linux-kernel - Re: [RFC PATCH bpf-next 0/2] Improve prog array uref semantics

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Message-Id: <7ec1a922-30c5-4899-a23f-11e3ef9d6fef@app.fastmail.com>
Date: Wed, 20 Nov 2024 07:55:03 -0800
From: "Daniel Xu" <dxu@...uu.xyz>
To: "Alexei Starovoitov" <alexei.starovoitov@...il.com>
Cc: "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
 LKML <linux-kernel@...r.kernel.org>,
 "Network Development" <netdev@...r.kernel.org>,
 "Daniel Borkmann" <daniel@...earbox.net>
Subject: Re: [RFC PATCH bpf-next 0/2] Improve prog array uref semantics



On Sat, Nov 16, 2024, at 2:17 PM, Alexei Starovoitov wrote:
> On Tue, Oct 29, 2024 at 11:36 PM Daniel Xu <dxu@...uu.xyz> wrote:
>>
>> Hey Daniel,
>>
>> On Wed, Aug 23, 2023, at 9:08 AM, Daniel Xu wrote:
>> > This patchset changes the behavior of TC and XDP hooks during attachment
>> > such that any BPF_MAP_TYPE_PROG_ARRAY that the prog uses has an extra
>> > uref taken.
>> >
>> > The goal behind this change is to try and prevent confusion for the
>> > majority of use cases. The current behavior where when the last uref is
>> > dropped the prog array map is emptied is quite confusing. Confusing
>> > enough for there to be multiple references to it in ebpf-go [0][1].
>> >
>> > Completely solving the problem is difficult. As stated in c9da161c6517
>> > ("bpf: fix clearing on persistent program array maps"), it is
>> > difficult-to-impossible to walk the full dependency graph b/c it is too
>> > dynamic.
>> >
>> > However in practice, I've found that all progs in a tailcall chain
>> > share the same prog array map. Knowing that, if we take a uref on any
>> > used prog array map when the program is attached, we can simplify the
>> > majority use case and make it more ergonomic.
>
> Are you proposing to inc map uref when prog is attached?
>
> But that re-adds the circular dependency that uref concept is solving.
> When prog is inserted into prog array prog refcnt is incremented.
> So if prog also incremented uref. The user space can exit
> but prog array and progs will stay there though nothing is using them.
> I guess I'm missing the idea.

IIRC the old-style tc/xdp attachment is the one incrementing the uref. Once
whatever program there is detached the uref is dropped. So I don't think
any circular refs can happen unless a prog can somehow prevent its own
detachment.

Could be mis-remembering though.

Thanks,
Daniel