linux-kernel - Re: [PATCH v2 3/4] printk/btrfs: Handle more message headers

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Date:   Tue, 13 Dec 2016 10:21:27 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Jaroslav Kysela <perex@...ex.cz>,
        Takashi Iwai <tiwai@...e.com>, Chris Mason <clm@...com>,
        Josef Bacik <jbacik@...com>, David Sterba <dsterba@...e.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 3/4] printk/btrfs: Handle more message headers

Hi Petr,

On Wed, Nov 9, 2016 at 1:41 PM, Petr Mladek <pmladek@...e.com> wrote:
> The commit 4bcc595ccd80decb4245096e ("printk: reinstate KERN_CONT for
> printing continuation lines") allows to define more message headers
> for a single message. The motivation is that continuous lines might
> get mixed. Therefore it make sense to define the right log level
> for every piece of a cont line.
>
> The current btrfs_printk() macros do not support continuous lines
> at the moment. But better be prepared for a custom messages and
> avoid potential "lvl" buffer overflow.
>
> This patch iterates over the entire message header. It is interested
> only into the message level like the original code.
>
> This patch also introduces PRINTK_MAX_SINGLE_HEADER_LEN. Three bytes
> are enough for the message level header at the moment. But it used to
> be three, see the commit 04d2c8c83d0e3ac5f ("printk: convert the format
> for KERN_<LEVEL> to a 2 byte pattern").
>
> Also I fixed the default ratelimit level. It looked very strange
> when it was different from the default log level.
>
> Signed-off-by: Petr Mladek <pmladek@...e.com>
> ---
>  fs/btrfs/super.c       | 26 +++++++++++++++-----------
>  include/linux/printk.h |  2 ++
>  2 files changed, 17 insertions(+), 11 deletions(-)
>
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 74ed5aae6cea..c083d84eaa32 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -202,27 +202,31 @@ void __btrfs_handle_fs_error(struct btrfs_fs_info *fs_info, const char *function
>  void btrfs_printk(const struct btrfs_fs_info *fs_info, const char *fmt, ...)
>  {
>         struct super_block *sb = fs_info->sb;
> -       char lvl[4];
> +       char lvl[PRINTK_MAX_SINGLE_HEADER_LEN + 1];
>         struct va_format vaf;
>         va_list args;
> -       const char *type = logtypes[4];
> +       const char *type = NULL;
>         int kern_level;
>         struct ratelimit_state *ratelimit;

warning: ‘ratelimit’ may be used uninitialized in this function

So this triggered my attention. It seems my gcc (4.1.2) is not smart enough
to notice that ratelimit will be set to a default value if !type.

Still, IMHO the code is too convoluted for the casual reader.

>         va_start(args, fmt);
>
> -       kern_level = printk_get_level(fmt);
> -       if (kern_level) {
> +       while ((kern_level = printk_get_level(fmt)) != 0) {
>                 size_t size = printk_skip_level(fmt) - fmt;
> -               memcpy(lvl, fmt,  size);
> -               lvl[size] = '\0';
> +
> +               if (kern_level >= '0' || kern_level <= '7') {
> +                       memcpy(lvl, fmt,  size);
> +                       lvl[size] = '\0';
> +                       type = logtypes[kern_level - '0'];
> +                       ratelimit = &printk_limits[kern_level - '0'];
> +               }
>                 fmt += size;
> -               type = logtypes[kern_level - '0'];
> -               ratelimit = &printk_limits[kern_level - '0'];
> -       } else {
> +       }
> +
> +       if (!type) {
>                 *lvl = '\0';
> -               /* Default to debug output */
> -               ratelimit = &printk_limits[7];
> +               type = logtypes[4];
> +               ratelimit = &printk_limits[4];
>         }
>
>         vaf.fmt = fmt;
> diff --git a/include/linux/printk.h b/include/linux/printk.h
> index a0859e169bc3..afe8ccec1672 100644
> --- a/include/linux/printk.h
> +++ b/include/linux/printk.h
> @@ -10,6 +10,8 @@
>  extern const char linux_banner[];
>  extern const char linux_proc_banner[];
>
> +#define PRINTK_MAX_SINGLE_HEADER_LEN 2

I think you want to use this definition instead of the hardcoded "2" in
printk_skip_level():

| static inline const char *printk_skip_level(const char *buffer)
| {
|         if (printk_get_level(buffer))
|                 return buffer + 2;

return buffer + PRINTK_MAX_SINGLE_HEADER_LEN;

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds