netdev - Re: [PATCH net 4/8] i40e: Fix XDP program unloading while removing the driver

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Date: Thu, 30 May 2024 09:38:04 -0700
From: Jacob Keller <jacob.e.keller@...el.com>
To: Jakub Kicinski <kuba@...nel.org>, Michal Kubiak <michal.kubiak@...el.com>
CC: David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>,
	Michal Kubiak <michal.kubiak@...el.com>, Wojciech Drewek
	<wojciech.drewek@...el.com>, George Kuruvinakunnel
	<george.kuruvinakunnel@...el.com>, Maciej Fijalkowski
	<maciej.fijalkowski@...el.com>
Subject: Re: [PATCH net 4/8] i40e: Fix XDP program unloading while removing
 the driver



On 5/29/2024 6:54 PM, Jakub Kicinski wrote:
> On Tue, 28 May 2024 15:06:07 -0700 Jacob Keller wrote:
>> +	/* Called from netdev unregister context. Unload the XDP program. */
>> +	if (vsi->netdev->reg_state == NETREG_UNREGISTERING) {
>> +		xdp_features_clear_redirect_target(vsi->netdev);
>> +		old_prog = xchg(&vsi->xdp_prog, NULL);
>> +		if (old_prog)
>> +			bpf_prog_put(old_prog);
>> +
>> +		return 0;
>> +	}
> 
> This is not great. The netdevice is closed at this stage, why is the xdp
> setup try to do work if the device is closed even when not
> unregistering?

The comment makes this seem like its happening during unregistration. It
looks like i40e_xdp_setup() is only called from i40e_xdp(), which is if
xdp->command is XDP_SETUP_PROG

>From the looks of things, ndo_bpf is called both for setup and teardown?

>    7 >-------/* Set or clear a bpf program used in the earliest stages of packet
>    6 >------- * rx. The prog will have been loaded as BPF_PROG_TYPE_XDP. The callee
>    5 >------- * is responsible for calling bpf_prog_put on any old progs that are
>    4 >------- * stored. In case of error, the callee need not release the new prog
>    3 >------- * reference, but on success it takes ownership and must bpf_prog_put
>    2 >------- * when it is no longer used.
>    1 >------- */

Indeed, dev_xdp_uninstall calls dev_xdp_install in a loop to remove
programs.

As far as I can tell, it looks like the .ndo_bpf call is made with a
program set to NULL during uninstall:

>    30 static void dev_xdp_uninstall(struct net_device *dev)
>    29 {
>    28 >-------struct bpf_xdp_link *link;
>    27 >-------struct bpf_prog *prog;
>    26 >-------enum bpf_xdp_mode mode;
>    25 >-------bpf_op_t bpf_op;
>    24
>    23 >-------ASSERT_RTNL();
>    22
>    21 >-------for (mode = XDP_MODE_SKB; mode < __MAX_XDP_MODE; mode++) {
>    20 >------->-------prog = dev_xdp_prog(dev, mode);
>    19 >------->-------if (!prog)
>    18 >------->------->-------continue;
>    17
>    16 >------->-------bpf_op = dev_xdp_bpf_op(dev, mode);
>    15 >------->-------if (!bpf_op)
>    14 >------->------->-------continue;
>    13
>    12 >------->-------WARN_ON(dev_xdp_install(dev, mode, bpf_op, NULL, 0, NULL));
>    11

Here, dev_xdp_install is called with a prog of NULL

>    10 >------->-------/* auto-detach link from net device */
>     9 >------->-------link = dev_xdp_link(dev, mode);
>     8 >------->-------if (link)
>     7 >------->------->-------link->dev = NULL;
>     6 >------->-------else
>     5 >------->------->-------bpf_prog_put(prog);
>     4
>     3 >------->-------dev_xdp_set_link(dev, mode, NULL);
>     2 >-------}
>     1 }

I think the semantics are confusing here.

Basically, the issue is this function needs to remove the XDP properly
when called by the netdev unregister flow but has a check against adding
a new program if its called during remove...

I think this is confusing and could be improved by refactoring how the
i40e flow works. If the passed-in prog is NULL, its a request to remove.
If its otherwise, its a request to add a new program (possibly replacing
an existing one?).

I think we ought to just be checking NULL and not needing to bother with
the netdev_unregister state at all here?

Hopefully Michal can chime in with a better understanding.