linux-hardening - RE: [PATCH v3] drivers/edac/edac

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Date:   Tue, 10 Aug 2021 08:56:51 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Joe Perches' <joe@...ches.com>, Robert Richter <rric@...nel.org>,
        "Len Baker" <len.baker@....com>
CC:     Borislav Petkov <bp@...en8.de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        James Morse <james.morse@....com>,
        Kees Cook <keescook@...omium.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        "linux-edac@...r.kernel.org" <linux-edac@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v3] drivers/edac/edac_mc: Remove all strcpy() uses

From: Joe Perches
> Sent: 09 August 2021 18:19
> 
> On Mon, 2021-08-09 at 12:05 +0200, Robert Richter wrote:
> > On 08.08.21 13:26:17, Len Baker wrote:
> >
> > > > Perhaps this should use scnprintf rather than strscpy
> > > > Something like:
> > > > 			n += scnprintf(buf + n, len - n, "%s",
> > > > 				       p == e->label ? dim->label : OTHER_LABEL);
> > > >
> > > In the first version [1] the scnprintf was used but Robert Richter don't
> > > see any benefit compared with the current implementation.
> > >
> > > [1] https://lore.kernel.org/linux-hardening/20210725162954.9861-1-len.baker@gmx.com/
> >
> > Reason is that there is the assumption that p must always point at the
> > end of the string and its trailing zero byte. I am not opposed using
> > the string function's return code instead of strlen() to get the
> > length. But why using formated output if strscpy() can be used?
> 
> strscpy and scnprintf have different return values and it's simpler
> and much more common to use scnprintf for appended strings that are
> limited to a specific buffer length.

scnprintf() will be a lot slower, but has a much better return value
than most of the strxxxcpy() functions.

The only slight problem is that you can't differentiate overflow
from a max-length output.

Trouble is fixing that adds 'yet another set of functions'.
Clearly we need the yellow with purple stripe ones :-)
Probably:
	offset = xxx(buf, len, offset, ......)
where offset == len on truncation.

	David

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