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

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Date: Mon, 29 Apr 2024 20:21:49 -0700
From: Joe Perches <joe@...ches.com>
To: Kees Cook <keescook@...omium.org>, Justin Stitt <justinstitt@...gle.com>
Cc: 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, 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.

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.