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Date: Fri, 10 May 2024 12:04:12 +0200
From: Benjamin Tissoires <bentiss@kernel.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>, 
	Alexei Starovoitov <ast@kernel.org>, John Fastabend <john.fastabend@gmail.com>, 
	Andrii Nakryiko <andrii@kernel.org>, Martin KaFai Lau <martin.lau@linux.dev>, 
	Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>, 
	Yonghong Song <yonghong.song@linux.dev>, KP Singh <kpsingh@kernel.org>, 
	Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>, 
	bpf <bpf@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] bpf: verifier: allow arrays of progs to be used in
 sleepable context
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On May 08 2024, Alexei Starovoitov wrote:
> On Wed, May 8, 2024 at 4:53 AM Benjamin Tissoires <bentiss@kernel.org> wrote:
> >
> > On May 07 2024, Alexei Starovoitov wrote:
> > > On Tue, May 7, 2024 at 6:32 AM Benjamin Tissoires <bentiss@kernel.org> wrote:
> > > >
> > > > Yes, exactly that. See [0] for my current WIP. I've just sent it, not
> > > > for reviews, but so you see what I meant here.
> > >
> > > The patches helped to understand, for sure, and on surface
> > > they kind of make sense, but without seeing what is that
> > > hid specific kfunc that will use it
> > > it's hard to make a call.
> >
> > I've posted my HID WIP on [1]. It probably won't compile as my local
> > original branch was having a merge of HID and bpf trees.
> 
> Thanks for this context.
> Now it makes a lot more sense.
> And the first patches look fine and simplification is impressive,
> but this part:
> +     SEC("syscall")
> +     int name(struct attach_prog_args *ctx)
> +     {
> +       ctx->retval = hid_bpf_attach_prog_impl(ctx->hid,
> +                                                 type,
> +                                                 __##name,
> +                           ctx->insert_head ? HID_BPF_FLAG_INSERT_HEAD :
> +                                                 HID_BPF_FLAG_NONE,
> +                                                 NULL);
> 
> is too odd.
> Essentially you're adding a kfunc on hid side just to call it
> from a temporary "syscall" program to register another prog.
> A fake prog just to call a kfunc is a bit too much.
> 
> The overall mechanism is pretty much a customized struct-ops.
> 
> I think struct-ops infra provides better api-s, safety guarantees,
> bpf_link support, prog management including reentrance check, etc.

Ack!

> It needs to be parametrized, so it's not just
> SEC("struct_ops/kern_callback_name")
> so that the skeleton loading phase can pass device id or something.

I'm not sure how to parametrize staticaly. I can rely on the modalias,
but then that might not be ideal. Right now my loader gets called by a
udev rule, and then call a .probe syscall. If this returns success, then
the bpf programs are attached to the given hid device.

I saw that the struct_ops can have "data" fields. If we can change the
static value before calling attach, I should have a register callback
being able to retrieve that hid id, and then attach the struct_ops to
the correct hid device in the jump table. Anyway, I'll test this later.

Something along:

SEC(".struct_ops")
struct hid_bpf_ops dummy_1 = {
        .input_event = (void *)test_1,
        .rdesc_fixup = (void *)test_2,
        .hw_raw_request = (void *)test_sleepable,
        .hid_id = 0,
};

And in the loader, I call __load(), change the value ->hid_id, and then
__attach().

> 
> > > The (u64)(long) casting concerns and prog lifetime are
> > > difficult to get right. The verifier won't help and it all
> > > will fall on code reviews.
> >
> > yeah, this is a concern.
> 
> Not only that. The special kfunc does migrate_disable
> before calling callback, but it needs rcu_lock or tracing lock,
> plus reentrance checks.
> 
> >
> > > So I'd rather not go this route.
> > > Let's explore first what exactly the goal here.
> > > We've talked about sleepable tail_calls, this async callbacks
> > > from hid kfuncs, and struct-ops.
> > > Maybe none of them fit well and we need something else.
> > > Could you please explain (maybe once again) what is the end goal?
> >
> > right now I need 4 hooks in HID, the first 2 are already upstream:
> > - whenever I need to retrieve the report descriptor (this happens in a
> >   sleepable context, but non sleepable is fine)
> > - whenever I receive an event from a device (non sleepable context, this
> >   is coming from a hard IRQ context)
> > - whenever someone tries to write to the device through
> >   hid_hw_raw_request (original context is sleepable, and for being able
> >   to communicate with the device we need sleepable context in bpf)
> > - same but from hid_hw_output_report
> >
> > Again, the first 2 are working just fine.
> >
> > Implementing the latter twos requires sleepable context because we
> > might:
> >
> > 1. a request is made from user-space
> > 2. we jump into hid-bpf
> > 3. the bpf program "converts" the request from report ID 1 to 2 (because
> > we export a slightly different API)
> > 4. the bpf program directly emits hid_bpf_raw_request (sleepable
> > operation)
> > 5. the bpf program returns the correct value
> > 6. hid-core doesn't attempt to communicate with the device as bpf
> > already did.
> >
> > In the series, I also realized that I need sleepable and non sleepable
> > contexts for this kind of situation, because I want tracing and
> > firewalling available (non sleepable context), while still allowing to
> > communicate with the device. But when you communicate with the device
> > from bpf, the sleepable bpf program is not invoked or this allows
> > infinite loops.
> 
> I don't get the point about infinite loops.

