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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 24 Jul 2019 10:08:57 -0700
Message-ID: <CAHk-=whPA-Vv-OHbUe4M5=ygTknQNOasnLAp-E3zSAaq=pue+g@mail.gmail.com>
Subject: Re: [PATCH 1/2] string: Add stracpy and stracpy_pad mechanisms
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     Yann Droneaud <ydroneaud@opteya.com>,
        David Laight <David.Laight@aculab.com>,
        Joe Perches <joe@perches.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Jonathan Corbet <corbet@lwn.net>, Stephen Kitt <steve@sk2.org>,
        Kees Cook <keescook@chromium.org>,
        Nitin Gote <nitin.r.gote@intel.com>,
        "jannh@google.com" <jannh@google.com>,
        "kernel-hardening@lists.openwall.com" 
        <kernel-hardening@lists.openwall.com>,
        Andrew Morton <akpm@linux-foundation.org>
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On Wed, Jul 24, 2019 at 6:09 AM Rasmus Villemoes
<linux@rasmusvillemoes.dk> wrote:
>
> The kernel's snprintf() does not behave in a non-standard way, at least
> not with respect to its return value.

Note that the kernels snprintf() *does* very much protect against the
overflow case - not by changing the return value, but simply by having

        /* Reject out-of-range values early.  Large positive sizes are
           used for unknown buffer sizes. */
        if (WARN_ON_ONCE(size > INT_MAX))
                return 0;

at the very top.

So you can't actually overflow in the kernel by using the repeated

        offset += vsnprintf( .. size - offset ..);

model.

Yes, it's the wrong thing to do, but it is still _safe_.

              Linus