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

From: Arvind Sankar <nivedita@alum.mit.edu>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	"Dávid Bolvanský" <david.bolvansky@gmail.com>,
	"Eli Friedman" <efriedma@quicinc.com>,
	stable@vger.kernel.org, "Sami Tolvanen" <samitolvanen@google.com>,
	"Joe Perches" <joe@perches.com>,
	"Tony Luck" <tony.luck@intel.com>,
	"Yury Norov" <yury.norov@gmail.com>,
	"Daniel Axtens" <dja@axtens.net>,
	"Arvind Sankar" <nivedita@alum.mit.edu>,
	"Dan Williams" <dan.j.williams@intel.com>,
	"Joel Fernandes (Google)" <joel@joelfernandes.org>,
	"Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
	"Kees Cook" <keescook@chromium.org>,
	"Alexandru Ardelean" <alexandru.ardelean@analog.com>,
	linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com
Subject: Re: [PATCH] lib/string.c: implement stpcpy
Date: Fri, 14 Aug 2020 21:33:10 -0400	[thread overview]
Message-ID: <20200815013310.GA99152@rani.riverdale.lan> (raw)
In-Reply-To: <20200815002417.1512973-1-ndesaulniers@google.com>

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.