netdev - Re: [PATCH 15/23] io_uring: enable BPF to submit SQEs

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Message-ID: <a5cc80c8-5dea-031f-703e-cc18d6625ad6@gmail.com>
Date:   Fri, 21 May 2021 10:33:50 +0100
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     io-uring@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jens Axboe <axboe@...nel.dk>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Horst Schirmeier <horst.schirmeier@...dortmund.de>,
        "Franz-B . Tuneke" <franz-bernhard.tuneke@...dortmund.de>,
        Christian Dietrich <stettberger@...ucode.de>
Subject: Re: [PATCH 15/23] io_uring: enable BPF to submit SQEs

On 5/21/21 2:07 AM, Alexei Starovoitov wrote:
> On Wed, May 19, 2021 at 03:13:26PM +0100, Pavel Begunkov wrote:
>>  
>> +BPF_CALL_3(io_bpf_queue_sqe, struct io_bpf_ctx *,		bpf_ctx,
>> +			     const struct io_uring_sqe *,	sqe,
>> +			     u32,				sqe_len)
>> +{
>> +	struct io_ring_ctx *ctx = bpf_ctx->ctx;
>> +	struct io_kiocb *req;
>> +
>> +	if (sqe_len != sizeof(struct io_uring_sqe))
>> +		return -EINVAL;
>> +
>> +	req = io_alloc_req(ctx);
> 
> that is GFP_KERNEL allocation.
> It's only allowed from sleepable bpf progs and further down
> there is a correct check for it, so all good.
> But submitting sqe is a fundemntal io_uring operation,
> so what is the use case for non-sleepable?
> In other words why bother? Allow sleepable only and simplify the code?

Actual submission may be moved out of BPF, so enabling it for both, but
the question I wonder about is what are the plans for sleepable
programs? E.g. if it's a marginal features much limited in
functionality, e.g. iirc as it's not allowed to use some BPF data
types, it may not worth doing.

> 
>> +	if (unlikely(!req))
>> +		return -ENOMEM;
>> +	if (!percpu_ref_tryget_many(&ctx->refs, 1)) {
>> +		kmem_cache_free(req_cachep, req);
>> +		return -EAGAIN;
>> +	}
>> +	percpu_counter_add(&current->io_uring->inflight, 1);
>> +	refcount_add(1, &current->usage);
>> +
>> +	/* returns number of submitted SQEs or an error */
>> +	return !io_submit_sqe(ctx, req, sqe);
> 
> A buggy bpf prog will be able to pass junk sizeof(struct io_uring_sqe)
> as 'sqe' here.
> What kind of validation io_submit_sqe() does to avoid crashing the kernel?

It works on memory rw shared with userspace, so it already assumes
the worst
 
> General comments that apply to all patches:
> - commit logs are way too terse. Pls expand with details.
> - describe new bpf helpers in comments in bpf.h. Just adding them to an enum is not enough.
> - selftest/bpf are mandatory for all new bpf features.
> - consider bpf_link style of attaching bpf progs. We had enough issues with progs
>   that get stuck due to application bugs. Auto-detach saves the day more often than not.

Thanks for taking a look! I have no idea what bpf_link is, need
to check it out

-- 
Pavel Begunkov