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From: "Toke Høiland-Jørgensen" <toke@redhat.com>
To: Stanislav Fomichev <sdf@google.com>
Cc: bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net,
	andrii@kernel.org, martin.lau@linux.dev, song@kernel.org,
	yhs@fb.com, john.fastabend@gmail.com, kpsingh@kernel.org,
	haoluo@google.com, jolsa@kernel.org,
	David Ahern <dsahern@gmail.com>, Jakub Kicinski <kuba@kernel.org>,
	Willem de Bruijn <willemb@google.com>,
	Jesper Dangaard Brouer <brouer@redhat.com>,
	Anatoly Burakov <anatoly.burakov@intel.com>,
	Alexander Lobakin <alexandr.lobakin@intel.com>,
	Magnus Karlsson <magnus.karlsson@gmail.com>,
	Maryam Tahhan <mtahhan@redhat.com>,
	xdp-hints@xdp-project.net, netdev@vger.kernel.org
Subject: Re: [xdp-hints] Re: [PATCH bpf-next v3 03/12] bpf: XDP metadata RX kfuncs
Date: Fri, 09 Dec 2022 01:07:37 +0100	[thread overview]
Message-ID: <87zgbxjv7a.fsf@toke.dk> (raw)
In-Reply-To: <CAKH8qBvgkTXFEhd9hOa+SFtqKAXuD=WM_h1TZYdQA0d70_drEA@mail.gmail.com>

Stanislav Fomichev <sdf@google.com> writes:

>> Another UX thing I ran into is that libbpf will bail out if it can't
>> find the kfunc in the kernel vmlinux, even if the code calling the
>> function is behind an always-false if statement (which would be
>> eliminated as dead code from the verifier). This makes it a bit hard to
>> conditionally use them. Should libbpf just allow the load without
>> performing the relocation (and let the verifier worry about it), or
>> should we have a bpf_core_kfunc_exists() macro to use for checking?
>> Maybe both?
>
> I'm not sure how libbpf can allow the load without performing the
> relocation; maybe I'm missing something.
> IIUC, libbpf uses the kfunc name (from the relocation?) and replaces
> it with the kfunc id, right?

Yeah, so if it can't find the kfunc in vmlinux, just write an id of 0.
This will trip the check at the top of fixup_kfunc_call() in the
verifier, but if the code is hidden behind an always-false branch (an
rodata variable set to zero, say) the instructions should get eliminated
before they reach that point. That way you can at least turn it off at
runtime (after having done some kind of feature detection) without
having to compile it out of your program entirely.

> Having bpf_core_kfunc_exists would help, but this probably needs
> compiler work first to preserve some of the kfunc traces in vmlinux.h?

I am not sure how the existing macros work, TBH. Hopefully someone else
can chime in :)

-Toke


  reply	other threads:[~2022-12-09  0:08 UTC|newest]

