Re: [PATCH] gtp: disable BH before calling udp_tunnel_xmit_skb()

David CARLIER
2026-04-20T19:44:51Z

Hi Julian,

On Mon, 20 Apr 2026 at 20:02, Justin Iurman <justin.iurman@gmail.com> wrote:
>
> On 4/17/26 07:54, David Carlier wrote:
> > gtp_genl_send_echo_req() runs as a generic netlink doit handler in
> > process context with BH not disabled. It calls udp_tunnel_xmit_skb(),
> > which eventually invokes iptunnel_xmit() — that uses __this_cpu_inc/dec
> > on softnet_data.xmit.recursion to track the tunnel xmit recursion level.
> >
> > Without local_bh_disable(), the task may migrate between
> > dev_xmit_recursion_inc() and dev_xmit_recursion_dec(), breaking the
> > per-CPU counter pairing. The result is stale or negative recursion
> > levels that can later produce false-positive
> > SKB_DROP_REASON_RECURSION_LIMIT drops on either CPU.
> >
> > The other udp_tunnel_xmit_skb() call sites in gtp.c are unaffected:
> > the data path runs under ndo_start_xmit and the echo response handlers
> > run from the UDP encap rx softirq, both with BH already disabled.
> >
> > Fix it by disabling BH around the udp_tunnel_xmit_skb() call, mirroring
> > commit 2cd7e6971fc2 ("sctp: disable BH before calling
> > udp_tunnel_xmit_skb()").
>
> Why not fix iptunnel_xmit() directly, rather than fixing all possible
> callers? Basically, jut like we did for lwtunnel_{output|xmit}(). The
> advantage would be that we no longer have to worry about BHs in the
> callers, and BHs would only be disabled when necessary.

Good point — your lwtunnel fix (c03a49f3093a) is a close parallel, and
a central fix would avoid chasing callers one by one (sctp was patched
last week, gtp is this one, and tipc/wireguard/ovpn genl paths look
similar).

Happy to respin as v2 with local_bh_disable/enable moved into
iptunnel_xmit() (and ip6tunnel_xmit() for symmetry), and drop the
gtp-local hunk. That would also supersede Xin Long's recent sctp
commit
(2cd7e6971fc2), so I'll make sure to Cc him.

One thing I'd like your take on before I send: iptunnel_xmit() feels
like the natural home since it owns the recursion counter, but would
you rather see it in udp_tunnel_xmit_skb()? I don't want to pick the
wrong spot if you already have a preference.

Cheers !