[PATCH v2] tracing: Use seq_buf for string concatenation

[PATCH v2] 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.

v1: https://lore.kernel.org/all/20260620175441.223342-1-woradorn.laon@gmail.com

 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

Re: [PATCH v2] tracing: Use seq_buf for string concatenation

[Steven Rostedt]

On Mon, 22 Jun 2026 16:46:23 +0700
Woradorn Laodhanadhaworn <woradorn.laon@gmail.com> wrote:

>  
>  #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;

Keep the above string and just assign it.

> +static struct seq_buf bootup_event_buf __initdata = {
> +	.buffer = (char[COMMAND_LINE_SIZE]) {},
> +	.size = COMMAND_LINE_SIZE,
> +};

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_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);

The above then would be:

	seq_buf_str(&bootup_event_seq);
	early_enable_events(tr, bootup_event_buf, false);

Don't typecast a const char* to non const.

>  
>  	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);

Same here.

-- Steve

>  
>  	return 0;
>  }

Re: [PATCH v2] tracing: Use seq_buf for string concatenation

[Woradorn Laodhanadhaworn]

On 30/6/2569 BE 01:39, Steven Rostedt wrote:
> On Mon, 22 Jun 2026 16:46:23 +0700
> Woradorn Laodhanadhaworn <woradorn.laon@gmail.com> wrote:
> 
>>  
>>  #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;
> 
> Keep the above string and just assign it.
> 
>> +static struct seq_buf bootup_event_buf __initdata = {
>> +	.buffer = (char[COMMAND_LINE_SIZE]) {},
>> +	.size = COMMAND_LINE_SIZE,
>> +};
> 
> 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_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);
> 
> The above then would be:
> 
> 	seq_buf_str(&bootup_event_seq);
> 	early_enable_events(tr, bootup_event_buf, false);
> 
> Don't typecast a const char* to non const.
> 
>>  
>>  	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);
> 
> Same here.
> 
> -- Steve
> 
>>  
>>  	return 0;
>>  }
> 

Thank you, Steven, for your review. I've sent v4:
https://lore.kernel.org/all/20260713045249.69942-1-woradorn.laon@gmail.com

Thanks,
Woradorn

Re: [PATCH v2] tracing: Use seq_buf for string concatenation

[kernel test robot]

Hi Woradorn,

kernel test robot noticed the following build warnings:

[auto build test WARNING on v7.2-rc7]
[also build test WARNING on linus/master]
[cannot apply to linux-review/Woradorn-Laodhanadhaworn/tracing-Use-seq_buf-for-string-concatenation/20260813-224524 trace/for-next next-20260814]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Woradorn-Laodhanadhaworn/tracing-Use-seq_buf-for-string-concatenation/20260815-195403
base:   v7.2-rc7
patch link:    https://lore.kernel.org/r/20260622094623.18469-1-woradorn.laon%40gmail.com
patch subject: [PATCH v2] tracing: Use seq_buf for string concatenation
config: hexagon-randconfig-r123-20260816 (https://download.01.org/0day-ci/archive/20260817/202608170556.RyB6XrT0-lkp@intel.com/config)
compiler: clang version 18.1.8 (https://github.com/llvm/llvm-project 3b5b5c1ec4a3095ab096dd780e84d7ab81f3d7ff)
sparse: v0.6.5-rc1
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260817/202608170556.RyB6XrT0-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202608170556.RyB6XrT0-lkp@intel.com/

All warnings (new ones prefixed by >>, old ones prefixed by <<):

>> WARNING: modpost: vmlinux: section mismatch in reference: seq_buf_str+0x4 (section: .text) -> bootup_event_buf (section: .init.data)
WARNING: modpost: vmlinux: section mismatch in reference: seq_buf_str+0x8 (section: .text) -> bootup_event_buf (section: .init.data)
>> WARNING: modpost: vmlinux: section mismatch in reference: seq_buf_str+0xe0 (section: .text) -> __TRACE_SYSTEM_ERROR_DETECTOR_KFENCE (section: .init.data)
WARNING: modpost: vmlinux: section mismatch in reference: seq_buf_str+0xe4 (section: .text) -> __TRACE_SYSTEM_ERROR_DETECTOR_KFENCE (section: .init.data)
WARNING: modpost: vmlinux: section mismatch in reference: seq_buf_str+0xfc (section: .text) -> bootup_event_buf (section: .init.data)
WARNING: modpost: vmlinux: section mismatch in reference: seq_buf_str+0x100 (section: .text) -> bootup_event_buf (section: .init.data)
WARNING: modpost: vmlinux: section mismatch in reference: seq_buf_str+0x1cc (section: .text) -> bootup_event_buf (section: .init.data)
WARNING: modpost: vmlinux: section mismatch in reference: seq_buf_str+0x1d0 (section: .text) -> bootup_event_buf (section: .init.data)
WARNING: modpost: vmlinux: section mismatch in reference: seq_buf_str+0x1f4 (section: .text) -> bootup_event_buf (section: .init.data)
WARNING: modpost: vmlinux: section mismatch in reference: seq_buf_str+0x1f8 (section: .text) -> bootup_event_buf (section: .init.data)
WARNING: modpost: vmlinux: section mismatch in reference: seq_buf_str+0x228 (section: .text) -> bootup_event_buf (section: .init.data)
WARNING: modpost: vmlinux: section mismatch in reference: seq_buf_str+0x22c (section: .text) -> bootup_event_buf (section: .init.data)

--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki