Re: [PATCH] tracing/filters: Treat zero-length dynamic strings as empty

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

On Tue, 06 Oct 2026 15:56:42 -0700, Kyle Zeng <kylebot@openai.com> wrote:
> The regex callbacks use a length of zero for NUL-terminated strings of
> unknown size, such as the trailing buffer in a trace_marker record.
> However, filter_pred_strloc() and filter_pred_strrelloc() pass the length
> from a data locator directly to those callbacks. A zero-length locator
> has no data and need not point at a NUL byte.
>
> In particular, syscall entry events can contain an empty captured array
> or a failed user-string copy. Matching such a field with an equality,
> prefix, or substring filter can read beyond the record. The per-CPU perf
> trace buffer is reused, so an unprivileged counting syscall event can
> disclose bytes left by a previous kernel tracepoint even when raw
> sampling is denied and exclude_kernel prevents counting that tracepoint.
>
> Handle both locator formats through a common helper. For a zero-length
> field, pass an empty string and its actual size to the regex callback.
> This gives all matching modes the empty-string semantics without reading
> the locator's target. Keep nonzero fields and the legacy trace_marker
> unknown-length convention unchanged.
>
> Fixes: 10f20e9f9da9 ("tracing: Have zero size length in filter logic be full string")
> Cc: stable@vger.kernel.org
> Assisted-by: Codex:gpt-6-astra

nit:
Assisted-by: LLM

> Signed-off-by: Kyle Zeng <kylebot@openai.com>

Looks good to me.

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

Thanks,

> ---
>  kernel/trace/trace_events_filter.c | 38 +++++++++++++++++++-----------
>  1 file changed, 24 insertions(+), 14 deletions(-)
>
> diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
> index 2b46ca536045..02aaa92e0361 100644
> --- a/kernel/trace/trace_events_filter.c
> +++ b/kernel/trace/trace_events_filter.c
> @@ -884,6 +884,25 @@ static int filter_pred_pchar_user(struct filter_pred *pred, void *event)
>  	return filter_pchar(pred, str);
>  }
>  
> +static __always_inline int filter_dyn_string(struct filter_pred *pred,
> +					     char *str, int len)
> +{
> +	int cmp;
> +
> +	/*
> +	 * The regex callbacks use zero for an unknown-length, NUL-terminated
> +	 * string. A zero-length data_loc or rel_loc field has no bytes at all.
> +	 * Match an empty string instead of reading past the end of the field.
> +	 */
> +	if (!len) {
> +		str = "";
> +		len = 1;
> +	}
> +
> +	cmp = pred->regex->match(str, pred->regex, len);
> +	return cmp ^ pred->not;
> +}
> +
>  /*
>   * Filter predicate for dynamic sized arrays of characters.
>   * These are implemented through a list of strings at the end
> @@ -900,13 +919,8 @@ static int filter_pred_strloc(struct filter_pred *pred, void *event)
>  	int str_loc = str_item & 0xffff;
>  	int str_len = str_item >> 16;
>  	char *addr = (char *)(event + str_loc);
> -	int cmp, match;
> -
> -	cmp = pred->regex->match(addr, pred->regex, str_len);
>  
> -	match = cmp ^ pred->not;
> -
> -	return match;
> +	return filter_dyn_string(pred, addr, str_len);
>  }
>  
>  /*
> @@ -923,13 +937,8 @@ static int filter_pred_strrelloc(struct filter_pred *pred, void *event)
>  	int str_loc = str_item & 0xffff;
>  	int str_len = str_item >> 16;
>  	char *addr = (char *)(&item[1]) + str_loc;
> -	int cmp, match;
> -
> -	cmp = pred->regex->match(addr, pred->regex, str_len);
>  
> -	match = cmp ^ pred->not;
> -
> -	return match;
> +	return filter_dyn_string(pred, addr, str_len);
>  }
>  
>  /* Filter predicate for CPUs. */
> @@ -1014,7 +1023,8 @@ static int filter_pred_function(struct filter_pred *pred, void *event)
>   *
>   * @str: the string to be searched
>   * @r:   the regex structure containing the pattern string
> - * @len: the length of the string to be searched (including '\0')
> + * @len: the length of the string to be searched (including '\0'), or zero
> + *       for a NUL-terminated string of unknown length
>   *
>   * Note:
>   * - @str might not be NULL-terminated if it's of type DYN_STRING
> @@ -1023,7 +1033,7 @@ static int filter_pred_function(struct filter_pred *pred, void *event)
>  
>  static int regex_match_full(char *str, struct regex *r, int len)
>  {
> -	/* len of zero means str is dynamic and ends with '\0' */
> +	/* Zero is reserved for a NUL-terminated string of unknown length. */
>  	if (!len)
>  		return strcmp(str, r->pattern) == 0;
>  
>

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