Commit e0a4f46c authored by Nikolay Kuratov's avatar Nikolay Kuratov Committed by Pu Lehui
Browse files

ftrace: Avoid potential division by zero in function_stat_show() 

mainline inclusion
from mainline-v6.14-rc5
commit a1a7eb89ca0b89dc1c326eeee2596f263291aca3
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/IBY5DV

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a1a7eb89ca0b

--------------------------------

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: e31f7939 ("ftrace: Avoid potential division by zero in function profiler")
Signed-off-by: default avatarNikolay Kuratov <kniv@yandex-team.ru>
Signed-off-by: default avatarSteven Rostedt (Google) <rostedt@goodmis.org>
Conflicts:
	kernel/trace/ftrace.c
[The conflicts were due to some minor issue]
Signed-off-by: default avatarPu Lehui <pulehui@huawei.com>
parent 7951fd32

kernel/trace/ftrace.c

+12 −15
Original line number Diff line number Diff line
@@ -544,6 +544,7 @@ 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

	mutex_lock(&ftrace_profile_lock);
@@ -565,23 +566,19 @@ 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:

	 * 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);