Date: Mon, 12 May 2025 10:22:43 +1200
From: Tao Liu <ltao@redhat.com>
Subject: [Crash-utility] Re: [PATCH] Fix incorrect task state during
        exit
To: Stephen Brennan <stephen.s.brennan@oracle.com>
Cc: devel@lists.crash-utility.osci.io
Message-ID:
        <CAO7dBbWHa5x+BdiVF3y_QL-JgQhDLgEqKzaQS+YfBrL=jY6N_w@mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"

Hi Stephen,

Thanks for your fix and detailed explanation!

On Sat, May 3, 2025 at 8:19 AM Stephen Brennan
<stephen.s.brennan@oracle.com> wrote:
>
> task_state() assumes that exit_state is a unsigned long, when in
> reality, it has been declared as an int since 97dc32cdb1b53 ("reduce
> size of task_struct on 64-bit machines"), in Linux 2.6.22. So on 64-bit
> machines, task_state() reads 8 bytes rather than 4, and gets the wrong
> exit_state value by including the next field.
>
> This has gone unnoticed because directly after exit_state comes
> exit_code, which is generally zero while the task is alive. When the
> exit_code is set, exit_state is usually set not long after. Since
> task_state_string() only checks whether exit_state bits are set, it
> never notices the presence of the exit code inside of the state.
>
> But this leaves open a window during the process exit, when the
> exit_code has been set (in do_exit()), but the exit_state has not (in
> exit_notify()). In this case, crash reports a state of "??", but in
> reality, the task is still running -- it's just running the exit()
> system call. This race window can be long enough to be observed in core
> dumps, for example if the mmput() takes a long time.
>
> This should be considered a bug. A task state of "??" or "(unknown)" is
> frequently of concern when debugging, as it could indicate that the
> state fields had some sort of corruption, and draw the attention of the
> debugger. To handle it properly, record the size of exit_state, and read
> it conditionally as a UINT or ULONG, just like the state. This ensures
> we retain compatibility with kernel before v2.6.22. Whether that is
> actually desirable is anybody's guess.
>
> Reported-by: Jeffery Yoder <jeffery.yoder@oracle.com>
> Signed-off-by: Stephen Brennan <stephen.s.brennan@oracle.com>
> ---
>  defs.h |  1 +
>  task.c | 11 +++++++++--
>  2 files changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/defs.h b/defs.h
> index 4cf169c..58362d0 100644
> --- a/defs.h
> +++ b/defs.h
> @@ -2435,6 +2435,7 @@ struct size_table {         /* stash of commonly-used sizes */
>         long prb_desc;
>         long wait_queue_entry;
>         long task_struct_state;
> +       long task_struct_exit_state;
>         long printk_safe_seq_buf_buffer;
>         long sbitmap_word;
>         long sbitmap;

The patch looks good to me, except for the above code. Any newly added

members for size/offset_table should go to the end of the struct,
rather than the middle of it. See the 2nd item of
https://github.com/crash-utility/crash/wiki#writing-patches. But this
is a slight error, I can get it corrected when merging. Other than
that, Ack for the patch.

Thanks,
Tao Liu

> diff --git a/task.c b/task.c
> index 3bafe79..e07b479 100644
> --- a/task.c
> +++ b/task.c
> @@ -306,6 +306,7 @@ task_init(void)
>                 MEMBER_SIZE_INIT(task_struct_state, "task_struct", "__state");
>         }
>          MEMBER_OFFSET_INIT(task_struct_exit_state, "task_struct", "exit_state");
> +        MEMBER_SIZE_INIT(task_struct_exit_state, "task_struct", "exit_state");
>          MEMBER_OFFSET_INIT(task_struct_pid, "task_struct", "pid");
>          MEMBER_OFFSET_INIT(task_struct_comm, "task_struct", "comm");
>          MEMBER_OFFSET_INIT(task_struct_next_task, "task_struct", "next_task");
> @@ -5965,8 +5966,14 @@ task_state(ulong task)
>                 state = ULONG(tt->task_struct + OFFSET(task_struct_state));
>         else
>                 state = UINT(tt->task_struct + OFFSET(task_struct_state));
> -       exit_state = VALID_MEMBER(task_struct_exit_state) ?
> -               ULONG(tt->task_struct + OFFSET(task_struct_exit_state)) : 0;
> +
> +       if (VALID_MEMBER(task_struct_exit_state)
> +           && SIZE(task_struct_exit_state) == sizeof(ulong))
> +           exit_state = ULONG(tt->task_struct + OFFSET(task_struct_exit_state));
> +       else if (VALID_MEMBER(task_struct_exit_state))
> +           exit_state = UINT(tt->task_struct + OFFSET(task_struct_exit_state));
> +       else
> +           exit_state = 0;
>
>          return (state | exit_state);
>  }
> --