Message-ID: <CAC1LvL1xgFMjjE+3wHH79_9rumwjNqDAS2Yg2NpSvmewHsYScA@mail.gmail.com>
Date:   Tue, 21 Sep 2021 10:31:24 -0700
From:   Zvi Effron <zeffron@...tgames.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     Lorenz Bauer <lmb@...udflare.com>,
        Lorenzo Bianconi <lbianconi@...hat.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        netdev@...r.kernel.org, bpf <bpf@...r.kernel.org>
Subject: Re: Redux: Backwards compatibility for XDP multi-buff

On Tue, Sep 21, 2021 at 9:06 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Hi Lorenz (Cc. the other people who participated in today's discussion)
>
> Following our discussion at the LPC session today, I dug up my previous
> summary of the issue and some possible solutions[0]. Seems no on
> actually replied last time, which is why we went with the "do nothing"
> approach, I suppose. I'm including the full text of the original email
> below; please take a look, and let's see if we can converge on a
> consensus here.
>
> First off, a problem description: If an existing XDP program is exposed
> to an xdp_buff that is really a multi-buffer, while it will continue to
> run, it may end up with subtle and hard-to-debug bugs: If it's parsing
> the packet it'll only see part of the payload and not be aware of that
> fact, and if it's calculating the packet length, that will also only be
> wrong (only counting the first fragment).
>
> So what to do about this? First of all, to do anything about it, XDP
> programs need to be able to declare themselves "multi-buffer aware" (but
> see point 1 below). We could try to auto-detect it in the verifier by
> which helpers the program is using, but since existing programs could be
> perfectly happy to just keep running, it probably needs to be something
> the program communicates explicitly. One option is to use the
> expected_attach_type to encode this; programs can then declare it in the
> source by section name, or the userspace loader can set the type for
> existing programs if needed.
>
> With this, the kernel will know if a given XDP program is multi-buff
> aware and can decide what to do with that information. For this we came
> up with basically three options:
>
> 1. Do nothing. This would make it up to users / sysadmins to avoid
>    anything breaking by manually making sure to not enable multi-buffer
>    support while loading any XDP programs that will malfunction if
>    presented with an mb frame. This will probably break in interesting
>    ways, but it's nice and simple from an implementation PoV. With this
>    we don't need the declaration discussed above either.
>
> 2. Add a check at runtime and drop the frames if they are mb-enabled and
>    the program doesn't understand it. This is relatively simple to
>    implement, but it also makes for difficult-to-understand issues (why
>    are my packets suddenly being dropped?), and it will incur runtime
>    overhead.
>
> 3. Reject loading of programs that are not MB-aware when running in an
>    MB-enabled mode. This would make things break in more obvious ways,
>    and still allow a userspace loader to declare a program "MB-aware" to
>    force it to run if necessary. The problem then becomes at what level
>    to block this?
>

I think there's another potential problem with this as well: what happens to
already loaded programs that are not MB-aware? Are they forcibly unloaded?

>    Doing this at the driver level is not enough: while a particular
>    driver knows if it's running in multi-buff mode, we can't know for
>    sure if a particular XDP program is multi-buff aware at attach time:
>    it could be tail-calling other programs, or redirecting packets to
>    another interface where it will be processed by a non-MB aware
>    program.
>
>    So another option is to make it a global toggle: e.g., create a new
>    sysctl to enable multi-buffer. If this is set, reject loading any XDP
>    program that doesn't support multi-buffer mode, and if it's unset,
>    disable multi-buffer mode in all drivers. This will make it explicit
>    when the multi-buffer mode is used, and prevent any accidental subtle
>    malfunction of existing XDP programs. The drawback is that it's a
>    mode switch, so more configuration complexity.
>

Could we combine the last two bits here into a global toggle that doesn't
require a sysctl? If any driver is put into multi-buffer mode, then the system
switches to requiring all programs be multi-buffer? When the last multi-buffer
enabled driver switches out of multi-buffer, remove the system-wide
restriction?

Regarding my above question, if non-MB-aware XDP programs are not forcibly
unloaded, then a global toggle is also insufficient. An existing non-MB-aware
XDP program would still beed to be rejected at attach time by the driver.

> None of these options are ideal, of course, but I hope the above
> explanation at least makes sense. If anyone has any better ideas (or can
> spot any flaws in the reasoning above) please don't hesitate to let us
> know!
>
> -Toke
>
> [0] https://lore.kernel.org/r/8735srxglb.fsf@toke.dk
>

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