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ftrace: Avoid potential division by zero in function_stat_show()

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Check whether denominator expression x * (x - 1) * 1000 mod {2^32, 2^64}
produce zero and skip stddev computation in that case.

For now don't care about rec->counter * rec->counter overflow because
rec->time * rec->time overflow will likely happen earlier.

Cc: stable@vger.kernel.org
Cc: Wen Yang <wenyang@linux.alibaba.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://lore.kernel.org/20250206090156.1561783-1-kniv@yandex-team.ru
Fixes: e31f7939c1c27 ("ftrace: Avoid potential division by zero in function profiler")
Signed-off-by: Nikolay Kuratov <kniv@yandex-team.ru>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
This commit is contained in:
Nikolay Kuratov 2025-02-06 12:01:56 +03:00 committed by Steven Rostedt (Google)
parent 3908b6baf2
commit a1a7eb89ca
1 changed files with 12 additions and 15 deletions
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27
kernel/trace/ftrace.c
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@ -540,6 +540,7 @@ static int function_stat_show(struct seq_file *m, void *v)
static struct trace_seq s;
unsigned long long avg;
unsigned long long stddev;
unsigned long long stddev_denom;
#endif
guard(mutex)(&ftrace_profile_lock);
@ -559,23 +560,19 @@ static int function_stat_show(struct seq_file *m, void *v)
#ifdef CONFIG_FUNCTION_GRAPH_TRACER
seq_puts(m, " ");
/* Sample standard deviation (s^2) */
if (rec->counter <= 1)
stddev = 0;
else {
/*
* Apply Welford's method:
* s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2)
*/
/*
* Variance formula:
* s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2)
* Maybe Welford's method is better here?
* Divide only by 1000 for ns^2 -> us^2 conversion.
* trace_print_graph_duration will divide by 1000 again.
*/
stddev = 0;
stddev_denom = rec->counter * (rec->counter - 1) * 1000;
if (stddev_denom) {
stddev = rec->counter * rec->time_squared -
rec->time * rec->time;
/*
* Divide only 1000 for ns^2 -> us^2 conversion.
* trace_print_graph_duration will divide 1000 again.
*/
stddev = div64_ul(stddev,
rec->counter * (rec->counter - 1) * 1000);
stddev = div64_ul(stddev, stddev_denom);
}
trace_seq_init(&s);