[PATCH] tracing: Suppress sparse warnings triggered by is_signed_type()

[Bart Van Assche <bvanassche@acm.org>]

Using a __bitwise type in a tracing __field() definition triggers four
sparse warnings in stage 4 of expanding the TRACE_EVENT() macro. These
warnings are triggered by the is_signed_type() macro implementation.
Since there is no known way of checking signedness of a bitwise type
without triggering sparse warnings, disable signedness checking when
verifying code with sparse.

Note: work is in progress to improve sparse but has not yet landed. See
also "[PATCH 0/5] allow -1 and compares in bitwise types"
(https://lore.kernel.org/all/20220627190540.13358-1-luc.vanoostenryck@gmail.com/ ).

Suggested-by: Christoph Hellwig <hch@lst.de>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 include/linux/trace_events.h | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
index e6e95a9f07a5..e43f0f37ef27 100644
--- a/include/linux/trace_events.h
+++ b/include/linux/trace_events.h
@@ -814,7 +814,19 @@ extern int trace_add_event_call(struct trace_event_call *call);
 extern int trace_remove_event_call(struct trace_event_call *call);
 extern int trace_event_get_offsets(struct trace_event_call *call);
 
+/*
+ * There is no known way to check signedness of __bitwise types without
+ * triggering a sparse warning. Hence the #ifdef __CHECKER__.
+ *
+ * Since there is another definition of is_signed_type() in <linux/overflow.h>,
+ * undefine is_signed_type() before redefining it.
+ */
+#undef is_signed_type
+#ifdef __CHECKER__
+#define is_signed_type(type)	0
+#else
 #define is_signed_type(type)	(((type)(-1)) < (type)1)
+#endif
 
 int ftrace_set_clr_event(struct trace_array *tr, char *buf, int set);
 int trace_set_clr_event(const char *system, const char *event, int set);