linux-kernel - Re: [PATCHv2] seq_buf: add seq_buf_do

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Message-ID: <ZDkWR2lq2MZ4r1aG@alley>
Date:   Fri, 14 Apr 2023 11:00:55 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2] seq_buf: add seq_buf_do_printk() helper

On Tue 2023-04-11 23:38:52, Sergey Senozhatsky wrote:
> Sometimes we use seq_buf to format a string buffer, which
> we then pass to printk(). However, in certain situations
> the seq_buf string buffer can get too big, exceeding the
> PRINTKRB_RECORD_MAX bytes limit, and causing printk() to
> truncate the string.
> 
> Add a new seq_buf helper. This helper prints the seq_buf
> string buffer line by line, using \n as a delimiter,
> rather than passing the whole string buffer to printk()
> at once.
> 
> --- a/lib/seq_buf.c
> +++ b/lib/seq_buf.c
> @@ -93,6 +93,40 @@ int seq_buf_printf(struct seq_buf *s, const char *fmt, ...)
>  }
>  EXPORT_SYMBOL_GPL(seq_buf_printf);
>  
> +/**
> + * seq_buf_do_printk - printk seq_buf line by line
> + * @s: seq_buf descriptor
> + * @lvl: printk level
> + *
> + * printk()-s a multi-line sequential buffer line by line. The function
> + * makes sure that the buffer in @s is nul terminated and safe to read
> + * as a string.
> + */
> +void seq_buf_do_printk(struct seq_buf *s, const char *lvl)
> +{
> +	const char *start, *lf;
> +	int len;
> +
> +	if (s->size == 0 || s->len == 0)
> +		return;
> +
> +	seq_buf_terminate(s);
> +
> +	start = s->buffer;
> +	while ((lf = strchr(start, '\n'))) {
> +		len = lf - start + 1;
> +		printk("%s%.*s", lvl, len, start);
> +		start = ++lf;
> +	}
> +
> +	/* No trailing LF */
> +	if (start < s->buffer + s->len) {
> +		len = s->buffer + s->len - start;
> +		printk("%s%.*s\n", lvl, len, start);

We know that the string is '\0' terminated, so the last print
might be easier:

	if (start < s->buffer + s->len)
		printk("%s%s\n", lvl, start);

Anyway, it looks good. With or without this change:

Reviewed-by: Petr Mladek <pmladek@...e.com>

Best Regards,
Petr