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Date:	Tue, 26 Aug 2014 10:48:33 +0200
From:	Jan Kara <jack@...e.cz>
To:	Patrick Palka <patrick@...cs.ath.cx>
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	pmladek@...e.cz, jack@...e.cz, rostedt@...dmis.org
Subject: Re: [PATCH] Fix faulty logic in the case of recursive printk

On Sun 24-08-14 12:01:36, Patrick Palka wrote:
> We shouldn't set text_len in the code path that detects printk recursion
> because text_len corresponds to the length of the string inside textbuf.
> A few lines down from the line
> 
>     text_len = strlen(recursion_msg);
> 
> is the line
> 
>     text_len += vscnprintf(text + text_len, ...);
> 
> So if printk detects recursion, it sets text_len to 29 and logs an
> error.  Then the message supplied by the caller of printk is stored
> inside textbuf but offset by 29 bytes.  This means that the output of
> the recursive call to printk will contain 29 bytes of garbage in front
> of it.
> 
> This defect is caused by commit 458df9fd ("printk: remove separate
> printk_sched buffers and use printk buf instead") which turned
> 
>     text_len = vscnprintf(text + text_len, ...);
> 
> into
> 
>     text_len += vscnprintf(text + text_len, ...);
> 
> To fix this, this patch avoids setting text_len when logging the printk
> recursion error.  This patch also performs a couple of local
> micro-optimizations (use unlikely() and ARRAY_SIZE()).
  Thanks for spotting this! Just one nit below:

> Signed-off-by: Patrick Palka <patrick@...cs.ath.cx>
> ---
>  kernel/printk/printk.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index e04c455..c101ec2 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -1665,15 +1665,15 @@ asmlinkage int vprintk_emit(int facility, int level,
>  	raw_spin_lock(&logbuf_lock);
>  	logbuf_cpu = this_cpu;
>  
> -	if (recursion_bug) {
> +	if (unlikely(recursion_bug)) {
>  		static const char recursion_msg[] =
>  			"BUG: recent printk recursion!";
>  
>  		recursion_bug = 0;
> -		text_len = strlen(recursion_msg);
>  		/* emit KERN_CRIT message */
>  		printed_len += log_store(0, 2, LOG_PREFIX|LOG_NEWLINE, 0,
> -					 NULL, 0, recursion_msg, text_len);
> +					 NULL, 0, recursion_msg,
> +					 ARRAY_SIZE(recursion_msg) - 1);
  I'd prefer to keep using strlen(recursion_msg). It is more obvious what
it does and is more futureproof.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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