linux-kernel - Re: [PATCH v2] checkpatch: add check for snprintf to scnprintf

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Date: Wed, 21 Feb 2024 17:05:30 -0800
From: Joe Perches <joe@...ches.com>
To: Justin Stitt <justinstitt@...gle.com>, Andy Whitcroft
 <apw@...onical.com>,  Dwaipayan Ray <dwaipayanray1@...il.com>, Lukas
 Bulwahn <lukas.bulwahn@...il.com>
Cc: linux-kernel@...r.kernel.org, Lee Jones <lee@...nel.org>, 
 linux-hardening@...r.kernel.org, Kees Cook <keescook@...omium.org>, Finn
 Thain <fthain@...ux-m68k.org>
Subject: Re: [PATCH v2] checkpatch: add check for snprintf to scnprintf

On Wed, 2024-02-21 at 22:11 +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>
> ---
> Changes in v2:
> - Had a vim moment and deleted a character before sending the patch.
> - Replaced the character :)
> - Link to v1: https://lore.kernel.org/r/20240221-snprintf-checkpatch-v1-1-3ac5025b5961@google.com
> ---
> From a discussion here [1].
> 
> [1]: https://lore.kernel.org/all/0f9c95f9-2c14-eee6-7faf-635880edcea4@linux-m68k.org/

> diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
[]
> @@ -7012,6 +7012,12 @@ sub process {
>  			     "Prefer strscpy, strscpy_pad, or __nonstring over strncpy - see: https://github.com/KSPP/linux/issues/90\n" . $herecurr);
>  		}
>  
> +# snprintf uses that should likely be {v}scnprintf
> +		if ($line =~ /\bsnprintf\s*\(\s*/) {
> +				WARN("SNPRINTF",
> +				     "Prefer scnprintf over snprintf\n" . $herecurr);

There really should be some sort of reference link here
similar to the one above this.

Also, I rather doubt _all_ of these should be changed just
for churn's sake.

Maybe add a test for some return value use like

		if (defined($stat) &&
		    $stat =~ /$Lval\s*=\s*snprintf\s*\(/) {
			etc...

Maybe offer to --fix it too.