linux-hardening - Re: [PATCH v4] checkpatch: add check for snprintf to scnprintf

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Date: Thu, 11 Apr 2024 13:56:27 -0700
From: Joe Perches <joe@...ches.com>
To: Christophe JAILLET <christophe.jaillet@...adoo.fr>, 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 v4] checkpatch: add check for snprintf to scnprintf

On Thu, 2024-04-11 at 22:01 +0200, Christophe JAILLET wrote:
> Le 08/04/2024 à 22:53, 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 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..a0fd681ea837 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*\(\s*/) {
> 
> Hi,
> 
> for my understanding, what is the purpose of the 2nd "\s*"?
> IMHO, it could be just removed.

It could.

# {v}snprintf uses that should likely be {v}scnprintf
		if ($line =~ /\b((v?)snprintf)\s*\(/) {
			WARN("SNPRINTF",
			     "Prefer ${2}scnprintf over $1 - see: https://github.com/KSPP/linux/issues/105\n" . $herecurr);
		}



Though I also think it's better to use lore rather than github