Messages in this thread | | | Date | Tue, 7 Jun 2022 20:20:35 +0800 (GMT+08:00) | From | duoming@zju ... | Subject | Re: [PATCH net-next] ax25: Fix deadlock caused by skb_recv_datagram in ax25_recvmsg |
| |
Hello,
On Mon, 6 Jun 2022 10:31:49 -0700 Eric Dumazet wrote:
> On Mon, Jun 6, 2022 at 9:21 AM Duoming Zhou <duoming@zju.edu.cn> wrote: > > > > The skb_recv_datagram() in ax25_recvmsg() will hold lock_sock > > and block until it receives a packet from the remote. If the client > > doesn`t connect to server and calls read() directly, it will not > > receive any packets forever. As a result, the deadlock will happen. > > > > The fail log caused by deadlock is shown below: > > > > [ 861.122612] INFO: task ax25_deadlock:148 blocked for more than 737 seconds. > > [ 861.124543] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. > > [ 861.127764] Call Trace: > > [ 861.129688] <TASK> > > [ 861.130743] __schedule+0x2f9/0xb20 > > [ 861.131526] schedule+0x49/0xb0 > > [ 861.131640] __lock_sock+0x92/0x100 > > [ 861.131640] ? destroy_sched_domains_rcu+0x20/0x20 > > [ 861.131640] lock_sock_nested+0x6e/0x70 > > [ 861.131640] ax25_sendmsg+0x46/0x420 > > [ 861.134383] ? ax25_recvmsg+0x1e0/0x1e0 > > [ 861.135658] sock_sendmsg+0x59/0x60 > > [ 861.136791] __sys_sendto+0xe9/0x150 > > [ 861.137212] ? __schedule+0x301/0xb20 > > [ 861.137710] ? __do_softirq+0x4a2/0x4fd > > [ 861.139153] __x64_sys_sendto+0x20/0x30 > > [ 861.140330] do_syscall_64+0x3b/0x90 > > [ 861.140731] entry_SYSCALL_64_after_hwframe+0x46/0xb0 > > [ 861.141249] RIP: 0033:0x7fdf05ee4f64 > > [ 861.141249] RSP: 002b:00007ffe95772fc0 EFLAGS: 00000246 ORIG_RAX: 000000000000002c > > [ 861.141249] RAX: ffffffffffffffda RBX: 0000565303a013f0 RCX: 00007fdf05ee4f64 > > [ 861.141249] RDX: 0000000000000005 RSI: 0000565303a01678 RDI: 0000000000000005 > > [ 861.141249] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000 > > [ 861.141249] R10: 0000000000000000 R11: 0000000000000246 R12: 0000565303a00cf0 > > [ 861.141249] R13: 00007ffe957730e0 R14: 0000000000000000 R15: 0000000000000000 > > > > This patch moves the skb_recv_datagram() before lock_sock() in order > > that other functions that need lock_sock could be executed. > > > > > Why is this targeting net-next tree ? > > 1) A fix should target net tree > 2) It should include a Fixes: tag
Thank you for your time and suggestions! I will change the target tree to net and add a Fixes: tag.
> Also: > - this patch bypasses tests in ax25_recvmsg() > - This might break applications depending on blocking read() operations. > > I feel a real fix is going to be slightly more difficult than that.
I think moving skb_recv_datagram() before lock_sock() is ok, because it does not hold lock_sock() and will not influence other operations. The applications would not break. What`s more, it is safe to move skb_recv_datagram() before lock_sock().
The check "if (sk->sk_type == SOCK_SEQPACKET && sk->sk_state != TCP_ESTABLISHED)" have to be protected by lock_sock(), because sk->sk_state may be changed by ax25_disconnect() in ax25_kill_by_device().
Best regards, Duoming Zhou
| |