linux-kernel - Re: [PATCH 0/2] vsprintf: ignore %n again

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Date:	18 Sep 2013 11:22:09 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	dan.carpenter@...cle.com, penguin-kernel@...ove.SAKURA.ne.jp
Cc:	akpm@...ux-foundation.org, JBeulich@...e.com, joe@...ches.com,
	keescook@...omium.org, kosaki.motohiro@...il.com,
	linux-kernel@...r.kernel.org, linux@...izon.com,
	viro@...iv.linux.org.uk
Subject: Re: [PATCH 0/2] vsprintf: ignore %n again

> +#define printk(fmt, ...) do {				  \
> +	compiletime_assert(__builtin_constant_p(fmt),     \
> +			   "Non-constant format string"); \
> +	printk(fmt, ##__VA_ARGS__);			  \
> +} while (0)

May I recommend __builtin_constant_p(*fmt).  Since:

char buf[OVERFLOW_VULNERABILITY];
strcpy(buf, malicious_format);
printk(buf, args)

has __builtin_constant_p(buf), but it's not a constant format string.
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