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

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Message-ID: <20130913194915.13780.qmail@science.horizon.com>
Date:	13 Sep 2013 15:49:15 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	JBeulich@...e.com, joe@...ches.com, keescook@...omium.org
Cc:	akpm@...ux-foundation.org, dan.carpenter@...cle.com,
	davem@...emloft.net, eldad@...refinery.com,
	linux-kernel@...r.kernel.org, linux@...izon.com,
	rdunlap@...radead.org
Subject: Re: [PATCH] vsprintf: drop comment claiming %n is ignored

> Why would you want to artificially make the function diverge
> from the spec?

Because %n make it easy to convert a not-uncommon format string bug into
a code injection.  Thus, poses a significant security vulnerability.

Since it's an obscure and  rarely-used feature, it is straightforward
to eliminate all users in the Linux kernel, making removing it possible.

I agree that if it were harmless, it would be useful to leave it
implemented just for simplicity (it's a trivial amount of code), but
it's not harmless.

> People shouldn't be caught by surprises if at all
> possible, and one can certainly not expect people to go look at
> the comment before the function implementation to find out
> what basic (standard) features _do not_ work (one can expect
> so when trying to find out about _extensions_).

This is why people propose implementing it as a kernel warning.

Strongly support this change.
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