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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: e31f7939c1 ("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; static struct trace_seq s;
unsigned long long avg; unsigned long long avg;
unsigned long long stddev; unsigned long long stddev;
unsigned long long stddev_denom;
#endif #endif
guard(mutex)(&ftrace_profile_lock); 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 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
seq_puts(m, " "); seq_puts(m, " ");
/* Sample standard deviation (s^2) */ /*
if (rec->counter <= 1) * Variance formula:
stddev = 0; * s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2)
else { * Maybe Welford's method is better here?
/* * Divide only by 1000 for ns^2 -> us^2 conversion.
* Apply Welford's method: * trace_print_graph_duration will divide by 1000 again.
* s^2 = 1 / (n * (n-1)) * (n * \Sum (x_i)^2 - (\Sum x_i)^2) */
*/ stddev = 0;
stddev_denom = rec->counter * (rec->counter - 1) * 1000;
if (stddev_denom) {
stddev = rec->counter * rec->time_squared - stddev = rec->counter * rec->time_squared -
rec->time * rec->time; rec->time * rec->time;
stddev = div64_ul(stddev, stddev_denom);
/*
* 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);
} }
trace_seq_init(&s); trace_seq_init(&s);