Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <CAEf4Bzbq0PSsc6xCGSF=Af-pcysjt8Lv76-4N65AJMpXOOpOcg@mail.gmail.com>
 <tencent_CD22930180507496CE65B30ACBABC4681506@qq.com>
In-Reply-To: <tencent_CD22930180507496CE65B30ACBABC4681506@qq.com>
From:   Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date:   Fri, 28 Oct 2022 10:08:25 -0700
Message-ID: <CAEf4BzazYVkVKrKzPD8a7tRZrcWDvvgoVksJHYk3+46V=8kZhw@mail.gmail.com>
Subject: Re: Re: [PATCH] selftests/bpf: Fix strncpy() fortify warning
To:     Rong Tao <rtoax@foxmail.com>
Cc:     andrii@kernel.org, ast@kernel.org, bpf@vger.kernel.org,
        daniel@iogearbox.net, haoluo@google.com, john.fastabend@gmail.com,
        jolsa@kernel.org, kpsingh@kernel.org, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org, martin.lau@linux.dev,
        mykolal@fb.com, rongtao@cestc.cn, sdf@google.com, shuah@kernel.org,
        song@kernel.org, yhs@fb.com
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Precedence: bulk

On Thu, Oct 27, 2022 at 5:26 PM Rong Tao <rtoax@foxmail.com> wrote:
>
> Thanks for your reply, `enable[0] =3D '\0';` at the beginning and then
> strncat() still has the same compile warning
>
> --- a/tools/testing/selftests/bpf/cgroup_helpers.c
> +++ b/tools/testing/selftests/bpf/cgroup_helpers.c
> @@ -77,7 +77,8 @@ static int __enable_controllers(const char *cgroup_path=
, const char *controllers
>                 enable[len] =3D 0;
>                 close(fd);
>         } else {
> -               strncpy(enable, controllers, sizeof(enable));
> +               enable[0] =3D '\0';
> +               strncat(enable, controllers, sizeof(enable));
>         }
>
> In function =E2=80=98__enable_controllers=E2=80=99:
> tools/testing/selftests/bpf/cgroup_helpers.c:81:17: warning: =E2=80=98str=
ncat=E2=80=99 specified bound 4097 equals destination size [-Wstringop-trun=
cation]
>    81 |                 strncat(enable, controllers, sizeof(enable));
>       |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> tools/testing/selftests/bpf/cgroup_helpers.c:81:17: warning: =E2=80=98str=
ncat=E2=80=99 specified bound 4097 equals destination size [-Wstringop-over=
flow=3D]
>
> So, i think just add '-1' for strncpy() is a good way.

no, it's not, see my previous email about ending up with
non-zero-terminated C string.

check strncat() API, it leaves the dst string zero terminated, and
yes, you need -1 for strncat as well, your compiler is right