netdev - Re: [PATCH v4 bpf-next 2/3] bpf: Support "%c" in bpf_bprintf

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Date:   Wed, 11 Aug 2021 14:15:50 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Kuniyuki Iwashima <kuniyu@...zon.co.jp>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...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>,
        Benjamin Herrenschmidt <benh@...zon.com>,
        Kuniyuki Iwashima <kuni1840@...il.com>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH v4 bpf-next 2/3] bpf: Support "%c" in bpf_bprintf_prepare().

On Tue, Aug 10, 2021 at 2:29 AM Kuniyuki Iwashima <kuniyu@...zon.co.jp> wrote:
>
> /proc/net/unix uses "%c" to print a single-byte character to escape '\0' in
> the name of the abstract UNIX domain socket.  The following selftest uses
> it, so this patch adds support for "%c".  Note that it does not support
> wide character ("%lc" and "%llc") for simplicity.
>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.co.jp>
> ---
>  kernel/bpf/helpers.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 15746f779fe1..6d3aaf94e9ac 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -907,6 +907,20 @@ int bpf_bprintf_prepare(char *fmt, u32 fmt_size, const u64 *raw_args,
>                         tmp_buf += err;
>                         num_spec++;
>
> +                       continue;
> +               } else if (fmt[i] == 'c') {

you are adding new features to printk-like helpers, please add
corresponding tests as well. I'm particularly curious how something
like "% 9c" (which is now allowed, along with a few other unusual
combinations) will work.

> +                       if (!tmp_buf)
> +                               goto nocopy_fmt;
> +
> +                       if (tmp_buf_end == tmp_buf) {
> +                               err = -ENOSPC;
> +                               goto out;
> +                       }
> +
> +                       *tmp_buf = raw_args[num_spec];
> +                       tmp_buf++;
> +                       num_spec++;
> +
>                         continue;
>                 }
>
> --
> 2.30.2
>