linux-kernel - Re: [PATCH v5 1/7] mm: Restore init_on

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Message-Id: <20210310155602.e005171dbecbc0be442f8aad@linux-foundation.org>
Date:   Wed, 10 Mar 2021 15:56:02 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>, stable@...r.kernel.org,
        Elena Reshetova <elena.reshetova@...el.com>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Potapenko <glider@...gle.com>,
        Alexander Popov <alex.popov@...ux.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Jann Horn <jannh@...gle.com>,
        kernel-hardening@...ts.openwall.com,
        linux-hardening@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Jonathan Corbet <corbet@....net>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH v5 1/7] mm: Restore init_on_* static branch defaults

On Tue,  9 Mar 2021 13:42:55 -0800 Kees Cook <keescook@...omium.org> wrote:

> Choosing the initial state of static branches changes the assembly layout
> (if the condition is expected to be likely, inline, or unlikely, out of
> line via a jump). The _TRUE/_FALSE defines for CONFIG_INIT_ON_*_DEFAULT_ON
> were accidentally removed. These need to stay so that the CONFIG controls
> the pessimization of the resulting static branch NOP/JMP locations.

Changelog doesn't really explain why anyone would want to apply this
patch.  This is especially important for -stable patches.

IOW, what is the user visible effect of the bug?