linux-kernel - [PATCH 2/2] tracing: Ignore -Wstring-compare with diagnostic macros

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Date: Tue, 19 Mar 2024 09:07:53 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: rostedt@...dmis.org, mhiramat@...nel.org
Cc: mathieu.desnoyers@...icios.com, ndesaulniers@...gle.com, 
 morbo@...gle.com, justinstitt@...gle.com, linux-kernel@...r.kernel.org, 
 linux-trace-kernel@...r.kernel.org, llvm@...ts.linux.dev, 
 patches@...ts.linux.dev, Linux Kernel Functional Testing <lkft@...aro.org>, 
 Nathan Chancellor <nathan@...nel.org>
Subject: [PATCH 2/2] tracing: Ignore -Wstring-compare with diagnostic
 macros

Commit b1afefa62ca9 ("tracing: Use strcmp() in __assign_str() WARN_ON()
check") addressed a clang warning, -Wstring-compare, with the use of
__builtin_constant_p() to dispatch to strcmp() if the source string is a
string literal and a direct comparison if not. Unfortunately, even with
this change, the warning is still present because __builtin_constant_p()
is not evaluated at this stage of the pipeline, so clang still thinks
the else branch could occur for this situation:

  include/trace/events/sunrpc.h:705:4: error: result of comparison against a string literal is unspecified (use an explicit string comparison function instead) [-Werror,-Wstring-compare]
  ...
  include/trace/stages/stage6_event_callback.h:40:15: note: expanded from macro '__assign_str'
     40 |                              (src) != __data_offsets.dst##_ptr_);       \
        |                                    ^
  ...

Use the compiler diagnostic macros to disable this warning around the
WARN_ON_ONCE() expression since a string comparison function, strcmp(),
will always be used for the comparison of string literals.

Fixes: b1afefa62ca9 ("tracing: Use strcmp() in __assign_str() WARN_ON() check")
Reported-by: Linux Kernel Functional Testing <lkft@...aro.org>
Closes: https://lore.kernel.org/all/CA+G9fYs=OTKAZS6g1P1Ewadfr0qoe6LgOVSohqkXmFXotEODdg@mail.gmail.com/
Signed-off-by: Nathan Chancellor <nathan@...nel.org>
---
 include/trace/stages/stage6_event_callback.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/include/trace/stages/stage6_event_callback.h b/include/trace/stages/stage6_event_callback.h
index 83da83a0c14f..56a4eea5a48e 100644
--- a/include/trace/stages/stage6_event_callback.h
+++ b/include/trace/stages/stage6_event_callback.h
@@ -35,9 +35,14 @@
 	do {								\
 		char *__str__ = __get_str(dst);				\
 		int __len__ = __get_dynamic_array_len(dst) - 1;		\
+		__diag_push();						\
+		__diag_ignore(clang, 11, "-Wstring-compare",		\
+			      "__builtin_constant_p() ensures strcmp()"	\
+			      "will be used for string literals");	\
 		WARN_ON_ONCE(__builtin_constant_p(src) ?		\
 			     strcmp((src), __data_offsets.dst##_ptr_) :	\
 			     (src) != __data_offsets.dst##_ptr_);	\
+		__diag_pop();						\
 		memcpy(__str__, __data_offsets.dst##_ptr_ ? :		\
 		       EVENT_NULL_STR, __len__);			\
 		__str__[__len__] = '\0';				\

-- 
2.44.0