linux-kernel - Re: [PATCH] sysctl: add register

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Date:   Mon, 6 Nov 2017 18:12:03 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Dave Martin <Dave.Martin@....com>
Cc:     Catalin Marinas <Catalin.Marinas@....com>,
        Will Deacon <Will.Deacon@....com>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Alex Bennée <alex.bennee@...aro.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] sysctl: add register_sysctl() dummy helper

On Mon, Nov 6, 2017 at 5:50 PM, Dave Martin <Dave.Martin@....com> wrote:
> On Mon, Nov 06, 2017 at 01:36:40PM +0000, Arnd Bergmann wrote:
>> The register_sysctl() function has been around for five years with commit
>> fea478d4101a ("sysctl: Add register_sysctl for normal sysctl users")
>> but now that arm64 started using it, I ran into a compile error:
>>
>> arch/arm64/kernel/armv8_deprecated.c: In function 'register_insn_emulation_sysctl':
>> arch/arm64/kernel/armv8_deprecated.c:257:2: error: implicit declaration of function 'register_sysctl'
>
> Hmmm, looks like I missed this combination in testing.
>
> I wonder whether ARMV8_DEPRECATED without SYSCTL is really a good idea
> though: in that config, we build a lot of dead code and leak some
> memory today.  The default emulation is still potentially useful, but
> all the support for runtime twiddling of the emulation modes becomes
> useless.
>
> For parallel reasons, the SVE sysctl stuff is protected by #ifdef
> CONFIG_SYSCTL, which is why I didn't get a similar splat there.
>
>
> So, although this patch is superficially sensible, it may tend to hide
> bugs: code that calls register_sysctl() when CONFIG_SYSCTL=n is
> suspicious and probably needs review... no?

I think your analysis for this code is correct, we waste a lot of memory
if we do it like this. However in the general case where we register a
statically allocated 'struct ctl_table', it wouldn't be an issue, because
gcc could then eliminate all the dead code.

Adding a CONFIG_SYSCTL #ifdef or Kconfig dependency would
probably be reasonable for armv8_deprecated.c, for the rest of
the kernel, having that wrapper is probably better.

I don't really care how this gets fixed, as long as some solution gets
merged.

       Arnd