Re: [PATCH v7 net-next 14/15] net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.

[Jesper Dangaard Brouer]

On 18/06/2024 09.13, Sebastian Andrzej Siewior wrote:
> The XDP redirect process is two staged:
> - bpf_prog_run_xdp() is invoked to run a eBPF program which inspects the
>    packet and makes decisions. While doing that, the per-CPU variable
>    bpf_redirect_info is used.
>
> - Afterwards xdp_do_redirect() is invoked and accesses bpf_redirect_info
>    and it may also access other per-CPU variables like xskmap_flush_list.
>
> At the very end of the NAPI callback, xdp_do_flush() is invoked which
> does not access bpf_redirect_info but will touch the individual per-CPU
> lists.
>
> The per-CPU variables are only used in the NAPI callback hence disabling
> bottom halves is the only protection mechanism. Users from preemptible
> context (like cpu_map_kthread_run()) explicitly disable bottom halves
> for protections reasons.
> Without locking in local_bh_disable() on PREEMPT_RT this data structure
> requires explicit locking.
>
> PREEMPT_RT has forced-threaded interrupts enabled and every
> NAPI-callback runs in a thread. If each thread has its own data
> structure then locking can be avoided.
>
> Create a struct bpf_net_context which contains struct bpf_redirect_info.
> Define the variable on stack, use bpf_net_ctx_set() to save a pointer to
> it, bpf_net_ctx_clear() removes it again.
> The bpf_net_ctx_set() may nest. For instance a function can be used from
> within NET_RX_SOFTIRQ/ net_rx_action which uses bpf_net_ctx_set() and
> NET_TX_SOFTIRQ which does not. Therefore only the first invocations
> updates the pointer.
> Use bpf_net_ctx_get_ri() as a wrapper to retrieve the current struct
> bpf_redirect_info. The returned data structure is zero initialized to
> ensure nothing is leaked from stack. This is done on first usage of the
> struct. bpf_net_ctx_set() sets bpf_redirect_info::kern_flags  to 0 to
> note that initialisation is required. First invocation of
> bpf_net_ctx_get_ri() will memset() the data structure and update
> bpf_redirect_info::kern_flags.
> bpf_redirect_info::nh  is excluded from memset because it is only used
> once BPF_F_NEIGH is set which also sets the nh member. The kern_flags is
> moved past nh to exclude it from memset.
>
> The pointer to bpf_net_context is saved task's task_struct. Using
> always the bpf_net_context approach has the advantage that there is
> almost zero differences between PREEMPT_RT and non-PREEMPT_RT builds.
>
> Cc: Alexei Starovoitov<ast@xxxxxxxxxx>
> Cc: Andrii Nakryiko<andrii@xxxxxxxxxx>
> Cc: Eduard Zingerman<eddyz87@xxxxxxxxx>
> Cc: Hao Luo<haoluo@xxxxxxxxxx>
> Cc: Jesper Dangaard Brouer<hawk@xxxxxxxxxx>
> Cc: Jiri Olsa<jolsa@xxxxxxxxxx>
> Cc: John Fastabend<john.fastabend@xxxxxxxxx>
> Cc: KP Singh<kpsingh@xxxxxxxxxx>
> Cc: Martin KaFai Lau<martin.lau@xxxxxxxxx>
> Cc: Song Liu<song@xxxxxxxxxx>
> Cc: Stanislav Fomichev<sdf@xxxxxxxxxx>
> Cc: Toke Høiland-Jørgensen<toke@xxxxxxxxxx>
> Cc: Yonghong Song<yonghong.song@xxxxxxxxx>
> Cc:bpf@xxxxxxxxxxxxxxx
> Acked-by: Alexei Starovoitov<ast@xxxxxxxxxx>
> Reviewed-by: Toke Høiland-Jørgensen<toke@xxxxxxxxxx>
> Signed-off-by: Sebastian Andrzej Siewior<bigeasy@xxxxxxxxxxxxx>

Acked-by: Jesper Dangaard Brouer <hawk@xxxxxxxxxx>

> ---
>   include/linux/filter.h | 56 ++++++++++++++++++++++++++++++++++--------
>   include/linux/sched.h  |  3 +++
>   kernel/bpf/cpumap.c    |  3 +++
>   kernel/bpf/devmap.c    |  9 ++++++-
>   kernel/fork.c          |  1 +
>   net/bpf/test_run.c     | 11 ++++++++-
>   net/core/dev.c         | 26 +++++++++++++++++++-
>   net/core/filter.c      | 44 +++++++++------------------------
>   net/core/lwt_bpf.c     |  3 +++
>   9 files changed, 111 insertions(+), 45 deletions(-)