netdev - Re: How do I avoid recvmsg races with IP

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Message-ID: <CALCETrU3EXkxANKQYHYpB-cELeOGKAwB-8wTxX+xJ0Ms+kx4Bg@mail.gmail.com>
Date:	Tue, 2 Jun 2015 17:03:40 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Hannes Frederic Sowa <hannes@...essinduktion.org>
Cc:	Andy Lutomirski <luto@...nel.org>,
	Network Development <netdev@...r.kernel.org>
Subject: Re: How do I avoid recvmsg races with IP_RECVERR?

On Tue, Jun 2, 2015 at 2:50 PM, Hannes Frederic Sowa
<hannes@...essinduktion.org> wrote:
> On Tue, Jun 2, 2015, at 23:42, Hannes Frederic Sowa wrote:
>> On Tue, Jun 2, 2015, at 23:33, Andy Lutomirski wrote:
>> > On Tue, Jun 2, 2015 at 2:17 PM, Hannes Frederic Sowa
>> > <hannes@...essinduktion.org> wrote:
>> > > On Tue, Jun 2, 2015, at 21:40, Andy Lutomirski wrote:
>> >
>> > [...]
>> >
>> > I do this already, which makes me think that there's a bug or another
>> > race somewhere.  I've only seen a failure once in several years of
>> > operation.
>> >
>> > The failure happened on a ping socket.  I suspect that the race is:
>> >
>> > ping_err: ip_icmp_error(...);
>> >
>> > user: recvmsg(MSG_ERRQUEUE) and dequeues the error.
>> >
>> > ping_err: sk_err = err;
>> >
>> > user: recvmsg(MSG_ERRQUEUE not set), and recvmsg sees and clears the
>> > error via sock_error.
>> >
>> > user: recvmsg(MSG_ERRQUEUE), and recvmsg returns -EAGAIN.
>> >
>> > Now the user code thinks that it was a real (non-transient) error and
>> > aborts.
>> >
>> > Shouldn't that sk->sk_err = err assignment at least use WRITE_ONCE?
>>
>> Hmm, I don't think this will help.
>>
>> > Even if this race were fixed, this interface still sucks IMO.
>>
>> Yes. :/
>>
>> My proposal would be to make the error conversion lazy:
>>
>> Keeping duplicate data is not a good idea in general: So we shouldn't
>> use sk->sk_err if IP_RECVERR is set at all but let sock_error just use
>> the sk_error_queue and extract the error code from there.
>>
>> Only if IP_RECVERR was not set, we use sk->sk_err logic.
>>
>> What do you think?
>
> I just noticed that this will probably break existing user space
> applications which require that icmp errors are transient even with
> IP_RECVERR. We can mark that with a bit in the sk_error_queue pointer
> and xchg the pointer, hmmm....

Do you mean to fix the race like this but to otherwise leave the semantics
alone?  That would be an improvement, but it might be nice to also add
a non-crappy API for this, too.

--Andy
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