linux-hardening - Re: [PATCH v6] checkpatch: add check for snprintf to scnprintf

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Date: Thu, 2 May 2024 15:57:35 -0700
From: Kees Cook <keescook@...omium.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, Lee Jones <lee@...nel.org>,
	linux-hardening@...r.kernel.org, Finn Thain <fthain@...ux-m68k.org>
Subject: Re: [PATCH v6] checkpatch: add check for snprintf to scnprintf

On Mon, Apr 29, 2024 at 08:21:49PM -0700, Joe Perches wrote:
> On Mon, 2024-04-29 at 12:49 -0700, Kees Cook wrote:
> > On Mon, Apr 29, 2024 at 06:39:28PM +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>
> > 
> > Thanks!
> > 
> > Reviewed-by: Kees Cook <keescook@...omium.org>
> > 
> 
> $ git grep -P '\b((v|)snprintf)\s*\(' | wc -l
> 7745
> $ git grep -P '(?:return\s+|=\s*)\b((v|)snprintf)\s*\(' | wc -l
> 1626
> 
> Given there are ~5000 uses of these that don't care
> whether or not it's snprintf or scnprintf, I think this
> is not great.

But let's not add more of either case. :)

> I'd much rather make sure the return value of the call
> is used before suggesting an alternative.
> 
> $ git grep  -P '\b((v|)snprintf)\s*\(.*PAGE_SIZE' | wc -l
> 515
> 
> And about 1/3 of these snprintf calls are for sysfs style
> output that ideally would be converted to sysfs_emit or
> sysfs_emit_at instead.

Detecting that we're in the right place for sysfs_emit seems out of
scope for here, but maybe it should be more clearly called out by the
contents at the reported URL?

-- 
Kees Cook