linux-kernel - Re: [PATCH] lib/string.c: implement stpcpy

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Message-ID: <20200815014006.GB99152@rani.riverdale.lan>
Date:   Fri, 14 Aug 2020 21:40:06 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dávid Bolvanský <david.bolvansky@...il.com>,
        Eli Friedman <efriedma@...cinc.com>, stable@...r.kernel.org,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Joe Perches <joe@...ches.com>, Tony Luck <tony.luck@...el.com>,
        Yury Norov <yury.norov@...il.com>,
        Daniel Axtens <dja@...ens.net>,
        Dan Williams <dan.j.williams@...el.com>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Kees Cook <keescook@...omium.org>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH] lib/string.c: implement stpcpy

On Fri, Aug 14, 2020 at 09:33:10PM -0400, Arvind Sankar wrote:
> On Fri, Aug 14, 2020 at 05:24:15PM -0700, Nick Desaulniers wrote:
> > +#ifndef __HAVE_ARCH_STPCPY
> > +/**
> > + * stpcpy - copy a string from src to dest returning a pointer to the new end
> > + *          of dest, including src's NULL terminator. May overrun dest.
> > + * @dest: pointer to end of string being copied into. Must be large enough
> > + *        to receive copy.
> > + * @src: pointer to the beginning of string being copied from. Must not overlap
> > + *       dest.
> > + *
> > + * stpcpy differs from strcpy in two key ways:
> > + * 1. inputs must not overlap.
> > + * 2. return value is the new NULL terminated character. (for strcpy, the
> > + *    return value is a pointer to src.
> > + */
> > +#undef stpcpy
> > +char *stpcpy(char *__restrict__ dest, const char *__restrict__ src)
> > +{
> > +	while ((*dest++ = *src++) != '\0')
> > +		/* nothing */;
> > +	return dest;
> > +}
> > +#endif
> > +
> 
> Won't this return a pointer that's one _past_ the terminating NUL? I
> think you need the increments to be inside the loop body, rather than as
> part of the condition.
> 
> Nit: NUL is more correct than NULL to refer to the string terminator.

Also, strcpy (at least the one in the C standard) is undefined if the
strings overlap, so that's not really a difference.