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

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Date: Fri, 23 Feb 2024 10:38:45 +0000
From: Lee Jones <lee@...nel.org>
To: Joe Perches <joe@...ches.com>
Cc: Justin Stitt <justinstitt@...gle.com>,
	Andy Whitcroft <apw@...onical.com>,
	Dwaipayan Ray <dwaipayanray1@...il.com>,
	Lukas Bulwahn <lukas.bulwahn@...il.com>,
	linux-kernel@...r.kernel.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, 21 Feb 2024, Joe Perches wrote:

> 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.

This is for new implementations only.

Kees is planning on changing all of the current instances kernel-wide.

> 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.
> 

-- 
Lee Jones [李琼斯]