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From:   Song Liu <song@kernel.org>
Date:   Fri, 15 Apr 2022 16:57:53 -0700
Message-ID: <CAPhsuW7AV+RSqmH3WOSB6uvS2oiXH-Tgfx84H0rEC9_NyZ3SNA@mail.gmail.com>
Subject: Re: [PATCH bpf-next 11/11] samples: bpf: xdpsock: fix -Wmaybe-uninitialized
To:     Alexander Lobakin <alobakin@pm.me>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
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        Martin KaFai Lau <kafai@fb.com>,
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On Thu, Apr 14, 2022 at 3:47 PM Alexander Lobakin <alobakin@pm.me> wrote:
>
> Fix two sort-of-false-positives in the xdpsock userspace part:
>
> samples/bpf/xdpsock_user.c: In function 'main':
> samples/bpf/xdpsock_user.c:1531:47: warning: 'tv_usec' may be used uninitialized in this function [-Wmaybe-uninitialized]
>  1531 |                         pktgen_hdr->tv_usec = htonl(tv_usec);
>       |                                               ^~~~~~~~~~~~~~
> samples/bpf/xdpsock_user.c:1500:26: note: 'tv_usec' was declared here
>  1500 |         u32 idx, tv_sec, tv_usec;
>       |                          ^~~~~~~
> samples/bpf/xdpsock_user.c:1530:46: warning: 'tv_sec' may be used uninitialized in this function [-Wmaybe-uninitialized]
>  1530 |                         pktgen_hdr->tv_sec = htonl(tv_sec);
>       |                                              ^~~~~~~~~~~~~
> samples/bpf/xdpsock_user.c:1500:18: note: 'tv_sec' was declared here
>  1500 |         u32 idx, tv_sec, tv_usec;
>       |                  ^~~~~~
>
> Both variables are always initialized when @opt_tstamp == true and
> they're being used also only when @opt_tstamp == true. However, that
> variable comes from the BSS and is being toggled from another
> function. They can't be executed simultaneously to actually trigger
> undefined behaviour, but purely technically it is a correct warning.
> Just initialize them with zeroes.
>
> Fixes: eb68db45b747 ("samples/bpf: xdpsock: Add timestamp for Tx-only operation")
> Signed-off-by: Alexander Lobakin <alobakin@pm.me>

Acked-by: Song Liu <songliubraving@fb.com>

> ---
>  samples/bpf/xdpsock_user.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/samples/bpf/xdpsock_user.c b/samples/bpf/xdpsock_user.c
> index 399b999fcec2..1dc7ad5dbef4 100644
> --- a/samples/bpf/xdpsock_user.c
> +++ b/samples/bpf/xdpsock_user.c
> @@ -1496,7 +1496,7 @@ static void rx_drop_all(void)
>  static int tx_only(struct xsk_socket_info *xsk, u32 *frame_nb,
>                    int batch_size, unsigned long tx_ns)
>  {
> -       u32 idx, tv_sec, tv_usec;
> +       u32 idx, tv_sec = 0, tv_usec = 0;
>         unsigned int i;
>
>         while (xsk_ring_prod__reserve(&xsk->tx, batch_size, &idx) <
> --
> 2.35.2
>
>