[PATCH] sched: fix possible divide by zero in avg_atom calculation

[PATCH] sched: fix possible divide by zero in avg_atom calculation

[Mateusz Guzik]

proc_sched_show_task does:
if (nr_switches)
	do_div(avg_atom, nr_switches);

nr_switches is unsigned long and do_div truncates it to 32 bits, which
means it can test non-zero on e.g. x86-64 and be truncated to zero for
division.

Fix the problem by using div64_ul instead.

As a side effect calculations of avg_atom for big nr_switches are now correct.

Signed-off-by: Mateusz Guzik <mguzik@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
---
 kernel/sched/debug.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 695f977..627b3c3 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -608,7 +608,7 @@ void proc_sched_show_task(struct task_struct *p, struct seq_file *m)
 
 		avg_atom = p->se.sum_exec_runtime;
 		if (nr_switches)
-			do_div(avg_atom, nr_switches);
+			avg_atom = div64_ul(avg_atom, nr_switches);
 		else
 			avg_atom = -1LL;
 
-- 
1.8.3.1

Re: [PATCH] sched: fix possible divide by zero in avg_atom calculation

[Peter Zijlstra]

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On Sat, Jun 14, 2014 at 03:00:09PM +0200, Mateusz Guzik wrote:
> proc_sched_show_task does:
> if (nr_switches)
> 	do_div(avg_atom, nr_switches);
> 
> nr_switches is unsigned long and do_div truncates it to 32 bits, which
> means it can test non-zero on e.g. x86-64 and be truncated to zero for
> division.
> 
> Fix the problem by using div64_ul instead.
> 
> As a side effect calculations of avg_atom for big nr_switches are now correct.
> 
> Signed-off-by: Mateusz Guzik <mguzik@redhat.com>

Thanks.

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[tip:sched/urgent] sched: Fix possible divide by zero in avg_atom () calculation

[tip-bot for Mateusz Guzik]

Commit-ID:  b0ab99e7736af88b8ac1b7ae50ea287fffa2badc
Gitweb:     http://git.kernel.org/tip/b0ab99e7736af88b8ac1b7ae50ea287fffa2badc
Author:     Mateusz Guzik <mguzik@redhat.com>
AuthorDate: Sat, 14 Jun 2014 15:00:09 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 16 Jul 2014 13:36:07 +0200

sched: Fix possible divide by zero in avg_atom() calculation

proc_sched_show_task() does:

  if (nr_switches)
	do_div(avg_atom, nr_switches);

nr_switches is unsigned long and do_div truncates it to 32 bits, which
means it can test non-zero on e.g. x86-64 and be truncated to zero for
division.

Fix the problem by using div64_ul() instead.

As a side effect calculations of avg_atom for big nr_switches are now correct.

Signed-off-by: Mateusz Guzik <mguzik@redhat.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1402750809-31991-1-git-send-email-mguzik@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/debug.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 695f977..627b3c3 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -608,7 +608,7 @@ void proc_sched_show_task(struct task_struct *p, struct seq_file *m)
 
 		avg_atom = p->se.sum_exec_runtime;
 		if (nr_switches)
-			do_div(avg_atom, nr_switches);
+			avg_atom = div64_ul(avg_atom, nr_switches);
 		else
 			avg_atom = -1LL;