From: Michal Schmidt <mschmidt@xxxxxxxxxx>
Date: Mon, 6 May 2024 16:50:22 +0200
> 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@xxxxxxxxxx>
> ---
> tools/testing/selftests/bpf/prog_tests/xdp_do_redirect.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_do_redirect.c b/tools/testing/selftests/bpf/prog_tests/xdp_do_redirect.c
> index 498d3bdaa4b0..bad0ea167be7 100644
> --- a/tools/testing/selftests/bpf/prog_tests/xdp_do_redirect.c
> +++ b/tools/testing/selftests/bpf/prog_tests/xdp_do_redirect.c
> @@ -107,8 +107,8 @@ void test_xdp_do_redirect(void)
> .attach_point = BPF_TC_INGRESS);
>
> memcpy(&data[sizeof(__u64)], &pkt_udp, sizeof(pkt_udp));
> - *((__u32 *)data) = 0x42; /* metadata test value */
> - *((__u32 *)data + 4) = 0;
> + ((__u32 *)data)[0] = 0x42; /* metadata test value */
> + ((__u32 *)data)[1] = 0;
Uff sorry. I was a bit in hurry to fix BPF CI and did this braino :z
I know pointer arithms obviously :D
I'd give a Rev-by, but it was applied already. Thanks!
>
> skel = test_xdp_do_redirect__open();
> if (!ASSERT_OK_PTR(skel, "skel"))
Thanks,
Olek
