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Date: Fri, 27 Mar 2026 17:12:20 -0700
From: Jakub Kicinski <kuba@kernel.org>
To: Leon Hwang <leon.hwang@linux.dev>
Cc: bpf@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>, Daniel
 Borkmann <daniel@iogearbox.net>, John Fastabend <john.fastabend@gmail.com>,
 Andrii Nakryiko <andrii@kernel.org>, Martin KaFai Lau
 <martin.lau@linux.dev>, Eduard Zingerman <eddyz87@gmail.com>, Song Liu
 <song@kernel.org>, Yonghong Song <yonghong.song@linux.dev>, KP Singh
 <kpsingh@kernel.org>, Stanislav Fomichev <sdf@fomichev.me>, Hao Luo
 <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>, Shuah Khan
 <shuah@kernel.org>, "David S . Miller" <davem@davemloft.net>, Jesper
 Dangaard Brouer <hawk@kernel.org>, Toke Hoiland-Jorgensen
 <toke@redhat.com>, Lorenzo Bianconi <lorenzo@kernel.org>,
 linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
 netdev@vger.kernel.org, kernel-patches-bot@fb.com
Subject: Re: [PATCH bpf-next 1/3] bpf: Disallow freplace on XDP with
 mismatched xdp_has_frags values
Message-ID: <20260327171220.6d10d710@kernel.org>
In-Reply-To: <785e0b04-91e4-457f-8d6c-e0a911c0fd88@linux.dev>
References: <20260324150444.68166-1-leon.hwang@linux.dev>
	<20260324150444.68166-2-leon.hwang@linux.dev>
	<20260326124205.1a3bb825@kernel.org>
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On Fri, 27 Mar 2026 14:42:19 +0800 Leon Hwang wrote:
> > It may be worth adding a selftest to
> > tools/testing/selftests/drivers/net/xdp.py
> > which sets MTU to 9k, tries to attach a non-frag-capable prog
> > if that fails attaches a frag-capable prog and then checks if
> > replacing the capable prog with non-capable fails.
> > Drivers may be buggy in this regard.  
> 
> Do you mean adding these two tests to xdp.py?
> 
> 1. Verify the failure of attaching non-frag-capable prog to mtu=9k
>    driver.
> 2. Verify the failure of updating frag-capable prog with non-frag-
>    capable prog via libbpf.c::bpf_link__update_program().

Not directly via libbpf just ip -f link, it uses libbpf internally
but testing with  ip link is simpler. Also - there are already XDP
progs which return XDP_PASS in frag-capable and non-capable mode,
so just use those.

> As for test #2, it may require a helper to attach prog then update prog
> using libbpf's APIs.

See above.