Re: [PATCH v6] tracing: Bound synthetic-field strings with seq_buf

Masami Hiramatsu
2026-04-30T05:14:16Z

On Thu, 30 Apr 2026 12:33:50 +0800
Pengpeng Hou <pengpeng@iscas.ac.cn> wrote:

> The synthetic field helpers build a prefixed synthetic variable name and
> a generated hist command in fixed MAX_FILTER_STR_VAL buffers. The
> current code appends those strings with raw strcat(), so long key lists,
> field names, or saved filters can run past the end of the staging
> buffers.
>
> Build both strings with seq_buf and propagate -E2BIG if either the
> synthetic variable name or the generated command exceeds
> MAX_FILTER_STR_VAL. This keeps the existing tracing-side limit while
> using the helper intended for bounded command construction.
>

Good catch! This looks good to me.

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

Thank you,

(BTW, we need to use __free(kfree) in this file too.)

> Fixes: 02205a6752f2 ("tracing: Add support for 'field variables'")
> Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
> ---
> Changes since v5: https://lore.kernel.org/all/20260424070104.1-tracing-synth-v5-pengpeng@iscas.ac.cn/
> - start a new thread for the new patch revision
> - use a lore link for the previous version in the changelog
> - simplify the synthetic-name construction with seq_buf_printf()
> - keep saved_filter as a normal local variable and avoid an anonymous block
>
> kernel/trace/trace_events_hist.c | 41 ++++++++++++++++++++++----------
> 1 file changed, 29 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/trace/trace_events_hist.c b/kernel/trace/trace_events_hist.c
> index 0dbbf6cca9bc..aa8e7f043ac0 100644
> --- a/kernel/trace/trace_events_hist.c
> +++ b/kernel/trace/trace_events_hist.c
> @@ -8,6 +8,7 @@
> #include <linux/module.h>
> #include <linux/kallsyms.h>
> #include <linux/security.h>
> +#include <linux/seq_buf.h>
> #include <linux/mutex.h>
> #include <linux/slab.h>
> #include <linux/stacktrace.h>
> @@ -2968,14 +2969,23 @@ find_synthetic_field_var(struct hist_trigger_data *target_hist_data,
> char *system, char *event_name, char *field_name)
> {
> struct hist_field *event_var;
> + struct seq_buf s;
> char *synthetic_name;
>
> synthetic_name = kzalloc(MAX_FILTER_STR_VAL, GFP_KERNEL);
> if (!synthetic_name)
> return ERR_PTR(-ENOMEM);
>
> - strcpy(synthetic_name, "synthetic_");
> - strcat(synthetic_name, field_name);
> + seq_buf_init(&s, synthetic_name, MAX_FILTER_STR_VAL);
> + seq_buf_printf(&s, "synthetic_%s", field_name);
> +
> + /* Terminate synthetic_name with a NUL. */
> + seq_buf_str(&s);
> +
> + if (seq_buf_has_overflowed(&s)) {
> + kfree(synthetic_name);
> + return ERR_PTR(-E2BIG);
> + }
>
> event_var = find_event_var(target_hist_data, system, event_name, synthetic_name);
>
> @@ -3020,6 +3030,7 @@ create_field_var_hist(struct hist_trigger_data *target_hist_data,
> struct trace_event_file *file;
> struct hist_field *key_field;
> struct hist_field *event_var;
> + struct seq_buf s;
> char *saved_filter;
> char *cmd;
> int ret;
> @@ -3065,28 +3076,34 @@ create_field_var_hist(struct hist_trigger_data *target_hist_data,
> return ERR_PTR(-ENOMEM);
> }
>
> + seq_buf_init(&s, cmd, MAX_FILTER_STR_VAL);
> +
> /* Use the same keys as the compatible histogram */
> - strcat(cmd, "keys=");
> + seq_buf_puts(&s, "keys=");
>
> for_each_hist_key_field(i, hist_data) {
> key_field = hist_data->fields[i];
> if (!first)
> - strcat(cmd, ",");
> - strcat(cmd, key_field->field->name);
> + seq_buf_putc(&s, ',');
> + seq_buf_puts(&s, key_field->field->name);
> first = false;
> }
>
> /* Create the synthetic field variable specification */
> - strcat(cmd, ":synthetic_");
> - strcat(cmd, field_name);
> - strcat(cmd, "=");
> - strcat(cmd, field_name);
> + seq_buf_printf(&s, ":synthetic_%s=%s", field_name, field_name);
>
> /* Use the same filter as the compatible histogram */
> saved_filter = find_trigger_filter(hist_data, file);
> - if (saved_filter) {
> - strcat(cmd, " if ");
> - strcat(cmd, saved_filter);
> + if (saved_filter)
> + seq_buf_printf(&s, " if %s", saved_filter);
> +
> + /* Terminate cmd with a NUL. */
> + seq_buf_str(&s);
> +
> + if (seq_buf_has_overflowed(&s)) {
> + kfree(cmd);
> + kfree(var_hist);
> + return ERR_PTR(-E2BIG);
> }
>
> var_hist->cmd = kstrdup(cmd, GFP_KERNEL);
> --
> 2.50.1 (Apple Git-155)
>

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