Re: [PATCH 1/4] printk/NMI: Handle continuous lines and missing newline

[Joe Perches]

On Thu, 2016-10-27 at 17:52 +0200, Petr Mladek wrote:
> The commit 4bcc595ccd80decb4245 ("printk: reinstate KERN_CONT for printing
> continuation lines") added back KERN_CONT message header. As a result
> it might appear in the middle of the line when the parts are squashed
> via the temporary NMI buffer.
> 
> A reasonable solution seems to be to split the text in the NNI temporary
> not only by newlines but also by the message headers.
> 
> Another solution would be to filter out KERN_CONT when writing to
> the temporary buffer. But this would complicate the lockless handling.
> Also it would not solve problems with a missing newline that was there
> even before the KERN_CONT stuff.

I believe the proper solution there is add the missing EOL/newlines
where appropriate.  There aren't many treewide.  Maybe a couple dozen
vs the 250,000 messages with newlines.

> This patch moves the temporary buffer handling into separate function.
> I played with it and it seems that using the char pointers make the
> code easier to read.
> 
> Also it moves handling of the s->len overflow into the paranoid check.
> And allows to recover from the disaster.
> 
> Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
> ---
>  kernel/printk/nmi.c | 78 +++++++++++++++++++++++++++++++++--------------------
>  1 file changed, 49 insertions(+), 29 deletions(-)
> 
> diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
> index 16bab471c7e2..740c90efc65d 100644
> --- a/kernel/printk/nmi.c
> +++ b/kernel/printk/nmi.c
> @@ -113,16 +113,49 @@ static void printk_nmi_flush_line(const char *text, int len)
>  
>  }
>  
> -/*
> - * printk one line from the temporary buffer from @start index until
> - * and including the @end index.
> - */
> -static void printk_nmi_flush_seq_line(struct nmi_seq_buf *s,
> -					int start, int end)
> +/* printk part of the temporary buffer line by line */
> +static int printk_nmi_flush_buffer(unsigned char *start, size_t len)

const unsigned char *?

>  {
> -	const char *buf = s->buffer + start;
> +	unsigned char *c, *end;
> +	bool header;
> +
> +	c = start;
> +	end = start + len;
> +	header = true;
> +
> +	/* Print line by line. */
> +	while (c < end) {
> +		if (*c == '\n') {
> +			printk_nmi_flush_line(start, c - start + 1);
> +			start = ++c;
> +			header = true;
> +			continue;
> +		}
>  
> -	printk_nmi_flush_line(buf, (end - start) + 1);
> +		/* Handle continuous lines or missing new line. */
> +		if ((c + 1 < end) && printk_get_level(c)) {
> +			if (header) {
> +				c += 2;

printk_skip_level

> +				continue;
> +			}
> +
> +			printk_nmi_flush_line(start, c - start);
> +			start = c++;
> +			header = true;
> +			continue;
> +		}
> +
> +		header = false;
> +		c++;
> +	}
> +
> +	/* Check if there was a partial line. Ignore pure header. */
> +	if (start < end && !header) {
> +		printk_nmi_flush_line(start, end - start);
> +		printk_nmi_flush_line("\n", strlen("\n"));
> +	}
> +
> +	return len;
>  }
>  
>  /*
> @@ -135,8 +168,8 @@ static void __printk_nmi_flush(struct irq_work *work)
>  		__RAW_SPIN_LOCK_INITIALIZER(read_lock);
>  	struct nmi_seq_buf *s = container_of(work, struct nmi_seq_buf, work);
>  	unsigned long flags;
> -	size_t len, size;
> -	int i, last_i;
> +	size_t len;
> +	int i;
>  
>  	/*
>  	 * The lock has two functions. First, one reader has to flush all
> @@ -154,35 +187,22 @@ static void __printk_nmi_flush(struct irq_work *work)
>  	/*
>  	 * This is just a paranoid check that nobody has manipulated
>  	 * the buffer an unexpected way. If we printed something then
> -	 * @len must only increase.
> +	 * @len must only increase. Also it should never overflow the
> +	 * buffer size.
>  	 */
> -	if (i && i >= len) {
> +	if ((i && i >= len) || len > sizeof(s->buffer)) {
>  		const char *msg = "printk_nmi_flush: internal error\n";
>  
>  		printk_nmi_flush_line(msg, strlen(msg));
> +		len = 0;
>  	}
>  
>  	if (!len)
>  		goto out; /* Someone else has already flushed the buffer. */
>  
> -	/* Make sure that data has been written up to the @len */
> +	/* Make sure the data has been written up to the @len */
>  	smp_rmb();
> -
> -	size = min(len, sizeof(s->buffer));
> -	last_i = i;
> -
> -	/* Print line by line. */
> -	for (; i < size; i++) {
> -		if (s->buffer[i] == '\n') {
> -			printk_nmi_flush_seq_line(s, last_i, i);
> -			last_i = i + 1;
> -		}
> -	}
> -	/* Check if there was a partial line. */
> -	if (last_i < size) {
> -		printk_nmi_flush_seq_line(s, last_i, size - 1);
> -		printk_nmi_flush_line("\n", strlen("\n"));
> -	}
> +	i += printk_nmi_flush_buffer(s->buffer + i, len - i);
>  
>  	/*
>  	 * Check that nothing has got added in the meantime and truncate