linux-kernel - Re: [PATCH v6] checkpatch: add check for snprintf to scnprintf

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Message-ID: <202404291249.078D924@keescook>
Date: Mon, 29 Apr 2024 12:49:48 -0700
From: Kees Cook <keescook@...omium.org>
To: Justin Stitt <justinstitt@...gle.com>
Cc: Andy Whitcroft <apw@...onical.com>, Joe Perches <joe@...ches.com>,
	Dwaipayan Ray <dwaipayanray1@...il.com>,
	Lukas Bulwahn <lukas.bulwahn@...il.com>,
	linux-kernel@...r.kernel.org, Lee Jones <lee@...nel.org>,
	linux-hardening@...r.kernel.org, Finn Thain <fthain@...ux-m68k.org>
Subject: Re: [PATCH v6] checkpatch: add check for snprintf to scnprintf

On Mon, Apr 29, 2024 at 06:39:28PM +0000, Justin Stitt wrote:
> I am going to quote Lee Jones who has been doing some snprintf ->
> scnprintf refactorings:
> 
> "There is a general misunderstanding amongst engineers that
> {v}snprintf() returns the length of the data *actually* encoded into the
> destination array.  However, as per the C99 standard {v}snprintf()
> really returns the length of the data that *would have been* written if
> there were enough space for it.  This misunderstanding has led to
> buffer-overruns in the past.  It's generally considered safer to use the
> {v}scnprintf() variants in their place (or even sprintf() in simple
> cases).  So let's do that."
> 
> To help prevent new instances of snprintf() from popping up, let's add a
> check to checkpatch.pl.
> 
> Suggested-by: Finn Thain <fthain@...ux-m68k.org>
> Signed-off-by: Justin Stitt <justinstitt@...gle.com>

Thanks!

Reviewed-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook