Date:   Tue, 21 Sep 2021 12:17:44 -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 11:23 AM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Zvi Effron <zeffron@...tgames.com> writes:
>
> > 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?
>
> I'd say probably the opposite: You can't toggle whatever switch we end
> up with if there are any non-MB-aware programs (you'd have to unload
> them first)...
>

How would we communicate that issue? dmesg? I'm not very familiar with how
sysctl change failure causes are communicated to users, so this might be a
solved problem, but if I run `sysctl -w net.xdp.multibuffer 1` (or whatever
ends up actually being the toggle) to active multi-buffer, and it fails because
there's a loaded non-aware program, that seems like a potential for a lot of
administrator pain.

> >>    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?
>
> Well, the trouble here is that we don't necessarily have an explicit
> "multi-buf mode" for devices. For instance, you could raise the MTU of a
> device without it necessarily involving any XDP multi-buffer stuff (if
> you're not running XDP on that device). So if we did turn "raising the
> MTU" into such a mode switch, we would end up blocking any MTU changes
> if any XDP programs are loaded. Or having an MTU change cause a
> force-unload of all XDP programs.

Maybe I missed something then, but you had stated that "while a particular
driver knows if it's running in multi-buff mode" so I assumed that the driver
would be able to tell when to toggle the mode on.

I had been thinking that when a driver turned multi-buffer off, it could
trigger a check of all drivers, but that also seems like it could just be a
global refcount of all the drivers that have requested multi-buffer mode. When
a driver enables multi-buffer for itself, it increments the refcount, and when
it disables, it decrements. A non-zero count means the system is in
multi-buffer mode.

Obviously this is more complex than just requiring the administrator to enable
the system-wide mode.

>
> Neither of those are desirable outcomes, I think; and if we add a
> separate "XDP multi-buff" switch, we might as well make it system-wide?
>

> > 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.
>
> See above.
>
> -Toke
>

--Zvi

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