Subject: Re: [PATCH v5 bpf 02/11] xsk: make xsk_buff_pool responsible for clearing xdp_buff::flags

From: Maciej Fijalkowski <[email-masked]>
Date: Wed, 24 Jan 2024 12:42:45 +0100

On Wed, Jan 24, 2024 at 09:20:26AM +0100, Magnus Karlsson wrote:
> On Mon, 22 Jan 2024 at 23:16, Maciej Fijalkowski
> <[email-masked]> wrote:
> >
> > XDP multi-buffer support introduced XDP_FLAGS_HAS_FRAGS flag that is
> > used by drivers to notify data path whether xdp_buff contains fragments
> > or not. Data path looks up mentioned flag on first buffer that occupies
> > the linear part of xdp_buff, so drivers only modify it there. This is
> > sufficient for SKB and XDP_DRV modes as usually xdp_buff is allocated on
> > stack or it resides within struct representing driver's queue and
> > fragments are carried via skb_frag_t structs. IOW, we are dealing with
> > only one xdp_buff.
> >
> > ZC mode though relies on list of xdp_buff structs that is carried via
> > xsk_buff_pool::xskb_list, so ZC data path has to make sure that
> > fragments do *not* have XDP_FLAGS_HAS_FRAGS set. Otherwise,
> > xsk_buff_free() could misbehave if it would be executed against xdp_buff
> > that carries a frag with XDP_FLAGS_HAS_FRAGS flag set. Such scenario can
> > take place when within supplied XDP program bpf_xdp_adjust_tail() is
> > used with negative offset that would in turn release the tail fragment
> > from multi-buffer frame.
> >
> > Calling xsk_buff_free() on tail fragment with XDP_FLAGS_HAS_FRAGS would
> > result in releasing all the nodes from xskb_list that were produced by
> > driver before XDP program execution, which is not what is intended -
> > only tail fragment should be deleted from xskb_list and then it should
> > be put onto xsk_buff_pool::free_list. Such multi-buffer frame will never
> > make it up to user space, so from AF_XDP application POV there would be
> > no traffic running, however due to free_list getting constantly new
> > nodes, driver will be able to feed HW Rx queue with recycled buffers.
> > Bottom line is that instead of traffic being redirected to user space,
> > it would be continuously dropped.
> >
> > To fix this, let us clear the mentioned flag on xsk_buff_pool side at
> > allocation time, which is what should have been done right from the
> > start of XSK multi-buffer support.
> >
> > Fixes: 1bbc04de607b ("ice: xsk: add RX multi-buffer support")
> > Fixes: 1c9ba9c14658 ("i40e: xsk: add RX multi-buffer support")
> > Fixes: 24ea50127ecf ("xsk: support mbuf on ZC RX")
> > Signed-off-by: Maciej Fijalkowski <[email-masked]>
> > ---
> >  drivers/net/ethernet/intel/i40e/i40e_xsk.c | 1 -
> >  drivers/net/ethernet/intel/ice/ice_xsk.c   | 1 -
> >  net/xdp/xsk_buff_pool.c                    | 3 +++
> >  3 files changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/ethernet/intel/i40e/i40e_xsk.c b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
> > index e99fa854d17f..fede0bb3e047 100644
> > --- a/drivers/net/ethernet/intel/i40e/i40e_xsk.c
> > +++ b/drivers/net/ethernet/intel/i40e/i40e_xsk.c
> > @@ -499,7 +499,6 @@ int i40e_clean_rx_irq_zc(struct i40e_ring *rx_ring, int budget)
> >                 xdp_res = i40e_run_xdp_zc(rx_ring, first, xdp_prog);
> >                 i40e_handle_xdp_result_zc(rx_ring, first, rx_desc, &rx_packets,
> >                                           &rx_bytes, xdp_res, &failure);
> > -               first->flags = 0;
> >                 next_to_clean = next_to_process;
> >                 if (failure)
> >                         break;
> > diff --git a/drivers/net/ethernet/intel/ice/ice_xsk.c b/drivers/net/ethernet/intel/ice/ice_xsk.c
> > index 5d1ae8e4058a..d9073a618ad6 100644
> > --- a/drivers/net/ethernet/intel/ice/ice_xsk.c
> > +++ b/drivers/net/ethernet/intel/ice/ice_xsk.c
> > @@ -895,7 +895,6 @@ int ice_clean_rx_irq_zc(struct ice_rx_ring *rx_ring, int budget)
> >
> >                 if (!first) {
> >                         first = xdp;
> > -                       xdp_buff_clear_frags_flag(first);
> >                 } else if (ice_add_xsk_frag(rx_ring, first, xdp, size)) {
> >                         break;
> >                 }
> > diff --git a/net/xdp/xsk_buff_pool.c b/net/xdp/xsk_buff_pool.c
> > index 28711cc44ced..dc5659da6728 100644
> > --- a/net/xdp/xsk_buff_pool.c
> > +++ b/net/xdp/xsk_buff_pool.c
> > @@ -555,6 +555,7 @@ struct xdp_buff *xp_alloc(struct xsk_buff_pool *pool)
> >
> >         xskb->xdp.data = xskb->xdp.data_hard_start + XDP_PACKET_HEADROOM;
> >         xskb->xdp.data_meta = xskb->xdp.data;
> > +       xskb->xdp.flags = 0;
> >
> >         if (pool->dma_need_sync) {
> >                 dma_sync_single_range_for_device(pool->dev, xskb->dma, 0,
> > @@ -601,6 +602,7 @@ static u32 xp_alloc_new_from_fq(struct xsk_buff_pool *pool, struct xdp_buff **xd
> >                 }
> >
> >                 *xdp = &xskb->xdp;
> > +               xskb->xdp.flags = 0;
> 
> Thanks for catching this. I am thinking we should have an if-statement
> here and only do this when multi-buffer is enabled. The reason that we
> have two different paths for aligned mode and unaligned mode here is
> that we do not have to touch the xdp_buff at all at allocation time in
> aligned mode, which provides a nice speed-up. So let us only do this
> when necessary. What do you think? Same goes for the line in
> xp_alloc_reused().
> 

Good point. How about keeping flags = 0 in xp_alloc() and adding it to
xsk_buff_set_size() ? We do touch xdp_buff there and these two paths cover
batched and non-batched APIs. I do agree that doing it in
xp_alloc_new_from_fq() and in xp_alloc_reused() is not really handy.

> >                 xdp++;
> >         }
> >
> > @@ -621,6 +623,7 @@ static u32 xp_alloc_reused(struct xsk_buff_pool *pool, struct xdp_buff **xdp, u3
> >                 list_del_init(&xskb->free_list_node);
> >
> >                 *xdp = &xskb->xdp;
> > +               xskb->xdp.flags = 0;
> >                 xdp++;
> >         }
> >         pool->free_list_cnt -= nb_entries;
> > --
> > 2.34.1
> >
> >