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Date:   Wed, 3 Oct 2018 20:58:06 +0200
From:   Eugene Syromiatnikov <esyr@redhat.com>
To:     Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc:     x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@amacapital.net>,
        Balbir Singh <bsingharora@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Florian Weimer <fweimer@redhat.com>,
        "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>
Subject: Re: [RFC PATCH v4 2/9] x86/cet/ibt: User-mode indirect branch
 tracking support
Message-ID: <20181003185802.GE32759@asgard.redhat.com>
References: <20180921150553.21016-1-yu-cheng.yu@intel.com>
 <20180921150553.21016-3-yu-cheng.yu@intel.com>
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On Fri, Sep 21, 2018 at 08:05:46AM -0700, Yu-cheng Yu wrote:

> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index bffa9ef47832..230f65ee881e 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c

> @@ -434,6 +435,23 @@ static __init int setup_disable_shstk(char *s)
>  __setup("no_cet_shstk", setup_disable_shstk);
>  #endif
>  
> +#ifdef CONFIG_X86_INTEL_BRANCH_TRACKING_USER
> +static __init int setup_disable_ibt(char *s)
> +{
> +	/* require an exact match without trailing characters */
> +	if (strlen(s))
> +		return 0;

"s[0] (!= '\0')" will do the same check in constant time.