linux-kernel - Re: [PATCH v2 05/17] add support for Clang's Shadow Call Stack (SCS)

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Message-ID: <CABCJKufR04dmzj3-7Uw0QkcHXvNd6h8XMPVV-hZ-AyOX-CJcjA@mail.gmail.com>
Date:   Fri, 25 Oct 2019 13:51:22 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 05/17] add support for Clang's Shadow Call Stack (SCS)

On Fri, Oct 25, 2019 at 9:22 AM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
> > +static void scs_free(void *s)
> > +{
> > +       int i;
> > +
> > +       for (i = 0; i < SCS_CACHE_SIZE; i++) {
> > +               if (this_cpu_cmpxchg(scs_cache[i], 0, s) != 0)
> > +                       continue;
> > +
> > +               return;
> > +       }
>
> prefer:
>
> for ...:
>   if foo() == 0:
>     return
>
> to:
>
> for ...:
>   if foo() != 0:
>     continue
>   return

This was essentially copied from free_thread_stack in kernel/fork.c,
but I agree, your way is cleaner. I'll change this in the next
version. Thanks!

Sami