linux-kernel - [PATCH 5.16 0984/1039] taskstats: Cleanup the use of task->exit

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Message-Id: <20220124184158.349632089@linuxfoundation.org>
Date:   Mon, 24 Jan 2022 19:46:13 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Balbir Singh <bsingharora@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH 5.16 0984/1039] taskstats: Cleanup the use of task->exit_code

From: Eric W. Biederman <ebiederm@...ssion.com>

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@...il.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@...ssion.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 kernel/tsacct.c |    7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

--- a/kernel/tsacct.c
+++ b/kernel/tsacct.c
@@ -38,11 +38,10 @@ void bacct_add_tsk(struct user_namespace
 	stats->ac_btime = clamp_t(time64_t, btime, 0, U32_MAX);
 	stats->ac_btime64 = btime;

-	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)