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

On 22/6/2569 BE 00:16, Jori Koolstra wrote:

quoted

Op 20-06-2026 19:54 CEST schreef Woradorn Laodhanadhaworn [off-list ref]:

 
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 <redacted>
---
 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;

Isn't this now unused?

quoted

+static struct seq_buf bootup_event_seq;
+static bool bootup_event_seq_initialized;
 

I think this can be refactored to avoid the bool. And should bootup_event_seq not be
__initdata?

quoted

 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);

What if trace_event is empty? Then setup_trace_event does not run AFAIK. See the
WARN_ON in seq_buf_str too. Have you tested this?

quoted

 
 	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

Thanks,
Jori.

Thank you, Jori, for your review. I will send v2.
I tested both empty and non-empty trace_event cases with QEMU, and both now boot successfully. 

The logs below demonstrate this.

Non-empty trace_event:

qemu-system-x86_64 \
    -kernel arch/x86/boot/bzImage \
    -initrd initramfs.cpio.gz \
    -append "console=ttyS0 trace_event=:mod:rproc_qcom_common,:mod:qrtr,:mod:qcom_aoss trace_event=:mod:rproc_qcom_common" \
    -nographic \
    -m 512M

[    0.082316] Kernel command line: console=ttyS0 trace_event=:mod:rproc_qcom_common,:mod:qrtr,:mod:qcom_aoss trace_event=:mod:rproc_qcom_common
[    0.083324] bootup_event_buf: :mod:rproc_qcom_common,:mod:qrtr,:mod:qcom_aoss
[    0.083413] bootup_event_buf: :mod:rproc_qcom_common,:mod:qrtr,:mod:qcom_aoss,:mod:rproc_qcom_common
[    0.083689] printk: log buffer data + meta data: 262144 + 917504 = 1179648 bytes

Empty trace_event:

qemu-system-x86_64 \
    -kernel arch/x86/boot/bzImage \
    -initrd initramfs.cpio.gz \
    -append "console=ttyS0" \
    -nographic \
    -m 512M

[    0.085213] Kernel command line: console=ttyS0
[    0.086458] printk: log buffer data + meta data: 262144 + 917504 = 1179648 bytes

Thanks,
Woradorn