linux-kernel - Re: [PATCHv3 2/2] arch: Rename CONFIG_DEBUG_RODATA and CONFIG_DEBUG_MODULE

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Date:   Mon, 20 Feb 2017 08:21:30 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Pavel Machek <pavel@....cz>, Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "James E.J. Bottomley" <jejb@...isc-linux.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        Rob Herring <robh@...nel.org>, Jessica Yu <jeyu@...hat.com>,
        Jonathan Corbet <corbet@....net>, Helge Deller <deller@....de>,
        "x86@...nel.org" <x86@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        Len Brown <len.brown@...el.com>,
        "linux-s390@...r.kernel.org" <linux-s390@...r.kernel.org>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        linux-parisc <linux-parisc@...r.kernel.org>,
        Linux PM list <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Jason Wessel <jason.wessel@...driver.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCHv3 2/2] arch: Rename CONFIG_DEBUG_RODATA and
 CONFIG_DEBUG_MODULE_RONX


* Kees Cook <keescook@...omium.org> wrote:

> On Thu, Feb 16, 2017 at 2:25 PM, Pavel Machek <pavel@....cz> wrote:
> > Hi!
> >
> >>
> >> -config DEBUG_RODATA
> >> +config STRICT_KERNEL_RWX
> >>       bool "Make kernel text and rodata read-only" if ARCH_OPTIONAL_KERNEL_RWX
> >>       depends on ARCH_HAS_STRICT_KERNEL_RWX
> >>       default !ARCH_OPTIONAL_KERNEL_RWX ||
> >
> > Debug features are expected to have runtime cost, so kconfig help is
> > silent about those. But there are runtime costs, right? It would be
> > nice to mention them in the help text...
> 
> It depends on the architecture. The prior help text for arm said:
> 
>          The tradeoff is that each region is padded to section-size (1MiB)
>          boundaries (because their permissions are different and splitting
>          the 1M pages into 4K ones causes TLB performance problems), which
>          can waste memory.
> 
> parisc (somewhat inaccurately) said:
> 
>          This option may have a slight performance impact because a
>          portion of the kernel code won't be covered by a TLB anymore.
> 
> IIUC, arm64 does what parisc is hinting at: mappings at the end are
> broken down to PAGE_SIZE. On x86, IIUC, there's actually no change to
> TLB performance due to how the mappings are already set up.

BTW., a good strategy with RAM sizes above say 4GB would be to just round up to 
the next large-TLB boundary (2MB) and waste 0-2MB of RAM - which is only 0.05% of 
4GB of RAM. On most workloads, especially with SSDs it's probably a positive RAM 
vs. performance trade-off.

Thanks,

	Ingo