[PATCH v2 3/6] proc: Use task_is_running() for wchan in /proc/$pid/stat

[Kees Cook]

The implementations of get_wchan() can be expensive. The only information
imparted here is whether or not a process is currently blocked in the
scheduler (and even this doesn't need to be exact). Avoid doing the
heavy lifting of stack walking and just report that information by using
task_is_running().

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Christian Brauner <christian.brauner@xxxxxxxxxx>
Cc: Anand K Mistry <amistry@xxxxxxxxxx>
Cc: "Kenta.Tada@xxxxxxxx" <Kenta.Tada@xxxxxxxx>
Cc: Alexey Gladkov <legion@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: "Michael Weiß" <michael.weiss@xxxxxxxxxxxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Helge Deller <deller@xxxxxx>
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 fs/proc/array.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/fs/proc/array.c b/fs/proc/array.c
index 49be8c8ef555..77cf4187adec 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -541,7 +541,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 	}
 
 	if (permitted && (!whole || num_threads < 2))
-		wchan = get_wchan(task);
+		wchan = !task_is_running(task);
 	if (!whole) {
 		min_flt = task->min_flt;
 		maj_flt = task->maj_flt;
@@ -606,10 +606,7 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
 	 *
 	 * This works with older implementations of procps as well.
 	 */
-	if (wchan)
-		seq_puts(m, " 1");
-	else
-		seq_puts(m, " 0");
+	seq_put_decimal_ull(m, " ", wchan);
 
 	seq_put_decimal_ull(m, " ", 0);
 	seq_put_decimal_ull(m, " ", 0);
-- 
2.30.2