[PATCH] 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.

The backing buffer bootup_event_buf allocation is unchanged.
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>
---
 kernel/trace/trace_events.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index c46e623e7e0d..15164723e028 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 +4502,23 @@ 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;
+static bool bootup_event_seq_initialized;
 
 static __init int setup_trace_event(char *str)
 {
-	if (bootup_event_buf[0] != '\0')
-		strlcat(bootup_event_buf, ",", COMMAND_LINE_SIZE);
+	if (!bootup_event_seq_initialized) {
+		seq_buf_init(&bootup_event_seq, bootup_event_buf, COMMAND_LINE_SIZE);
+		bootup_event_seq_initialized = true;
+	}
+
+	if (seq_buf_used(&bootup_event_seq) > 0)
+		seq_buf_puts(&bootup_event_seq, ",");
 
-	strlcat(bootup_event_buf, str, COMMAND_LINE_SIZE);
+	seq_buf_puts(&bootup_event_seq, str);
+
+	if (seq_buf_has_overflowed(&bootup_event_seq))
+		return -ENOMEM;
 
 	trace_set_ring_buffer_expanded(NULL);
 	disable_tracing_selftest("running event tracing");
@@ -4766,7 +4777,7 @@ 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_seq), false);
 
 	trace_printk_start_comm();
 
@@ -4794,7 +4805,7 @@ 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_seq), true);
 
 	return 0;
 }
-- 
2.43.0