linux-hardening - Re: [PATCH v5] checkpatch: add check for snprintf to scnprintf

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Message-ID: <c0aac58a-1164-49ff-ad6c-060cd883a133@wanadoo.fr>
Date: Tue, 23 Apr 2024 10:17:09 +0200
From: Christophe JAILLET <christophe.jaillet@...adoo.fr>
To: Justin Stitt <justinstitt@...gle.com>, Andy Whitcroft
 <apw@...onical.com>, Joe Perches <joe@...ches.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 v5] checkpatch: add check for snprintf to scnprintf

Le 22/04/2024 à 23:33, Justin Stitt a écrit :
> 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 v5:
> - use capture groups to let the user know which variation they used
> - Link to v4: https://lore.kernel.org/r/20240408-snprintf-checkpatch-v4-1-8697c96ac94b@google.com
> 
> Changes in v4:
> - also check for vsnprintf variant (thanks Bill)
> - Link to v3: https://lore.kernel.org/r/20240315-snprintf-checkpatch-v3-1-a451e7664306@google.com
> 
> Changes in v3:
> - fix indentation
> - add reference link (https://github.com/KSPP/linux/issues/105) (thanks Joe)
> - Link to v2: https://lore.kernel.org/r/20240221-snprintf-checkpatch-v2-1-9baeb59dae30@google.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/
> ---
>   scripts/checkpatch.pl | 6 ++++++
>   1 file changed, 6 insertions(+)
> 
> diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
> index 9c4c4a61bc83..b7404e53c554 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" . $herecurr);
>   		}
>   
> +# {v}snprintf uses that should likely be {v}scnprintf
> +		if ($line =~ /\b((v|)snprintf\s*\()/) {

Nit: I think that /\b((v|)snprintf)\s*\(/) would be nice.
Otherwise, <spaces>( would be added to the message.

CJ

> +			WARN("SNPRINTF",
> +			     "Prefer ${2}scnprintf over $1 - see: https://github.com/KSPP/linux/issues/105\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@...gle.com>
> 
> 
>