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From: "Björn Töpel" <bjorn.topel@intel.com>
To: John Fastabend <john.fastabend@gmail.com>,
	Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
	daniel@iogearbox.net, ast@kernel.org, bpf@vger.kernel.org,
	netdev@vger.kernel.org
Cc: andrii@kernel.org, toke@redhat.com, magnus.karlsson@intel.com,
	ciara.loftus@intel.com
Subject: Re: [PATCH bpf-next 1/3] libbpf: xsk: use bpf_link
Date: Tue, 16 Feb 2021 10:20:52 +0100	[thread overview]
Message-ID: <4a52d09a-363b-e69e-41d3-7918f0204901@intel.com> (raw)
In-Reply-To: <602ade57ddb9c_3ed41208a1@john-XPS-13-9370.notmuch>

On 2021-02-15 21:49, John Fastabend wrote:
> Maciej Fijalkowski wrote:
>> Currently, if there are multiple xdpsock instances running on a single
>> interface and in case one of the instances is terminated, the rest of
>> them are left in an inoperable state due to the fact of unloaded XDP
>> prog from interface.
>>
>> To address that, step away from setting bpf prog in favour of bpf_link.
>> This means that refcounting of BPF resources will be done automatically
>> by bpf_link itself.
>>
>> When setting up BPF resources during xsk socket creation, check whether
>> bpf_link for a given ifindex already exists via set of calls to
>> bpf_link_get_next_id -> bpf_link_get_fd_by_id -> bpf_obj_get_info_by_fd
>> and comparing the ifindexes from bpf_link and xsk socket.
>>
>> If there's no bpf_link yet, create one for a given XDP prog and unload
>> explicitly existing prog if XDP_FLAGS_UPDATE_IF_NOEXIST is not set.
>>
>> If bpf_link is already at a given ifindex and underlying program is not
>> AF-XDP one, bail out or update the bpf_link's prog given the presence of
>> XDP_FLAGS_UPDATE_IF_NOEXIST.
>>
>> Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
>> ---
>>   tools/lib/bpf/xsk.c | 143 +++++++++++++++++++++++++++++++++++++-------
>>   1 file changed, 122 insertions(+), 21 deletions(-)
> 
> [...]
> 
>> +static int xsk_create_bpf_link(struct xsk_socket *xsk)
>> +{
>> +	/* bpf_link only accepts XDP_FLAGS_MODES, but xsk->config.xdp_flags
>> +	 * might have set XDP_FLAGS_UPDATE_IF_NOEXIST
>> +	 */
>> +	DECLARE_LIBBPF_OPTS(bpf_link_create_opts, opts,
>> +			    .flags = (xsk->config.xdp_flags & XDP_FLAGS_MODES));
>> +	struct xsk_ctx *ctx = xsk->ctx;
>> +	__u32 prog_id;
>> +	int link_fd;
>> +	int err;
>> +
>> +	/* for !XDP_FLAGS_UPDATE_IF_NOEXIST, unload the program first, if any,
>> +	 * so that bpf_link can be attached
>> +	 */
>> +	if (!(xsk->config.xdp_flags & XDP_FLAGS_UPDATE_IF_NOEXIST)) {
>> +		err = bpf_get_link_xdp_id(ctx->ifindex, &prog_id, xsk->config.xdp_flags);
>> +		if (err) {
>> +			pr_warn("getting XDP prog id failed\n");
>> +			return err;
>> +		}
>> +		if (prog_id) {
>> +			err = bpf_set_link_xdp_fd(ctx->ifindex, -1, 0);
>> +			if (err < 0) {
>> +				pr_warn("detaching XDP prog failed\n");
>> +				return err;
>> +			}
>> +		}
>>   	}
>>   
>> -	ctx->prog_fd = prog_fd;
>> +	link_fd = bpf_link_create(ctx->prog_fd, xsk->ctx->ifindex, BPF_XDP, &opts);
>> +	if (link_fd < 0) {
>> +		pr_warn("bpf_link_create failed: %s\n", strerror(errno));
>> +		return link_fd;
>> +	}
>> +
> 
> This can leave the system in a bad state where it unloaded the XDP program
> above, but then failed to create the link. So we should somehow fix that
> if possible or at minimum put a note somewhere so users can't claim they
> shouldn't know this.
> 
> Also related, its not good for real systems to let XDP program go missing
> for some period of time. I didn't check but we should make
> XDP_FLAGS_UPDATE_IF_NOEXIST the default if its not already.
>

This is the default for XDP sockets library. The
"bpf_set_link_xdp_fd(...-1)" way is only when a user sets it explicitly.
One could maybe argue that the "force remove" would be out of scope for
AF_XDP; Meaning that if an XDP program is running, attached via netlink,
the AF_XDP library simply cannot remove it. The user would need to rely
on some other mechanism.


Björn


[...]

  parent reply	other threads:[~2021-02-16  9:21 UTC|newest]

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-15 15:46 [PATCH bpf-next 0/3] Introduce bpf_link in libbpf's xsk Maciej Fijalkowski
2021-02-15 15:46 ` [PATCH bpf-next 1/3] libbpf: xsk: use bpf_link Maciej Fijalkowski
2021-02-15 17:07   ` Toke Høiland-Jørgensen
2021-02-15 17:38     ` Björn Töpel
2021-02-15 19:35       ` Toke Høiland-Jørgensen
2021-02-16  2:01         ` Maciej Fijalkowski
2021-02-16  9:15           ` Björn Töpel
2021-02-16 10:27           ` Toke Høiland-Jørgensen
2021-02-16 20:15             ` Maciej Fijalkowski
2021-02-15 20:22       ` John Fastabend
2021-02-15 21:38         ` Toke Høiland-Jørgensen
2021-02-16  0:18           ` John Fastabend
2021-02-16  2:23             ` Maciej Fijalkowski
2021-02-16  9:23               ` Björn Töpel
2021-02-16 10:36             ` Toke Høiland-Jørgensen
2021-02-23  1:15               ` Andrii Nakryiko
2021-02-17  2:23           ` Dan Siemon
2021-02-17  7:16             ` Magnus Karlsson
2021-02-17  7:36               ` Magnus Karlsson
2021-02-16  2:10         ` Maciej Fijalkowski
2021-02-15 20:49   ` John Fastabend
2021-02-16  2:38     ` Maciej Fijalkowski
2021-02-16 18:19       ` John Fastabend
2021-02-16 20:10         ` Maciej Fijalkowski
2021-02-16  9:20     ` Björn Töpel [this message]
2021-02-16 10:39       ` Toke Høiland-Jørgensen
2021-02-16 19:15         ` John Fastabend
2021-02-16 20:50           ` Maciej Fijalkowski
2021-02-16 21:17             ` John Fastabend
2021-02-15 15:46 ` [PATCH bpf-next 2/3] libbpf: clear map_info before each bpf_obj_get_info_by_fd Maciej Fijalkowski
2021-02-15 20:33   ` John Fastabend
2021-02-16  2:42     ` Maciej Fijalkowski
2021-02-15 15:46 ` [PATCH bpf-next 3/3] samples: bpf: do not unload prog within xdpsock Maciej Fijalkowski
2021-02-15 20:24   ` John Fastabend
2021-02-16  9:22     ` Björn Töpel
2021-02-16 14:15       ` Maciej Fijalkowski
2021-02-15 16:07 ` [PATCH bpf-next 0/3] Introduce bpf_link in libbpf's xsk Björn Töpel

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