linux-kernel - Re: [PATCH] libbpf: Initialize *nl

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Date:   Sun, 5 Apr 2020 19:46:38 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jeremy Cline <jcline@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] libbpf: Initialize *nl_pid so gcc 10 is happy

On Fri, Apr 3, 2020 at 10:15 PM Jeremy Cline <jcline@...hat.com> wrote:
>
> Builds of Fedora's kernel-tools package started to fail with "may be
> used uninitialized" warnings for nl_pid in bpf_set_link_xdp_fd() and
> bpf_get_link_xdp_info() on the s390 architecture.
>
> Although libbpf_netlink_open() always returns a negative number when it
> does not set *nl_pid, the compiler does not determine this and thus
> believes the variable might be used uninitialized. Assuage gcc's fears
> by explicitly initializing nl_pid.
>
> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1807781
> Signed-off-by: Jeremy Cline <jcline@...hat.com>
> ---

Yep, unfortunately compiler is not that smart.

Acked-by: Andrii Nakryiko <andriin@...com>

>  tools/lib/bpf/netlink.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/tools/lib/bpf/netlink.c b/tools/lib/bpf/netlink.c
> index 18b5319025e19..9a14694176de0 100644
> --- a/tools/lib/bpf/netlink.c
> +++ b/tools/lib/bpf/netlink.c
> @@ -142,7 +142,7 @@ static int __bpf_set_link_xdp_fd_replace(int ifindex, int fd, int old_fd,
>                 struct ifinfomsg ifinfo;
>                 char             attrbuf[64];
>         } req;
> -       __u32 nl_pid;
> +       __u32 nl_pid = 0;
>
>         sock = libbpf_netlink_open(&nl_pid);
>         if (sock < 0)
> @@ -288,7 +288,7 @@ int bpf_get_link_xdp_info(int ifindex, struct xdp_link_info *info,
>  {
>         struct xdp_id_md xdp_id = {};
>         int sock, ret;
> -       __u32 nl_pid;
> +       __u32 nl_pid = 0;
>         __u32 mask;
>
>         if (flags & ~XDP_FLAGS_MASK || !info_size)
> --
> 2.26.0
>