Michal Schmidt <mschmidt@xxxxxxxxxx> 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@xxxxxxxxxx>
Reviewed-by: Toke Høiland-Jørgensen <toke@xxxxxxxxxx>
