Michal Schmidt <[email protected]> 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 <[email protected]>
Reviewed-by: Toke Høiland-Jørgensen <[email protected]>
