Re: FAILED: patch "[PATCH] taskstats: Cleanup the use of task->exit_code" failed to apply to 5.4-stable tree

[Sudip Mukherjee <sudipm.mukherjee@gmail.com>]

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Hi Greg,

On Mon, Jan 24, 2022 at 02:11:16PM +0100, gregkh@linuxfoundation.org wrote:
> 
> The patch below does not apply to the 5.4-stable tree.
> If someone wants it applied there, or to any other stable or longterm
> tree, then please email the backport, including the original git commit
> id to <stable@vger.kernel.org>.

Here is the backport, will also apply to all branches till 4.9-stable.

--
Regards
Sudip

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From ca66fe9c13da7f1d14149b4a11bbd5dc2950180c Mon Sep 17 00:00:00 2001
From: "Eric W. Biederman" <ebiederm@xmission.com>
Date: Mon, 3 Jan 2022 11:32:36 -0600
Subject: [PATCH] taskstats: Cleanup the use of task->exit_code

commit 1b5a42d9c85f0e731f01c8d1129001fd8531a8a0 upstream.

In the function bacct_add_task the code reading task->exit_code was
introduced in commit f3cef7a99469 ("[PATCH] csa: basic accounting over
taskstats"), and it is not entirely clear what the taskstats interface
is trying to return as only returning the exit_code of the first task
in a process doesn't make a lot of sense.

As best as I can figure the intent is to return task->exit_code after
a task exits.  The field is returned with per task fields, so the
exit_code of the entire process is not wanted.  Only the value of the
first task is returned so this is not a useful way to get the per task
ptrace stop code.  The ordinary case of returning this value is
returning after a task exits, which also precludes use for getting
a ptrace value.

It is common to for the first task of a process to also be the last
task of a process so this field may have done something reasonable by
accident in testing.

Make ac_exitcode a reliable per task value by always returning it for
every exited task.

Setting ac_exitcode in a sensible mannter makes it possible to continue
to provide this value going forward.

Cc: Balbir Singh <bsingharora@gmail.com>
Fixes: f3cef7a99469 ("[PATCH] csa: basic accounting over taskstats")
Link: https://lkml.kernel.org/r/20220103213312.9144-5-ebiederm@xmission.com
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
[sudip: adjust context]
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
---
 kernel/tsacct.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/kernel/tsacct.c b/kernel/tsacct.c
index 7be3e7530841..33a4093306f9 100644
--- a/kernel/tsacct.c
+++ b/kernel/tsacct.c
@@ -35,11 +35,10 @@ void bacct_add_tsk(struct user_namespace *user_ns,
 	/* Convert to seconds for btime */
 	do_div(delta, USEC_PER_SEC);
 	stats->ac_btime = get_seconds() - delta;
-	if (thread_group_leader(tsk)) {
+	if (tsk->flags & PF_EXITING)
 		stats->ac_exitcode = tsk->exit_code;
-		if (tsk->flags & PF_FORKNOEXEC)
-			stats->ac_flag |= AFORK;
-	}
+	if (thread_group_leader(tsk) && (tsk->flags & PF_FORKNOEXEC))
+		stats->ac_flag |= AFORK;
 	if (tsk->flags & PF_SUPERPRIV)
 		stats->ac_flag |= ASU;
 	if (tsk->flags & PF_DUMPCORE)
-- 
2.30.2