Thread overview: 61+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-12-06  2:45 [PATCH bpf-next v3 00/12] xdp: hints via kfuncs Stanislav Fomichev
2022-12-06  2:45 ` [PATCH bpf-next v3 01/12] bpf: Document XDP RX metadata Stanislav Fomichev
2022-12-08  4:25   ` Jakub Kicinski
2022-12-08 19:06     ` Stanislav Fomichev
2022-12-06  2:45 ` [PATCH bpf-next v3 02/12] bpf: Rename bpf_{prog,map}_is_dev_bound to is_offloaded Stanislav Fomichev
2022-12-08  4:26   ` Jakub Kicinski
2022-12-06  2:45 ` [PATCH bpf-next v3 03/12] bpf: XDP metadata RX kfuncs Stanislav Fomichev
2022-12-07  4:29   ` Alexei Starovoitov
2022-12-07  4:52     ` Stanislav Fomichev
2022-12-07  7:23       ` Martin KaFai Lau
2022-12-07 18:05         ` Stanislav Fomichev
2022-12-08  2:47   ` Martin KaFai Lau
2022-12-08 19:07     ` Stanislav Fomichev
2022-12-08 22:53       ` Martin KaFai Lau
2022-12-08 23:45         ` Stanislav Fomichev
2022-12-08  5:00   ` Jakub Kicinski
2022-12-08 19:07     ` Stanislav Fomichev
2022-12-09  1:30       ` Jakub Kicinski
2022-12-09  2:57         ` Stanislav Fomichev
2022-12-08 22:39   ` [xdp-hints] " Toke Høiland-Jørgensen
2022-12-08 23:46     ` Stanislav Fomichev
2022-12-09  0:07       ` Toke Høiland-Jørgensen [this message]
2022-12-09  2:57         ` [xdp-hints] " Stanislav Fomichev
2022-12-10  0:42           ` Martin KaFai Lau
2022-12-10  1:12             ` Martin KaFai Lau
2022-12-09 11:10   ` Jesper Dangaard Brouer
2022-12-09 17:47     ` Stanislav Fomichev
2022-12-11 11:09       ` Jesper Dangaard Brouer
2022-12-06  2:45 ` [PATCH bpf-next v3 04/12] veth: Introduce veth_xdp_buff wrapper for xdp_buff Stanislav Fomichev
2022-12-06  2:45 ` [PATCH bpf-next v3 05/12] veth: Support RX XDP metadata Stanislav Fomichev
2022-12-06  2:45 ` [PATCH bpf-next v3 06/12] selftests/bpf: Verify xdp_metadata xdp->af_xdp path Stanislav Fomichev
2022-12-06  2:45 ` [PATCH bpf-next v3 07/12] mlx4: Introduce mlx4_xdp_buff wrapper for xdp_buff Stanislav Fomichev
2022-12-08  6:11   ` Tariq Toukan
2022-12-08 19:07     ` Stanislav Fomichev
2022-12-06  2:45 ` [PATCH bpf-next v3 08/12] mxl4: Support RX XDP metadata Stanislav Fomichev
2022-12-08  6:09   ` Tariq Toukan
2022-12-08 19:07     ` Stanislav Fomichev
2022-12-08 20:23       ` Tariq Toukan
2022-12-06  2:45 ` [PATCH bpf-next v3 09/12] xsk: Add cb area to struct xdp_buff_xsk Stanislav Fomichev
2022-12-06  2:45 ` [PATCH bpf-next v3 10/12] mlx5: Introduce mlx5_xdp_buff wrapper for xdp_buff Stanislav Fomichev
2022-12-06  2:45 ` [PATCH bpf-next v3 11/12] mlx5: Support RX XDP metadata Stanislav Fomichev
2022-12-08 22:59   ` Toke Høiland-Jørgensen
2022-12-08 23:45     ` Stanislav Fomichev
2022-12-09  0:02       ` [xdp-hints] " Toke Høiland-Jørgensen
2022-12-09  0:07         ` Alexei Starovoitov
2022-12-09  0:29           ` Toke Høiland-Jørgensen
2022-12-09  0:32             ` Alexei Starovoitov
2022-12-09  0:53               ` Toke Høiland-Jørgensen
2022-12-09  2:57                 ` Stanislav Fomichev
2022-12-09  5:24                   ` Saeed Mahameed
2022-12-09 12:59                     ` Jesper Dangaard Brouer
2022-12-09 14:37                       ` Toke Høiland-Jørgensen
2022-12-09 15:19                       ` Dave Taht
2022-12-09 14:42                   ` Toke Høiland-Jørgensen
2022-12-09 16:45                     ` Jakub Kicinski
2022-12-09 17:46                       ` Stanislav Fomichev
2022-12-09 22:13                         ` Jakub Kicinski
2022-12-06  2:45 ` [PATCH bpf-next v3 12/12] selftests/bpf: Simple program to dump XDP RX metadata Stanislav Fomichev
2022-12-08 22:28 ` [xdp-hints] [PATCH bpf-next v3 00/12] xdp: hints via kfuncs Toke Høiland-Jørgensen
2022-12-08 23:47   ` Stanislav Fomichev
2022-12-09  0:14     ` [xdp-hints] " Toke Høiland-Jørgensen

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