linux-kernel - Re: [PATCH bpf-next 11/11] samples: bpf: xdpsock: fix -Wmaybe-uninitialized

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Date:   Fri, 15 Apr 2022 14:15:15 +0200
From:   Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To:     Alexander Lobakin <alobakin@...me>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Björn Töpel <bjorn@...nel.org>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Dmitrii Dolgov <9erthalion6@...il.com>,
        Quentin Monnet <quentin@...valent.com>,
        Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Kumar Kartikeya Dwivedi <memxor@...il.com>,
        Chenbo Feng <fengc@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Daniel Wagner <daniel.wagner@...-carit.de>,
        Thomas Graf <tgraf@...g.ch>,
        Ong Boon Leong <boon.leong.ong@...el.com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH bpf-next 11/11] samples: bpf: xdpsock: fix
 -Wmaybe-uninitialized

On Thu, Apr 14, 2022 at 10:47:20PM +0000, Alexander Lobakin 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@...me>

Acked-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com>

Magnus would tell you that you should fix this on libxdp side instead :)

> ---
>  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
> 
>