Re: [PATCH v3 02/16] tracing/filters: Fix MATCH_END_ONLY regex matching for fixed-size strings

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] tracing/filters: unhandled zero len in regex_match_end_str()
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commit 92251abf42afdb93bef9298fec46088f078a4aa8
Author: Masami Hiramatsu (Google) <mhiramat@kernel.org>

tracing/filters: Fix MATCH_END_ONLY regex matching for fixed-size strings

This commit introduces regex_match_end_str() to handle suffix regex matching
on fixed-size string buffers using strnlen(), and assigns it in parse_pred()
for comm and static string fields.

> diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c
> index 59a0d3ba72740..f7c067a58ba41 100644
> --- a/kernel/trace/trace_events_filter.c
> +++ b/kernel/trace/trace_events_filter.c
> @@ -1087,6 +1087,20 @@ static int regex_match_end(char *str, struct regex *r, int len)
>  	return 0;
>  }
>  
> +static int regex_match_end_str(char *str, struct regex *r, int len)
> +{
> +	int slen;
> +
> +	if (len < r->len)
> +		return 0;
> +
> +	slen = strnlen(str, len);
> +	if (slen >= r->len &&
> +	    memcmp(str + slen - r->len, r->pattern, r->len) == 0)
> +		return 1;
> +	return 0;
> +}

[Severity: Medium]
How does regex_match_end_str() behave when len is 0?

For the ftrace/print event, the buf field is defined in
kernel/trace/trace_export.c with a size of 0:

#undef __dynamic_array
#define __dynamic_array(_type, _item) {					\
	.type = #_type "[]", .name = #_item,				\
	.size = 0, .align = __alignof__(_type),				\
	is_signed_type(_type), .filter_type = FILTER_OTHER },

Because its type is "char[]", filter_assign_type() classifies it as a static
string field while field->size remains 0. In parse_pred(),
pred->regex->field_len is set to field->size (0) and pred->regex->match is
set to regex_match_end_str():

		} else if (field->filter_type == FILTER_STATIC_STRING) {
			pred->fn_num = FILTER_PRED_FN_STRING;
			pred->regex->field_len = field->size;
			if (pred->regex->match == regex_match_end)
				pred->regex->match = regex_match_end_str;

When a trigger or eprobe with a suffix filter (such as buf ~ "*foo") is
evaluated via write_marker_to_buffer() -> event_triggers_call() ->
filter_match_preds() -> filter_pred_string(), regex_match_end_str() is called
with len == 0.

As noted in the comment above regex_match_full() (added in commit
10f20e9f9da9 ("tracing: Have zero size length in filter logic be full
string")), a len of zero indicates that str is a NUL-terminated string, which
regex_match_full(), regex_match_front(), and regex_match_middle() handle
explicitly:

static int regex_match_full(char *str, struct regex *r, int len)
{
	/* len of zero means str is dynamic and ends with '\0' */
	if (!len)
		return strcmp(str, r->pattern) == 0;

In regex_match_end_str(), however, when len is 0, the (len < r->len) check
returns 0 for any non-empty pattern, and strnlen(str, 0) also returns 0.

Should regex_match_end_str() check for !len and use strlen(str) in that case,
so that suffix filters on ftrace/print work as expected?

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Sashiko AI review · https://sashiko.dev/#/patchset/179314820431.684037.4147763044424284198.stgit@devnote2?part=2