linux-kernel - Re: [PATCH v3] lib/string.c: implement stpcpy

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Message-ID: <202008271523.88796F201F@keescook>
Date:   Thu, 27 Aug 2020 15:26:11 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joe Perches <joe@...ches.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        stable <stable@...r.kernel.org>, Andy Lavr <andy.lavr@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        Yury Norov <yury.norov@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] lib/string.c: implement stpcpy

On Thu, Aug 27, 2020 at 11:05:42PM +0300, Andy Shevchenko wrote:
> In general it's better to have a robust API, but what may go wrong
> with the interface where we have no length of  the buffer passed, but
> we all know that it's PAGE_SIZE?
> So, what's wrong with doing something like
> strcpy(buf, "Yes, we know we won't overflow here\n");

(There's a whole thread[1] about this right now, actually.)

The problem isn't the uses where it's safe (obviously), it's about the
uses where it is NOT safe. (Or _looks_ safe but isn't.) In order to
eliminate bug classes, we need remove the APIs that are foot-guns. Even
if one developer never gets it wrong, others might.

[1] https://lore.kernel.org/lkml/c256eba42a564c01a8e470320475d46f@AcuMS.aculab.com/T/#mac95487d7ae427de03251b49b75dd4de40c2462d

-- 
Kees Cook