_log_format' is unsafe}} \
+			         expected-note{{string argument is not guaranteed to be null-terminated}} 
+
+  __builtin_os_log_format(nullptr, "hello %"); // expected-format-warning{{incomplete format specifier}}
+  __builtin_os_log_format(nullptr, "hello %d", .42); // expected-format-warning{{format specifies type 'int' but the argument has type 'double'}} 
+}

>From eee98588a3b11d6b0c985bb5f5d2e258c50b77da Mon Sep 17 00:00:00 2001
From: Ziqing Luo <ziqing_luo at apple.com>
Date: Wed, 28 Jan 2026 17:16:30 -0800
Subject: [PATCH 3/4] add release note

---
 clang/docs/ReleaseNotes.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index f0d3d81f14e43..c1cd683847ae3 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -210,6 +210,7 @@ Bug Fixes to AST Handling
 
 Miscellaneous Bug Fixes
 ^^^^^^^^^^^^^^^^^^^^^^^
+- Fixed the arguments of the format attribute on ``__builtin_os_log_format``.  Previously, they were off by 1.
 
 Miscellaneous Clang Crashes Fixed
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

>From 5abe8ea1a094abf4238850ccf47666175dc82fc7 Mon Sep 17 00:00:00 2001
From: Ziqing Luo <ziqing_luo at apple.com>
Date: Thu, 29 Jan 2026 10:15:17 -0800
Subject: [PATCH 4/4] simplify test command

---
 .../warn-unsafe-buffer-usage-format-attr-builtins.cpp       | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/clang/test/SemaCXX/warn-unsafe-buffer-usage-format-attr-builtins.cpp b/clang/test/SemaCXX/warn-unsafe-buffer-usage-format-attr-builtins.cpp
index efa10096775ae..71426aa9ef992 100644
--- a/clang/test/SemaCXX/warn-unsafe-buffer-usage-format-attr-builtins.cpp
+++ b/clang/test/SemaCXX/warn-unsafe-buffer-usage-format-attr-builtins.cpp
@@ -1,7 +1,5 @@
-// RUN: %clang_cc1 -std=c++20 -Wno-all -Wformat \
-// RUN:            -verify=expected-format %s
-// RUN: %clang_cc1 -std=c++20 -Wno-all -Wunsafe-buffer-usage -Wunsafe-buffer-usage-in-format-attr-call \
-// RUN:            -verify %s
+// RUN: %clang_cc1 -Wformat -verify=expected-format %s
+// RUN: %clang_cc1 -Wunsafe-buffer-usage -Wunsafe-buffer-usage-in-format-attr-call -verify=expected,expected-format %s
 
 namespace std {
   template<typename T>