new generic strscpy API? (was Re: [PATCH v2] tile: avoid errors from truncating long strings in mpipe gxio)

[Chris Metcalf]

On 8/7/2014 2:43 AM, Randy Dunlap wrote:
> On 08/06/14 11:16, Chris Metcalf wrote:
> > Using strncpy() will just silently truncate long strings; we should
> > instead return an appropriate error.  Using strlcpy() would suffer from
> > the same problem.  Instead, use strnlen()+memcpy(), and add an
> > error-checking step to make sure the lengths are reasonable.
> >
> > I called the convenience wrapper strscpy(), and a case could be made for
> > making it more generic (possibly with a better name), but that seems
> > outside the scope of this initial commit.
> Well, having looked at the function before I read this comment, my first
> thought was that it should be added to lib/string.c for general
> availability.

I'm happy to do that, but it probably shouldn't go through the linux-tile
tree in that case, since I'd be touching platform-independent code.
If someone wants to volunteer to push a new lib/strscpy.c change to Linus
(presumably including the arch/tile caller) I'm happy to redo this commit
in that form.

My guess is that we also haven't hit the mandatory minimum of
bike-shedding around function name and precise semantics yet, anyway :-)

I will hold off on pushing this change until a bit later in the merge
window to see if anyone wants to jump in.

> > diff --git a/arch/tile/gxio/mpipe.c b/arch/tile/gxio/mpipe.c
> > index 5301a9ffbae1..27a56be8d583 100644
> > --- a/arch/tile/gxio/mpipe.c
> > +++ b/arch/tile/gxio/mpipe.c
> > @@ -29,6 +29,25 @@
> >   /* HACK: Avoid pointless "shadow" warnings. */
> >   #define link link_shadow
> >   
> > +/*
> > + * Use this routine to avoid copying too-long strings.  Unlike strncpy
> > + * or strlcpy, we don't enable programmers who don't check return codes;
> > + * partially-copied strings can be problematic.  The routine returns
> > + * the total number of bytes copied (including the trailing NUL) or
> > + * zero if the buffer wasn't big enough.
> > + */
> > +static size_t strscpy(char *dest, const char *src, size_t size)
> > +{
> > +	size_t ret = strnlen(src, size) + 1;
> > +	if (ret > size) {
> > +		if (size)
> > +			dest[0] = '\0';
> > +		return 0;
> > +	}
> > +	memcpy(dest, src, ret);
> > +	return ret;
> > +}

--
Chris Metcalf, Tilera Corp.
http://www.tilera.com

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