linux-kernel - Re: [PATCH v2 2/8] io_uring/cmd: Introduce SOCKET_URING_OP

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Date:   Thu, 10 Aug 2023 13:57:54 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Breno Leitao <leitao@...ian.org>, sdf@...gle.com,
        axboe@...nel.dk
Cc:     bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, io-uring@...r.kernel.org, kuba@...nel.org,
        pabeni@...hat.com
Subject: Re: [PATCH v2 2/8] io_uring/cmd: Introduce SOCKET_URING_OP_GETSOCKOPT

On 8/9/23 14:21, Willem de Bruijn wrote:
> Breno Leitao wrote:
>> Add support for getsockopt command (SOCKET_URING_OP_GETSOCKOPT), where
>> level is SOL_SOCKET. This is leveraging the sockptr_t infrastructure,
>> where a sockptr_t is either userspace or kernel space, and handled as
>> such.
>>
>> Function io_uring_cmd_getsockopt() is inspired by __sys_getsockopt().
>>
>> Differently from the getsockopt(2), the optlen field is not a userspace
>> pointers. In getsockopt(2), userspace provides optlen pointer, which is
>> overwritten by the kernel.  In this implementation, userspace passes a
>> u32, and the new value is returned in cqe->res. I.e., optlen is not a
>> pointer.
>>
>> Important to say that userspace needs to keep the pointer alive until
>> the CQE is completed.
> 
> What bad things can happen otherwise?
> 
> The kernel is not depending on a well behaved process for its
> correctness here, is it? Any user pages have to be pinned while

Right, it's the user api thing. There are always userspace progs
that would try to do:

submit_async() {
	char buf[20];
	do_submit(sqe = {buf = buf, ...});
}

submit_async();
wait_completions();


> kernel might refer to them, for instance.

fwiw, it's passed down as a user ptr, which will be eventually
used in copy_[from,to]_user() or so.

-- 
Pavel Begunkov