Re: [RFC][PATCH v2 19/31] timers: net: Use del_timer_shutdown() before freeing timer

[Steven Rostedt]

On Thu, 27 Oct 2022 12:38:16 -0700
Guenter Roeck <linux@xxxxxxxxxxxx> wrote:

> On 10/27/22 12:27, Steven Rostedt wrote:
> > On Thu, 27 Oct 2022 15:20:58 -0400
> > Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> >   
> >>> (many more of those)
> >>> ...
> >>> [   16.329989]  timer_fixup_free+0x40/0x54  
> >>
> >> Ah, I see the issue here. Looks like the timer_fixup_free() is calling
> >> itself and crashing.
> >>
> >> Let me take a look into that. I didn't touch the fixup code, and there
> >> could be an assumption there that it's behaving with the old approach.  
> > 
> > Can you add this and see if it makes this issue go away?
> >   
> 
> Yes, that fixes the crash. However, it still reports
> 
> [   12.235054] ------------[ cut here ]------------
> [   12.235240] ODEBUG: free active (active state 0) object type: timer_list hint: tcp_write_timer+0x0/0x190
> [   12.237331] WARNING: CPU: 0 PID: 310 at lib/debugobjects.c:502 debug_print_object+0xb8/0x100
> ...
> [   12.255251] Call trace:
> [   12.255305]  debug_print_object+0xb8/0x100
> [   12.255385]  __debug_check_no_obj_freed+0x1d0/0x25c
> [   12.255474]  debug_check_no_obj_freed+0x20/0x90
> [   12.255555]  slab_free_freelist_hook.constprop.0+0xac/0x1b0
> [   12.255650]  kmem_cache_free+0x1ac/0x500
> [   12.255728]  __sk_destruct+0x140/0x2a0
> [   12.255805]  sk_destruct+0x54/0x64
> [   12.255877]  __sk_free+0x74/0x120
> [   12.255944]  sk_free+0x64/0x8c
> [   12.256009]  tcp_close+0x94/0xc0
> [   12.256076]  inet_release+0x50/0xb0
> [   12.256145]  __sock_release+0x44/0xbc
> [   12.256219]  sock_close+0x18/0x30
> [   12.256292]  __fput+0x84/0x270
> [   12.256361]  ____fput+0x10/0x20
> [   12.256426]  task_work_run+0x88/0xf0
> [   12.256499]  do_exit+0x334/0xafc
> [   12.256566]  do_group_exit+0x34/0x90
> [   12.256634]  __arm64_sys_exit_group+0x18/0x20
> [   12.256713]  invoke_syscall+0x48/0x114
> [   12.256789]  el0_svc_common.constprop.0+0x60/0x11c
> [   12.256874]  do_el0_svc+0x30/0xd0
> [   12.256943]  el0_svc+0x48/0xc0
> [   12.257008]  el0t_64_sync_handler+0xbc/0x13c
> [   12.257086]  el0t_64_sync+0x18c/0x190
> 
> Is that a real problem or a false positive ? I didn't see that
> without your patch series (which of course might be the whole point
> of the series).
> 

I think this is indeed an issue, and I'm replying to the net patch as it
has the necessary folks Cc'd.

The ipv4 tcp code has:

void tcp_init_xmit_timers(struct sock *sk)
{
	inet_csk_init_xmit_timers(sk, &tcp_write_timer, &tcp_delack_timer,
				  &tcp_keepalive_timer);

And from the above back trace:

tcp_close() where I'm assuming that tcp_disconnect() or tcp_done() was
called that both calls:

  tcp_clear_xmit_timers(sk);

That calls:

	inet_csk_clear_xmit_timers(sk);

That has:

void inet_csk_clear_xmit_timers(struct sock *sk)
{
	struct inet_connection_sock *icsk = inet_csk(sk);

	icsk->icsk_pending = icsk->icsk_ack.pending = 0;

	sk_stop_timer(sk, &icsk->icsk_retransmit_timer);
	sk_stop_timer(sk, &icsk->icsk_delack_timer);
	sk_stop_timer(sk, &sk->sk_timer);
}

Where:

void sk_stop_timer(struct sock *sk, struct timer_list* timer)
{
	if (del_timer(timer))
		__sock_put(sk);
}


Hence, this is a case where we have timers that have been disabled with
only del_timer() before the timers are freed.

I think we need to update this code to squeeze in a del_timer_shutdown() to
make sure that the timers are never restarted.

There is a sk_stop_timer_sync() that I changed to use del_timer_shutdown()
but that's only used in one file: net/mptcp/pm_netlink.c

-- Steve