[PATCH net 0/3] Fix broken TC_ACT_REDIRECT

[Jamal Hadi Salim <jhs@mojatatu.com>]

When sashiko-gemini[1] reviewed commit a8a02897f2b4
("net/sched: cls_api: Handle TC_ACT_CONSUMED in tcf_qevent_handle") it
 correctly pointed out the following:

"
This is a pre-existing issue, but does executing a redirect via a qevent
filter cause a NULL pointer dereference?
When tcf_qevent_handle() processes a TC_ACT_REDIRECT, it calls
skb_do_redirect(). This eventually calls bpf_net_ctx_get_ri() which
dereferences the task bpf_net_context:
include/linux/filter.h:bpf_net_ctx_get_ri() {
    ...
    struct bpf_net_context *bpf_net_ctx = bpf_net_ctx_get();
    if (!(bpf_net_ctx->ri.kern_flags & BPF_RI_F_RI_INIT)) {
    ...
}
Since qevents are evaluated during enqueue, which runs within
__dev_queue_xmit() after sch_handle_egress() has already executed and
cleared the bpf_net_context pointer, will this dereference a NULL pointer?
"

That issue is fixed in patch 1. See the commit log for details.

Upon further investigation it turns out that TC_ACT_REDIRECT being returned
from the egress qdiscs never actually worked. When an action returns that
code we would silently loose it and the packet will never be redirected.
After all those years, if nobody complained, my gut feel is it was never
used by anyone with serious need for it.
Patch 2 fixes it by 1) putting a warning out when someone does and 2) asking
the core to drop the packet. At least this would help whoever is
misconfiguring to diagnose the issue much faster.
I had initially attempted to "fix" this and make it work, but unfortunately
it's a bit ugly so i left i didnt think it was worth fixing

Apologies for the shotgun Cc - its what get_maintainer.pl told me to use.

[1] https://sashiko.dev/#/patchset/20260620130749.226642-1-jhs%40mojatatu.com