[PATCH] tracing: Use strcmp() in __assign_str() WARN_ON() check

[Steven Rostedt]

From: "Steven Rostedt (Google)" <rostedt@xxxxxxxxxxx>

The WARN_ON() check in __assign_str() to catch where the source variable
to the macro doesn't match the source variable to __string() gives an
error in clang:

>> include/trace/events/sunrpc.h:703:4: warning: result of comparison against a string literal is unspecified (use an explicit string comparison function instead) [-Wstring-compare]
     670 |                         __assign_str(progname, "unknown");

That's because the __assign_str() macro has:

   WARN_ON_ONCE((src) != __data_offsets.dst##_ptr_);

Where "src" is a string literal. Clang warns when comparing a string
literal directly as it is undefined to what the value of the literal is.

Since this is still to make sure the same string that goes to __string()
is the same as __assign_str(), for string literals do a test for that and
then use strcmp() in those cases

Note that this depends on commit 51270d573a8d ("tracing/net_sched: Fix
tracepoints that save qdisc_dev() as a string") being applied, as this was
what found that bug.

Reported-by: kernel test robot <lkp@xxxxxxxxx>
Closes: https://lore.kernel.org/oe-kbuild-all/202402292111.KIdExylU-lkp@xxxxxxxxx/
Fixes: 433e1d88a3be ("tracing: Add warning if string in __assign_str() does not match __string()")
Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
---
 include/trace/stages/stage6_event_callback.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/trace/stages/stage6_event_callback.h b/include/trace/stages/stage6_event_callback.h
index a0c15f67eabe..83da83a0c14f 100644
--- a/include/trace/stages/stage6_event_callback.h
+++ b/include/trace/stages/stage6_event_callback.h
@@ -35,7 +35,9 @@
 	do {								\
 		char *__str__ = __get_str(dst);				\
 		int __len__ = __get_dynamic_array_len(dst) - 1;		\
-		WARN_ON_ONCE((src) != __data_offsets.dst##_ptr_);	\
+		WARN_ON_ONCE(__builtin_constant_p(src) ?		\
+			     strcmp((src), __data_offsets.dst##_ptr_) :	\
+			     (src) != __data_offsets.dst##_ptr_);	\
 		memcpy(__str__, __data_offsets.dst##_ptr_ ? :		\
 		       EVENT_NULL_STR, __len__);			\
 		__str__[__len__] = '\0';				\
-- 
2.43.0