linux-kernel - Re: [PATCH v2 1/4] printk/NMI: Handle continuous lines and missing newline

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Date:   Fri, 11 Nov 2016 19:07:09 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Steven Rostedt <rostedt@...dmis.org>
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>,
        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
Subject: Re: [PATCH v2 1/4] printk/NMI: Handle continuous lines and missing
 newline

On Fri 2016-11-11 12:28:51, Steven Rostedt wrote:
> On Wed,  9 Nov 2016 13:41:28 +0100
> Petr Mladek <pmladek@...e.com> wrote:
> 
> 
> >  /*
> > @@ -135,8 +170,8 @@ 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
> > @@ -154,12 +189,14 @@ 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)) {
> 
> What's wrong with using s->len? Isn't that what is inside the buffer?
> Couldn't just checking against the buffer size print garbage?

Note that this is not the classic seq_buf. It is struct nmi_seq_buf
where "len" is atomic_t and buffer is defined as
buffer[NMI_LOG_BUF_LEN].

It is just a paranoid check that should newer be true if the
implementation is correct. I believe that it makes sense as is.

Best Regards,
Petr