[PATCH v3] tracing: Use seq_buf for string concatenation

[PATCH v3] tracing: Use seq_buf for string concatenation

[Woradorn Laodhanadhaworn]

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.

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
Sashiko: https://sashiko.dev/#/patchset/20260622094623.18469-1-woradorn.laon%40gmail.com

 kernel/trace/trace_events.c | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index c46e623e7e0d..5ab630155ab6 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 = 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_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");
@@ -4766,7 +4774,7 @@ 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 +4802,7 @@ 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

Re: [PATCH v3] tracing: Use seq_buf for string concatenation

[Masami Hiramatsu]

On Tue, 23 Jun 2026 21:51:47 +0700
Woradorn Laodhanadhaworn <woradorn.laon@gmail.com> wrote:

> 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]
> 

Looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thanks,

> 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.
> 
> 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
> Sashiko: https://sashiko.dev/#/patchset/20260622094623.18469-1-woradorn.laon%40gmail.com
> 
>  kernel/trace/trace_events.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
> index c46e623e7e0d..5ab630155ab6 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 = 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_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");
> @@ -4766,7 +4774,7 @@ 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 +4802,7 @@ 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
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>

Re: [PATCH v3] tracing: Use seq_buf for string concatenation

[Markus Elfring]

…
> +++ b/kernel/trace/trace_events.c
…
> @@ -4501,13 +4502,20 @@ extern struct trace_event_call *__start_ftrace_events[];
…
>  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, ",");
…

I suggest to use the function “seq_buf_putc” instead at this source code place.
https://elixir.bootlin.com/linux/v7.1.1/source/lib/seq_buf.c#L203-L221

Is there a need for corresponding error detection?

Regards,
Markus