Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20200815002417.1512973-1-ndesaulniers@google.com> <20200815013310.GA99152@rani.riverdale.lan>
In-Reply-To: <20200815013310.GA99152@rani.riverdale.lan>
From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 14 Aug 2020 19:00:59 -0700
Message-ID: <CAKwvOdkcq4TeFQ_Cw39DyA+N6rhyx2q9jpvN+Cv9n7x+MwijmA@mail.gmail.com>
Subject: Re: [PATCH] lib/string.c: implement stpcpy
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        =?UTF-8?B?RMOhdmlkIEJvbHZhbnNrw70=?= <david.bolvansky@gmail.com>,
        Eli Friedman <efriedma@quicinc.com>,
        "# 3.4.x" <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>,
        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>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Aug 14, 2020 at 6:33 PM Arvind Sankar <nivedita@alum.mit.edu> 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.

Yep, looks like I had a bug in my test program that masked this.
Thanks for triple checking.

>
> Nit: NUL is more correct than NULL to refer to the string terminator.

TIL.

-- 
Thanks,
~Nick Desaulniers