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From: Arnd Bergmann <arnd@arndb.de>
To: Dave Martin <Dave.Martin@arm.com>
Cc: "Catalin Marinas" <Catalin.Marinas@arm.com>,
	"Will Deacon" <Will.Deacon@arm.com>,
	"Luis R. Rodriguez" <mcgrof@kernel.org>,
	"Kees Cook" <keescook@chromium.org>,
	"Alex Bennée" <alex.bennee@linaro.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH] sysctl: add register_sysctl() dummy helper
Date: Mon, 6 Nov 2017 18:12:03 +0100	[thread overview]
Message-ID: <CAK8P3a25CP-FgezD1S-8kNMCZF3CGraKAvDdb6KaV6HJXATZJw@mail.gmail.com> (raw)
In-Reply-To: <20171106165021.GV19485@e103592.cambridge.arm.com>

On Mon, Nov 6, 2017 at 5:50 PM, Dave Martin <Dave.Martin@arm.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

  reply	other threads:[~2017-11-06 17:12 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-06 13:36 Arnd Bergmann
2017-11-06 16:50 ` Dave Martin
2017-11-06 17:12   ` Arnd Bergmann [this message]
2017-11-06 17:41     ` Dave Martin
2017-11-06 20:36       ` Arnd Bergmann
2017-11-06 16:51 ` Kees Cook
2017-11-07  1:01 ` Will Deacon

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