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Date: Wed, 21 Feb 2024 14:33:14 -0800
From: Kees Cook <keescook@chromium.org>
To: Justin Stitt <justinstitt@google.com>
Cc: Andy Whitcroft <apw@canonical.com>, Joe Perches <joe@perches.com>,
	Dwaipayan Ray <dwaipayanray1@gmail.com>,
	Lukas Bulwahn <lukas.bulwahn@gmail.com>,
	linux-kernel@vger.kernel.org, Lee Jones <lee@kernel.org>,
	linux-hardening@vger.kernel.org, Finn Thain <fthain@linux-m68k.org>
Subject: Re: [PATCH v2] checkpatch: add check for snprintf to scnprintf
Message-ID: <202402211433.BCFF1FFD8@keescook>
References: <20240221-snprintf-checkpatch-v2-1-9baeb59dae30@google.com>
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In-Reply-To: <20240221-snprintf-checkpatch-v2-1-9baeb59dae30@google.com>

On Wed, Feb 21, 2024 at 10:11:59PM +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@linux-m68k.org>
> Signed-off-by: Justin Stitt <justinstitt@google.com>

Yes please! :)

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

-Kees

> ---
> 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/
> ---
>  scripts/checkpatch.pl | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
> index 9c4c4a61bc83..64025a6e6155 100755
> --- 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" . $herecurr);
> +		}
> +
>  # ethtool_sprintf uses that should likely be ethtool_puts
>  		if ($line =~ /\bethtool_sprintf\s*\(\s*$FuncArg\s*,\s*$FuncArg\s*\)/) {
>  			if (WARN("PREFER_ETHTOOL_PUTS",
> 
> ---
> base-commit: b401b621758e46812da61fa58a67c3fd8d91de0d
> change-id: 20240221-snprintf-checkpatch-a864ed67ebd0
> 
> Best regards,
> --
> Justin Stitt <justinstitt@google.com>
> 

-- 
Kees Cook