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Date:   Tue, 5 Feb 2019 14:29:53 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Borislav Petkov <bp@alien8.de>
Cc:     Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        x86@kernel.org, hpa@zytor.com,
        Thomas Gleixner <tglx@linutronix.de>,
        Nadav Amit <nadav.amit@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        linux_dti@icloud.com, linux-integrity@vger.kernel.org,
        linux-security-module@vger.kernel.org, akpm@linux-foundation.org,
        kernel-hardening@lists.openwall.com, linux-mm@kvack.org,
        will.deacon@arm.com, ard.biesheuvel@linaro.org,
        kristen@linux.intel.com, deneen.t.dock@intel.com,
        Nadav Amit <namit@vmware.com>,
        Kees Cook <keescook@chromium.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>
Subject: Re: [PATCH v2 06/20] x86/alternative: use temporary mm for text
 poking
Message-ID: <20190205132953.GF17582@hirez.programming.kicks-ass.net>
References: <20190129003422.9328-1-rick.p.edgecombe@intel.com>
 <20190129003422.9328-7-rick.p.edgecombe@intel.com>
 <20190205095853.GJ21801@zn.tnic>
 <20190205113146.GP17528@hirez.programming.kicks-ass.net>
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On Tue, Feb 05, 2019 at 12:31:46PM +0100, Peter Zijlstra wrote:
> In general, text_poke() cannot fail:
> 
>  - suppose changing a single jump label requires poking multiple sites
>    (not uncommon), we fail halfway through and then have to undo the
>    first pokes, but those pokes fail again.
> 
>  - this then leaves us no way forward and no way back, we've got
>    inconsistent text state -> FAIL.

Note that this exact fail scenario still exists in the CPU hotplug code.
See kernel/cpu.c:cpuhp_thread_fun():

		/*
		 * If we fail on a rollback, we're up a creek without no
		 * paddle, no way forward, no way back. We loose, thanks for
		 * playing.
		 */
		WARN_ON_ONCE(st->rollback);