linux-kernel - Re: [PATCH] tracing: Use strcmp() in __assign_str() WARN

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Date: Wed, 13 Mar 2024 16:14:20 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Nathan Chancellor <nathan@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Linux Trace Kernel
 <linux-trace-kernel@...r.kernel.org>, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 kernel test robot <lkp@...el.com>, llvm@...ts.linux.dev
Subject: Re: [PATCH] tracing: Use strcmp() in __assign_str() WARN_ON() check

On Wed, 13 Mar 2024 13:45:50 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> Let me test to make sure that when src is a string "like this" that it does
> the strcmp(). Otherwise, we may have to always do the strcmp(), which I
> really would like to avoid.

I added the below patch and enabled sched_switch and it triggered the
warning (expected if it went the strcmp() path). I then changed it to be:

#define __assign_str(dst, src)						\
	do {								\
		char *__str__ = __get_str(dst);				\
		int __len__ = __get_dynamic_array_len(dst) - 1;		\
		WARN_ON_ONCE(__builtin_constant_p(src) ?		\
			     strcmp((src), __data_offsets.dst##_ptr_) :	\
-			     (src) != __data_offsets.dst##_ptr_);	\
+			     (src) == __data_offsets.dst##_ptr_);	\
		memcpy(__str__, __data_offsets.dst##_ptr_ ? :		\
		       EVENT_NULL_STR, __len__);			\
		__str__[__len__] = '\0';				\
	} while (0)

And the sched_switch did not trigger (expected). So it seems that it should
not be a problem.

Note, I only tested this with gcc and not clang.

But I guess there's also the case where we have:

	__assign_str(str, field ? field : "NULL")

But hopefully that's not an issue.

-- Steve

diff --git a/include/trace/events/sched.h b/include/trace/events/sched.h
index dbb01b4b7451..eaacd0c4e899 100644
--- a/include/trace/events/sched.h
+++ b/include/trace/events/sched.h
@@ -236,6 +236,7 @@ TRACE_EVENT(sched_switch,
 		__array(	char,	next_comm,	TASK_COMM_LEN	)
 		__field(	pid_t,	next_pid			)
 		__field(	int,	next_prio			)
+		__string( test, "this")
 	),
 
 	TP_fast_assign(
@@ -246,6 +247,7 @@ TRACE_EVENT(sched_switch,
 		memcpy(__entry->prev_comm, prev->comm, TASK_COMM_LEN);
 		__entry->next_pid	= next->pid;
 		__entry->next_prio	= next->prio;
+		__assign_str(test, "this");
 		/* XXX SCHED_DEADLINE */
 	),
 
diff --git a/include/trace/stages/stage6_event_callback.h b/include/trace/stages/stage6_event_callback.h
index 83da83a0c14f..cf301c723fd0 100644
--- a/include/trace/stages/stage6_event_callback.h
+++ b/include/trace/stages/stage6_event_callback.h
@@ -36,7 +36,7 @@
 		char *__str__ = __get_str(dst);				\
 		int __len__ = __get_dynamic_array_len(dst) - 1;		\
 		WARN_ON_ONCE(__builtin_constant_p(src) ?		\
-			     strcmp((src), __data_offsets.dst##_ptr_) :	\
+			     !strcmp((src), __data_offsets.dst##_ptr_) :	\
 			     (src) != __data_offsets.dst##_ptr_);	\
 		memcpy(__str__, __data_offsets.dst##_ptr_ ? :		\
 		       EVENT_NULL_STR, __len__);			\