linux-kernel - Re: [Cocci] [PATCH] usb: atm: don't use snprintf() for sysfs attrs

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Date:   Fri, 28 Aug 2020 01:22:23 -0700
From:   Joe Perches <joe@...ches.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Denis Efremov <efremov@...ux.com>,
        Julia Lawall <julia.lawall@...ia.fr>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-usb@...r.kernel.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org, cocci <cocci@...teme.lip6.fr>,
        Alex Dewar <alex.dewar90@...il.com>
Subject: Re: [Cocci] [PATCH] usb: atm: don't use snprintf() for sysfs attrs

On Fri, 2020-08-28 at 01:10 -0700, Joe Perches wrote:
> On Fri, 2020-08-28 at 00:58 -0700, Kees Cook wrote:
> > On Thu, Aug 27, 2020 at 09:12:06PM -0700, Joe Perches wrote:
> > > Perhaps something like the below with a sample conversion
> > > that uses single and multiple sysfs_emit uses.
> > 
> > On quick review, I like it. :)
> > 
> > > [...]
> > > +int sysfs_emit(char *buf, char *pos, const char *fmt, ...)
> > > +{
> > > +	int len;
> > > +	va_list args;
> > > +
> > > +	WARN(pos < buf, "pos < buf\n");
> > > +	WARN(pos - buf >= PAGE_SIZE, "pos >= PAGE_SIZE (%tu > %lu)\n",
> > > +	     pos - buf, PAGE_SIZE);
> > > +	if (pos < buf || pos - buf >= PAGE_SIZE)
> > > +		return 0;
> > 
> > This can be:
> > 
> > 	if (WARN(pos < buf, "pos < buf\n") ||
> > 	    WARN(pos - buf >= PAGE_SIZE, "pos >= PAGE_SIZE (%tu > %lu)\n",
> > 		 pos - buf, PAGE_SIZE))
> > 		return 0;
> 
> I have some vague recollection that WARN could be compiled
> away to nothing somehow.  True or false?
> 
> If false, sure, of course, it'd be faster too.

I can't find an instance where WARN doesn't return the
condition.

And likely even faster would be to just show "invalid pos"
instead of specific messages.

	if (WARN(pos < buf || (pos - buf) >= PAGE_SIZE,
		 "Invalid pos\n");
		return 0;

or maybe use WARN_ONCE or no WARN at all.