Michal Schmidt mschmidt@redhat.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@redhat.com
Reviewed-by: Toke Høiland-Jørgensen toke@redhat.com