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

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Date: Wed, 21 Feb 2024 14:13:08 -0800
From: Justin Stitt <justinstitt@...gle.com>
To: 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] checkpatch: add check for snprintf to scnprintf

On Wed, Feb 21, 2024 at 2:05 PM Justin Stitt <justinstitt@...gle.com> 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>
> ---
> 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..bb4e99c818a9 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 =~ /\snprintf\s*\(\s*/) {
> +                               WARN("SNPRINTF",
> +                                    "Prefer scnprintf over snprintf\n" . $herecurr);

Whoops, I dropped the \b with some poor vim skills.

v2 is up.

[v2]: https://lore.kernel.org/r/20240221-snprintf-checkpatch-v2-1-9baeb59dae30@google.com

> +               }
> +
>  # 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>
>