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

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Message-ID: <20240223104121.GQ10170@google.com>
Date: Fri, 23 Feb 2024 10:41:21 +0000
From: Lee Jones <lee@...nel.org>
To: David Laight <David.Laight@...lab.com>
Cc: 'Justin Stitt' <justinstitt@...gle.com>,
	Andy Whitcroft <apw@...onical.com>, Joe Perches <joe@...ches.com>,
	Dwaipayan Ray <dwaipayanray1@...il.com>,
	Lukas Bulwahn <lukas.bulwahn@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-hardening@...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 Thu, 22 Feb 2024, David Laight wrote:

> From: Justin Stitt
> > Sent: 21 February 2024 22:12
> > 
> > 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."
> 
> While generally true, there are places that really do want to
> detect (and error) overflow.
> That isn't possible with scnprintf().
> 
> I'm not sure what the solution is though.
> Having a function that returns a negative value on overflow is also
> likely to get misused.
> seq_printf() (or whatever it is called) may let you check,
> but it is hardly a cheap wrapper and a bit of a PITA to use.

I agree.

spprinf() was my favorite solution, but it seems that the lib string
people don't like to accept new functionality, even if it's a clear
improvement over the currently available solutions.

[0] https://lore.kernel.org/all/20240130160953.766676-1-lee@kernel.org/

-- 
Lee Jones [李琼斯]