linux-kernel - Re: [PATCH] NFC: llcp: fix nfc_llcp_send_ui

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Date:   Mon, 25 Jun 2018 23:44:22 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Samuel Ortiz <sameo@...ux.intel.com>,
        "David S. Miller" <davem@...emloft.net>,
        Steven Rostedt <rostedt@...dmis.org>,
        Petr Mladek <pmladek@...e.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        linux-wireless@...r.kernel.org, netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzbot <syzbot+d29d18215e477cfbfbdd@...kaller.appspotmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [PATCH] NFC: llcp: fix nfc_llcp_send_ui_frame() lockup



On 06/25/2018 10:12 PM, Sergey Senozhatsky wrote:
> On (06/26/18 07:07), Dmitry Vyukov wrote:
> [..]
>>>  #include <net/nfc/nfc.h>
>>> @@ -755,7 +756,8 @@ int nfc_llcp_send_ui_frame(struct nfc_llcp_sock *sock, u8 ssap, u8 dsap,
>>>                 pdu = nfc_alloc_send_skb(sock->dev, &sock->sk, MSG_DONTWAIT,
>>>                                          frag_len + LLCP_HEADER_SIZE, &err);
>>>                 if (pdu == NULL) {
>>> -                       pr_err("Could not allocate PDU\n");
>>> +                       pr_err_ratelimited("Could not allocate PDU\n");
>>> +                       cond_resched();
>>>                         continue;
>>>                 }
>>
>>
>> But this thread is still in an infinite (unkillable?) loop? If yes, we
>> are waiting for the next syzbot report ;)
> 
> The loop is still infinite, correct, but we have a preemption point now.
> Sure, net people can come with a much better solution, I'll be happy to
> scratch my patch.
> 

This can not be the right solution, think about current thread being real time,
cond_resched() might be a nop.

We should probably not loop at all, or not use MSG_DONTWAIT.

(And remove this useless "Could not allocate PDU" message)

NFC maintainers should really take a look at this.