[PATCH v4] tracing: Use seq_buf for string concatenation

[Woradorn Laodhanadhaworn <woradorn.laon@gmail.com>]

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 <woradorn.laon@gmail.com>
---
v1 -> v2: 
	- Fixed WARN_ON when booting with empty trace_event.
v2 -> v3: 
	- Addressed Sashiko's concern about the compound literal backing buffer.
	- Replaced the compund literal with an explicit declared buffer and pointed
	seq_buf.buffer to it. This guarantees the backing storage is placed in 
	`.init.data` and reclaimed after boot.
v3 -> v4:
	- Removed typecast a const char* to non const.

v1: https://lore.kernel.org/all/20260620175441.223342-1-woradorn.laon@gmail.com
v2: https://lore.kernel.org/all/20260622094623.18469-1-woradorn.laon@gmail.com
v3: https://lore.kernel.org/all/20260623145147.12145-1-woradorn.laon@gmail.com
Sashiko: https://sashiko.dev/#/patchset/20260622094623.18469-1-woradorn.laon%40gmail.com

 kernel/trace/trace_events.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index c46e623e7e0d..4f5a0e73a89b 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>
@@ -4501,13 +4502,20 @@ 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_seq __initdata = {
+	.buffer = bootup_event_buf,
+	.size = sizeof(bootup_event_buf),
+};
 
 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_seq) > 0)
+		seq_buf_puts(&bootup_event_seq, ",");
+
+	seq_buf_puts(&bootup_event_seq, str);
 
-	strlcat(bootup_event_buf, str, COMMAND_LINE_SIZE);
+	if (seq_buf_has_overflowed(&bootup_event_seq))
+		return -ENOMEM;
 
 	trace_set_ring_buffer_expanded(NULL);
 	disable_tracing_selftest("running event tracing");
@@ -4766,6 +4774,7 @@ static __init int event_trace_enable(void)
 	 */
 	__trace_early_add_events(tr);
 
+	seq_buf_str(&bootup_event_seq);
 	early_enable_events(tr, bootup_event_buf, false);
 
 	trace_printk_start_comm();
@@ -4794,6 +4803,7 @@ static __init int event_trace_enable_again(void)
 	if (!tr)
 		return -ENODEV;
 
+	seq_buf_str(&bootup_event_seq);
 	early_enable_events(tr, bootup_event_buf, true);
 
 	return 0;
-- 
2.43.0