Re: [PATCH bpf] libbpf: fix passing uninitialized bytes to setsockopt

[Andrii Nakryiko]

On Wed, Oct 9, 2019 at 8:43 AM Ilya Maximets <i.maximets@xxxxxxx> wrote:
>
> 'struct xdp_umem_reg' has 4 bytes of padding at the end that makes
> valgrind complain about passing uninitialized stack memory to the
> syscall:
>
>   Syscall param socketcall.setsockopt() points to uninitialised byte(s)
>     at 0x4E7AB7E: setsockopt (in /usr/lib64/libc-2.29.so)
>     by 0x4BDE035: xsk_umem__create@@LIBBPF_0.0.4 (xsk.c:172)
>   Uninitialised value was created by a stack allocation
>     at 0x4BDDEBA: xsk_umem__create@@LIBBPF_0.0.4 (xsk.c:140)
>
> Padding bytes appeared after introducing of a new 'flags' field.
>
> Fixes: 10d30e301732 ("libbpf: add flags to umem config")
> Signed-off-by: Ilya Maximets <i.maximets@xxxxxxx>
> ---
>  tools/lib/bpf/xsk.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
> index a902838f9fcc..26d9db783560 100644
> --- a/tools/lib/bpf/xsk.c
> +++ b/tools/lib/bpf/xsk.c
> @@ -139,7 +139,7 @@ int xsk_umem__create_v0_0_4(struct xsk_umem **umem_ptr, void *umem_area,
>                             const struct xsk_umem_config *usr_config)
>  {
>         struct xdp_mmap_offsets off;
> -       struct xdp_umem_reg mr;
> +       struct xdp_umem_reg mr = {};

well, guess what, even with this explicit initialization, padding is
not guaranteed to be initialized (and it's sometimes is not in
practice, I ran into such problems), only since C11 standard it is
specified that padding is also zero-initialized. You have to do memset
to 0.

>         struct xsk_umem *umem;
>         socklen_t optlen;
>         void *map;
> --
> 2.17.1
>