Re: [PATCH bpf] cpumap: disable page_pool direct xdp_return need larger scope

[Chris Arges <carges@cloudflare.com>]

On 2025-08-14 20:24:37, Jesper Dangaard Brouer wrote:
> When running an XDP bpf_prog on the remote CPU in cpumap code
> then we must disable the direct return optimization that
> xdp_return can perform for mem_type page_pool.  This optimization
> assumes code is still executing under RX-NAPI of the original
> receiving CPU, which isn't true on this remote CPU.
> 
> The cpumap code already disabled this via helpers
> xdp_set_return_frame_no_direct() and xdp_clear_return_frame_no_direct(),
> but the scope didn't include xdp_do_flush().
> 
> When doing XDP_REDIRECT towards e.g devmap this causes the
> function bq_xmit_all() to run with direct return optimization
> enabled. This can lead to hard to find bugs.  The issue
> only happens when bq_xmit_all() cannot ndo_xdp_xmit all
> frames and them frees them via xdp_return_frame_rx_napi().
> 
> Fix by expanding scope to include xdp_do_flush().
> 
> Fixes: 11941f8a8536 ("bpf: cpumap: Implement generic cpumap")
> Found-by: Dragos Tatulea <dtatulea@nvidia.com>
> Reported-by: Chris Arges <carges@cloudflare.com>
> Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
> ---
>  kernel/bpf/cpumap.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
> index b2b7b8ec2c2a..c46360b27871 100644
> --- a/kernel/bpf/cpumap.c
> +++ b/kernel/bpf/cpumap.c
> @@ -186,7 +186,6 @@ static int cpu_map_bpf_prog_run_xdp(struct bpf_cpu_map_entry *rcpu,
>  	struct xdp_buff xdp;
>  	int i, nframes = 0;
>  
> -	xdp_set_return_frame_no_direct();
>  	xdp.rxq = &rxq;
>  
>  	for (i = 0; i < n; i++) {
> @@ -231,7 +230,6 @@ static int cpu_map_bpf_prog_run_xdp(struct bpf_cpu_map_entry *rcpu,
>  		}
>  	}
>  
> -	xdp_clear_return_frame_no_direct();
>  	stats->pass += nframes;
>  
>  	return nframes;
> @@ -255,6 +253,7 @@ static void cpu_map_bpf_prog_run(struct bpf_cpu_map_entry *rcpu, void **frames,
>  
>  	rcu_read_lock();
>  	bpf_net_ctx = bpf_net_ctx_set(&__bpf_net_ctx);
> +	xdp_set_return_frame_no_direct();
>  
>  	ret->xdp_n = cpu_map_bpf_prog_run_xdp(rcpu, frames, ret->xdp_n, stats);
>  	if (unlikely(ret->skb_n))
> @@ -264,6 +263,7 @@ static void cpu_map_bpf_prog_run(struct bpf_cpu_map_entry *rcpu, void **frames,
>  	if (stats->redirect)
>  		xdp_do_flush();
>  
> +	xdp_clear_return_frame_no_direct();
>  	bpf_net_ctx_clear(bpf_net_ctx);
>  	rcu_read_unlock();
>  
> 
>

FWIW, I tested this patch and could no longer reproduce the original issue.

Tested-By: Chris Arges <carges@cloudflare.com>

--chris