netdev - Re: [PATCH RFC net-next 1/2] net: Reference bpf_redirect_info via task_struct on PREEMPT

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Message-ID: <87y1bndvsx.fsf@toke.dk>
Date: Wed, 14 Feb 2024 14:23:10 +0100
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Jesper Dangaard
 Brouer <hawk@...nel.org>
Cc: bpf@...r.kernel.org, netdev@...r.kernel.org, Björn Töpel
 <bjorn@...nel.org>, "David S. Miller" <davem@...emloft.net>, Alexei
 Starovoitov <ast@...nel.org>, Andrii Nakryiko <andrii@...nel.org>, Daniel
 Borkmann <daniel@...earbox.net>, Eric Dumazet <edumazet@...gle.com>, Hao
 Luo <haoluo@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Jiri Olsa
 <jolsa@...nel.org>, John Fastabend <john.fastabend@...il.com>, Jonathan
 Lemon <jonathan.lemon@...il.com>, KP Singh <kpsingh@...nel.org>, Maciej
 Fijalkowski <maciej.fijalkowski@...el.com>, Magnus Karlsson
 <magnus.karlsson@...el.com>, Martin KaFai Lau <martin.lau@...ux.dev>,
 Paolo Abeni <pabeni@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Song Liu <song@...nel.org>, Stanislav Fomichev <sdf@...gle.com>, Thomas
 Gleixner <tglx@...utronix.de>, Yonghong Song <yonghong.song@...ux.dev>
Subject: Re: [PATCH RFC net-next 1/2] net: Reference bpf_redirect_info via
 task_struct on PREEMPT_RT.

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

> On 2024-02-13 21:50:51 [+0100], Jesper Dangaard Brouer wrote:
>> I generally like the idea around bpf_xdp_storage.
>> 
>> I only skimmed the code, but noticed some extra if-statements (for
>> !NULL). I don't think they will make a difference, but I know Toke want
>> me to test it...
>
> I've been looking at the assembly for the return value of
> bpf_redirect_info() and there is a NULL pointer check. I hoped it was
> obvious to be nun-NULL because it is a static struct.
>
> Should this become a problem I could add
> "__attribute__((returns_nonnull))" to the declaration of the function
> which will optimize the NULL check away.

If we know the function will never return NULL (I was wondering about
that, actually), why have the check in the C code at all? Couldn't we just
omit it entirely instead of relying on the compiler to optimise it out?

-Toke