Re: [PATCH 6.6 1/2] net: Reference bpf_redirect_info via task_struct on PREEMPT_RT.

[Greg KH <greg@kroah.com>]

On Mon, Feb 24, 2025 at 01:00:01PM +0100, Ricardo Cañuelo Navarro wrote:
> From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> 
> [ Upstream commit 401cb7dae8130fd34eb84648e02ab4c506df7d5e ]
> 
> 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: Andrii Nakryiko <andrii@kernel.org>
> Cc: Eduard Zingerman <eddyz87@gmail.com>
> Cc: Hao Luo <haoluo@google.com>
> Cc: Jiri Olsa <jolsa@kernel.org>
> Cc: John Fastabend <john.fastabend@gmail.com>
> Cc: KP Singh <kpsingh@kernel.org>
> Cc: Martin KaFai Lau <martin.lau@linux.dev>
> Cc: Song Liu <song@kernel.org>
> Cc: Stanislav Fomichev <sdf@google.com>
> Cc: Yonghong Song <yonghong.song@linux.dev>
> Acked-by: Alexei Starovoitov <ast@kernel.org>
> Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>
> Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Link: https://patch.msgid.link/20240620132727.660738-15-bigeasy@linutronix.de
> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
> ---
>  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         | 28 ++++++++++++++++++++++++-
>  net/core/filter.c      | 41 +++++++++++-------------------------
>  net/core/lwt_bpf.c     |  3 +++
>  9 files changed, 113 insertions(+), 42 deletions(-)

You did major changes to this and you didn't sign off and explain what
you did differently from the original commit?  You know we can't take
that...

thanks,

greg k-h