netdev - RE: [PATCH bpf] xsk: Initialise xskb free_list

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Message-ID: <PH0PR11MB479171AF2D4CE0B118B47A208E7C9@PH0PR11MB4791.namprd11.prod.outlook.com>
Date:   Tue, 21 Dec 2021 08:32:13 +0000
From:   "Loftus, Ciara" <ciara.loftus@...el.com>
To:     Magnus Karlsson <magnus.karlsson@...il.com>
CC:     "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        Björn Töpel <bjorn@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
        bpf <bpf@...r.kernel.org>
Subject: RE: [PATCH bpf] xsk: Initialise xskb free_list_node

> On Mon, Dec 20, 2021 at 9:10 PM Ciara Loftus <ciara.loftus@...el.com>
> wrote:
> >
> > This commit initialises the xskb's free_list_node when the xskb is
> > allocated. This prevents a potential false negative returned from a call
> > to list_empty for that node, such as the one introduced in commit
> > 199d983bc015 ("xsk: Fix crash on double free in buffer pool")
> >
> > In my environment this issue caused packets to not be received by
> > the xdpsock application if the traffic was running prior to application
> > launch. This happened when the first batch of packets failed the xskmap
> > lookup and XDP_PASS was returned from the bpf program. This action is
> > handled in the i40e zc driver (and others) by allocating an skbuff,
> > freeing the xdp_buff and adding the associated xskb to the
> > xsk_buff_pool's free_list if it hadn't been added already. Without this
> > fix, the xskb is not added to the free_list because the check to determine
> > if it was added already returns an invalid positive result. Later, this
> > caused allocation errors in the driver and the failure to receive packets.
> 
> Thank you for this fix Ciara! Though I do think the Fixes tag should
> be the one above: 199d983bc015 ("xsk: Fix crash on double free in
> buffer pool"). Before that commit, there was no test for an empty list
> in the xp_free path. The entry was unconditionally put on the list and
> "initialized" in that way, so that code will work without this patch.
> What do you think?

Agree - that makes sense.
Can the fixes tag be updated when pulled into the tree with:
Fixes: 199d983bc015 ("xsk: Fix crash on double free in buffer pool")

If I need to submit a v2 with the change please let me know.

Thanks,
Ciara

> 
> Acked-by: Magnus Karlsson <magnus.karlsson@...el.com>
> 
> > Fixes: 2b43470add8c ("xsk: Introduce AF_XDP buffer allocation API")
> >
> > Signed-off-by: Ciara Loftus <ciara.loftus@...el.com>
> > ---
> >  net/xdp/xsk_buff_pool.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c
> > index bc4ad48ea4f0..fd39bb660ebc 100644
> > --- a/net/xdp/xsk_buff_pool.c
> > +++ b/net/xdp/xsk_buff_pool.c
> > @@ -83,6 +83,7 @@ struct xsk_buff_pool
> *xp_create_and_assign_umem(struct xdp_sock *xs,
> >                 xskb = &pool->heads[i];
> >                 xskb->pool = pool;
> >                 xskb->xdp.frame_sz = umem->chunk_size - umem->headroom;
> > +               INIT_LIST_HEAD(&xskb->free_list_node);
> >                 if (pool->unaligned)
> >                         pool->free_heads[i] = xskb;
> >                 else
> > --
> > 2.17.1
> >