[PATCH v2] tracing: Use seq_buf for string concatenation

In preparation for removing the strlcat API[1],
replace the string concatenation logic with a struct seq_buf,
which tracks the current position and the remaining space internally.

Use seq_buf_str() to NUL-terminate before passing to early_enable_events().

Link: https://github.com/KSPP/linux/issues/370 [1]

Signed-off-by: Woradorn Laodhanadhaworn <[email protected]>
---
v1 -> v2: Fixed WARN_ON when booting with empty trace_event.

v1: https://lore.kernel.org/all/[email protected]

 kernel/trace/trace_events.c | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index c46e623e7e0d..1be62a46e49a 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -22,6 +22,7 @@
 #include <linux/sort.h>
 #include <linux/slab.h>
 #include <linux/delay.h>
+#include <linux/seq_buf.h>
 
 #include <trace/events/sched.h>
 #include <trace/syscall.h>
@@ -4500,14 +4501,20 @@ static void __add_event_to_tracers(struct trace_event_call *call)
 extern struct trace_event_call *__start_ftrace_events[];
 extern struct trace_event_call *__stop_ftrace_events[];
 
-static char bootup_event_buf[COMMAND_LINE_SIZE] __initdata;
+static struct seq_buf bootup_event_buf __initdata = {
+	.buffer = (char[COMMAND_LINE_SIZE]) {},
+	.size = COMMAND_LINE_SIZE,
+};
 
 static __init int setup_trace_event(char *str)
 {
-	if (bootup_event_buf[0] != '\0')
-		strlcat(bootup_event_buf, ",", COMMAND_LINE_SIZE);
+	if (seq_buf_used(&bootup_event_buf) > 0)
+		seq_buf_puts(&bootup_event_buf, ",");
+
+	seq_buf_puts(&bootup_event_buf, str);
 
-	strlcat(bootup_event_buf, str, COMMAND_LINE_SIZE);
+	if (seq_buf_has_overflowed(&bootup_event_buf))
+		return -ENOMEM;
 
 	trace_set_ring_buffer_expanded(NULL);
 	disable_tracing_selftest("running event tracing");
@@ -4766,7 +4773,7 @@ static __init int event_trace_enable(void)
 	 */
 	__trace_early_add_events(tr);
 
-	early_enable_events(tr, bootup_event_buf, false);
+	early_enable_events(tr, (char *)seq_buf_str(&bootup_event_buf), false);
 
 	trace_printk_start_comm();
 
@@ -4794,7 +4801,7 @@ static __init int event_trace_enable_again(void)
 	if (!tr)
 		return -ENODEV;
 
-	early_enable_events(tr, bootup_event_buf, true);
+	early_enable_events(tr, (char *)seq_buf_str(&bootup_event_buf), true);
 
 	return 0;
 }
-- 
2.43.0