If I don´t put restrictions on how the bpf program communicate with the
device I might have:

1. someone calls hid_hw_raw_request from hidraw
2. bpf jumps into filter for hid_hw_raw_request
3. the bpf program calls hid_bpf_raw_request (which internally calls
hid_hw_raw_request)
4. go back to 2.

But again, not a big deal: if I do not allow entering a sleepable bpf
program from hid_bpf_raw_request (so from a bpf program), instead of 4.
above, we prevent entering the same bpf program as the program in 2.
needs to be sleepable.

> fyi struct_ops already supports sleepable and non-sleepable callbacks.
> See progs/dummy_st_ops_success.c
> SEC(".struct_ops")
> struct bpf_dummy_ops dummy_1 = {
>         .test_1 = (void *)test_1,
>         .test_2 = (void *)test_2,
>         .test_sleepable = (void *)test_sleepable,
> };
> 
> two callbacks are normal and another one is sleepable.
> 
> The generated bpf trampoline will have the right
> __bpf_prog_enter* wrappers for all 3 progs,
> so the kernel code will be just do ops->callback_name().

Great!

So I think I have most of the pieces available... I just need to write
the code :)

> 
> > >
> > > > Last time I checked, I thought struct_ops were only for defining one set
> > > > of operations. And you could overwrite them exactly once.
> > > > But after reading more carefully how it was used in tcp_cong.c, it seems
> > > > we can have multiple programs which define the same struct_ops, and then
> > > > it's the kernel which will choose which one needs to be run.
> > >
> > > struct-ops is pretty much a mechanism for kernel to define
> > > a set of callbacks and bpf prog to provide implementation for
> > > these callbacks. The kernel choses when to call them.
> > > tcp-bpf is one such user. sched_ext is another and more advanced.
> > > Currently struct-ops bpf prog loading/attaching mechanism
> > > only specifies the struct-ops. There is no device-id argument,
> > > but that can be extended and kernel can keep per-device a set
> > > of bpf progs.
> > > struct-ops is a bit of overkill if you have only one callback.
> > > It's typically for a set of callbacks.
> >
> > In the end I have 4. However, I might have programs that overwrite twice
> > the same callback (see the 2 SEC("fmod_ret/hid_bpf_device_event") in
> > [2]).
> >
> > >
> > > > Last, I'm not entirely sure how I can specify which struct_ops needs to be
> > > > attached to which device, but it's worth a shot. I've already realized
> > > > that I would probably have to drop the current way of HID-BPF is running,
> > > > so now it's just technical bits to assemble :)
> > >
> > > You need to call different bpf progs per device, right?
> >
> > yes
> >
> > > If indirect call is fine from performance pov,
> > > then tailcall or struct_ops+device_argument might fit.
> >
> > performance is not a requirement. It's better if we have low latency but
> > we are not talking the same requirements than network.
> >
> > >
> > > If you want max perf with direct calls then
> > > we'd need to generalize xdp dispatcher.
> >
> > I'll need to have a deeper look at it, yeah.
> >
> > >
> > > So far it sounds that tailcalls might be the best actually,
> > > since prog lifetime is handled by prog array map.
> > > Maybe instead of bpf_tail_call helper we should add a kfunc that
> > > will operate on prog array differently?
> > > (if current bpf_tail_call semantics don't fit).
> >
> > Actually I'd like to remove bpf_tail_call entirely, because it requires
> > to pre-load a BPF program at boot, and in some situations (RHEL) this
> > creates issues. I haven't been able to debug what was happening, I
> > couldn't reproduce it myself, but removing that bit would be nice :)
> 
> We probably need to debug it anyway, since it sounds that it's
> related to preloaded bpf skeleton and not tail_call logic itself.

I really think this happens with RHEL because some part are backported and
some are not. So unless we get upstream reports, it's more likely a RHEL
only issue (which makes things even harder to debug for me).

> 
> After looking through all that it seems to me that
> parametrized struct-ops is the way to go.

Yeah, I think so too. I'll proabbly start working on it next Monday.

Thanks a lot for all of your feedback :)

Cheers,
Benjamin