linux-kernel - Re: [PATCH net-next 14/15] net: Reference bpf_redirect_info via task_struct on PREEMPT

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Date: Tue, 07 May 2024 15:50:49 +0200
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, "David S. Miller"
 <davem@...emloft.net>, Boqun Feng <boqun.feng@...il.com>, Daniel Borkmann
 <daniel@...earbox.net>, Eric Dumazet <edumazet@...gle.com>, Frederic
 Weisbecker <frederic@...nel.org>, Ingo Molnar <mingo@...hat.com>, Jakub
 Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Peter
 Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de>,
 Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>, Alexei
 Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, Eduard
 Zingerman <eddyz87@...il.com>, Hao Luo <haoluo@...gle.com>, Jesper
 Dangaard Brouer <hawk@...nel.org>, Jiri Olsa <jolsa@...nel.org>, John
 Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
 Stanislav Fomichev <sdf@...gle.com>, Yonghong Song
 <yonghong.song@...ux.dev>, bpf@...r.kernel.org
Subject: Re: [PATCH net-next 14/15] net: Reference bpf_redirect_info via
 task_struct on PREEMPT_RT.

Sebastian Andrzej Siewior <bigeasy@...utronix.de> writes:

>> > +static inline struct bpf_redirect_info *bpf_net_ctx_get_ri(void)
>> > +{
>> > +	struct bpf_net_context *bpf_net_ctx = bpf_net_ctx_get();
>> > +
>> > +	if (!bpf_net_ctx)
>> > +		return NULL;
>> 
>> ... do we really need all the NULL checks?
>> 
>> (not just here, but in the code below as well).
>> 
>> I'm a little concerned that we are introducing a bunch of new branches
>> in the XDP hot path. Which is also why I'm asking for benchmarks :)
>
> We could hide the WARN behind CONFIG_DEBUG_NET. The only purpose is to
> see the backtrace where the context is missing. Having just an error
> somewhere will make it difficult to track.
>
> The NULL check is to avoid a crash if the context is missing. You could
> argue that this should be noticed in development and never hit
> production. If so, then we get the backtrace from NULL-pointer
> dereference and don't need the checks and WARN.

Yup, this (relying on the NULL deref) SGTM :)

-Toke