Re: [PATCH 01/10] tracing/filters: Fix MATCH_END_ONLY regex matching for fixed-size strings

[Steven Rostedt <rostedt@goodmis.org>]

On Sun, 25 Oct 2026 01:25:33 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> regex_match_end() calculated string length as "len - 1", assuming @len is
> the string length including trailing '\0'. However, for fixed-size string
> fields, filter_pred_string() passes field->size as @len, and for "comm",
> filter_pred_comm() passes TASK_COMM_LEN as @len.
> 
> Because @len in these cases represents the buffer capacity rather than the
> actual length of the string, regex_match_end() tests against the tail of
> the buffer (typically trailing '\0' padding) rather than the end of the
> string. Furthermore, if @len is 0 (denoting a dynamic string ending with
> '\0'), "len - 1" underflows to -1 and never matches.
> 
> Compute the actual string length using strnlen(str, len) when @len is
> non-zero, and strlen(str) when @len is zero.
> 
> Fixes: a3291c14ecf0 ("tracing/filters: Fix MATCH_END_ONLY filter matching")
> Cc: stable@vger.kernel.org
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
>  kernel/trace/trace_events_filter.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
> index 5e74d2dae40a..18ce3ec00d34 100644
> --- a/kernel/trace/trace_events_filter.c
> +++ b/kernel/trace/trace_events_filter.c
> @@ -1075,10 +1075,10 @@ static int regex_match_middle(char *str, struct regex *r, int len)
>  
>  static int regex_match_end(char *str, struct regex *r, int len)
>  {
> -	int strlen = len - 1;
> +	int slen = len ? strnlen(str, len) : strlen(str);
>  
> -	if (strlen >= r->len &&
> -	    memcmp(str + strlen - r->len, r->pattern, r->len) == 0)
> +	if (slen >= r->len &&
> +	    memcmp(str + slen - r->len, r->pattern, r->len) == 0)
>  		return 1;

This slows down the code as it adds more strlen() calculations (it could
have already been done.

Let's fix the callers instead and have them pass in the real length.

-- Steve


>  	return 0;
>  }