linux-kernel - Re: [PATCH] vsprintf: drop comment claiming %n is ignored

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Message-Id: <201309141149.HGF39054.QLJVHFtMFOSOOF@I-love.SAKURA.ne.jp>
Date:	Sat, 14 Sep 2013 11:49:51 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	keescook@...omium.org, joe@...ches.com
Cc:	linux@...izon.com, akpm@...ux-foundation.org,
	dan.carpenter@...cle.com, davem@...emloft.net,
	eldad@...refinery.com, jbeulich@...e.com, jkosina@...e.cz,
	linux-kernel@...r.kernel.org, rdunlap@...radead.org,
	viro@...iv.linux.org.uk
Subject: Re: [PATCH] vsprintf: drop comment claiming %n is ignored

Kees Cook wrote:
> 3- some callers of seq_printf (incorrectly) use the return value as a
> length indication

Are there really?

Is somebody using the return value from seq_printf() like

  pos = snprintf(buf, sizeof(buf) - 1, "%s", foo);
  snprintf(buf + pos, sizeof(buf) - 1 - pos, "%s", bar);

? Since the caller cannot pass the return value from seq_printf() like

  pos = seq_printf(m, "%s", foo);
  seq_printf(m + pos, "%s", bar);

, I wonder who would interpret the return value as a length indication.

Even bad code which has never tested failure case, the authors should already
know that "seq_printf() returns 0 on success case".

I think that

	pos += seq_printf(m, "%s", foo);
	pos += seq_printf(m, "%s", bar);

is used as the equivalent to

	if (seq_printf(m, "%s", foo))
		pos = -1;
	if (seq_printf(m, "%s", bar))
		pos = -1;

.

Joe Perches wrote:
> @@ -174,8 +171,8 @@ static int dbg_show_state(struct seq_file *s, void *p)
>  	int pos = 0;
>  
>  	/* basic device status */
> -	pos += seq_printf(s, "DMA engine status\n");
> -	pos += seq_printf(s, "\tChannel number: %d\n", num_dma_channels);
> +	seq_puts(s, "DMA engine status\n");
> +	seq_printf(s, "\tChannel number: %d\n", num_dma_channels);
>  
>  	return pos;
>  }

As I described above, I think this change breaks the functionality.
We need to change like

  -	pos += seq_printf(s, "DMA engine status\n");
  -	pos += seq_printf(s, "\tChannel number: %d\n", num_dma_channels);
  +	pos |= seq_puts(s, "DMA engine status\n");
  +	pos |= seq_printf(s, "\tChannel number: %d\n", num_dma_channels);

or

  -	pos += seq_printf(s, "DMA engine status\n");
  -	pos += seq_printf(s, "\tChannel number: %d\n", num_dma_channels);
  +	seq_puts(s, "DMA engine status\n");
  +	seq_printf(s, "\tChannel number: %d\n", num_dma_channels);
   
  - 	return pos;
  +	return seq_overflow(s) : -1 : 0;

for keeping the functionality.
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