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

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Message-ID: <20130914045313.GB13318@ZenIV.linux.org.uk>
Date:	Sat, 14 Sep 2013 05:53:14 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	keescook@...omium.org, joe@...ches.com, 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
Subject: Re: [PATCH] vsprintf: drop comment claiming %n is ignored

On Sat, Sep 14, 2013 at 04:48:02AM +0100, Al Viro wrote:

> Overall: I suspect that Joe might be right.  The very few callers that
> use the return value and use it correctly can bloody well call
> seq_overflow(), preferably with a detailed comment about the reasons
> for doing so.  Anything that really wants the length of output (if we
> have such places at all) can use %n or see Figure 1.  I haven't
> crawled through lib/*, net/* and sound/* yet, but that's how the things
> look so far.

The same goes for seq_puts, seq_escape, seq_vprintf, seq_dentry,
seq_bitmap*, seq_cpumask*, seq_nodemask*, seq_putc, seq_put_decimal*

seq_puts() has one buggy user trying to return its return value from
->show().  seq_putc() has several such.

seq_path() returns length and in one case its return value is used
(right-padded pathname in /proc/swaps).

seq_path_root() returns what would be a valid return value for ->show()
(0 or 1, actually).

seq_write() return value is mostly ignored; kernel/trace/* is using it
to check for overflows, but its reaction to said overflows is odd.

The bottom line: most of these guys could as well return void; we have
few overflow checks and those could be made explicit.  As it is,
"return -1 on overflow" had been a mistake.  Mea culpa.
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