linux-kernel - Re: [PATCH net] selftests/bpf: fix pointer arithmetic in test_xdp_do

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Message-ID: <87v83qoh86.fsf@toke.dk>
Date: Mon, 06 May 2024 21:47:21 +0200
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: Michal Schmidt <mschmidt@...hat.com>, Alexei Starovoitov
 <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, "David S.
 Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Jesper
 Dangaard Brouer <hawk@...nel.org>, John Fastabend
 <john.fastabend@...il.com>, Andrii Nakryiko <andrii@...nel.org>, Martin
 KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>, KP
 Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...gle.com>, Hao Luo
 <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko
 <mykolal@...com>, Shuah Khan <shuah@...nel.org>, Alexander Lobakin
 <aleksander.lobakin@...el.com>
Cc: netdev@...r.kernel.org, bpf@...r.kernel.org,
 linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] selftests/bpf: fix pointer arithmetic in
 test_xdp_do_redirect

Michal Schmidt <mschmidt@...hat.com> writes:

> Cast operation has a higher precedence than addition. The code here
> wants to zero the 2nd half of the 64-bit metadata, but due to a pointer
> arithmetic mistake, it writes the zero at offset 16 instead.
>
> Just adding parentheses around "data + 4" would fix this, but I think
> this will be slightly better readable with array syntax.
>
> I was unable to test this with tools/testing/selftests/bpf/vmtest.sh,
> because my glibc is newer than glibc in the provided VM image.
> So I just checked the difference in the compiled code.
> objdump -S tools/testing/selftests/bpf/xdp_do_redirect.test.o:
>   -	*((__u32 *)data) = 0x42; /* metadata test value */
>   +	((__u32 *)data)[0] = 0x42; /* metadata test value */
>         be7:	48 8d 85 30 fc ff ff 	lea    -0x3d0(%rbp),%rax
>         bee:	c7 00 42 00 00 00    	movl   $0x42,(%rax)
>   -	*((__u32 *)data + 4) = 0;
>   +	((__u32 *)data)[1] = 0;
>         bf4:	48 8d 85 30 fc ff ff 	lea    -0x3d0(%rbp),%rax
>   -     bfb:	48 83 c0 10          	add    $0x10,%rax
>   +     bfb:	48 83 c0 04          	add    $0x4,%rax
>         bff:	c7 00 00 00 00 00    	movl   $0x0,(%rax)
>
> Fixes: 5640b6d89434 ("selftests/bpf: fix "metadata marker" getting overwritten by the netstack")
> Signed-off-by: Michal Schmidt <mschmidt@...hat.com>

Reviewed-by: Toke Høiland-Jørgensen <toke@...hat.com>