Re: [PATCH] arch/arm/Kconfig: enable ARM_MODULE_PLTS when LOCKDEP=y

[Anders Roxell]

On 31 January 2018 at 12:46, Russell King - ARM Linux
<linux@xxxxxxxxxxxxxxx> wrote:
> On Wed, Jan 31, 2018 at 12:25:33PM +0100, Arnd Bergmann wrote:
>> On Tue, Jan 30, 2018 at 12:57 AM, Russell King - ARM Linux
>> <linux@xxxxxxxxxxxxxxx> wrote:
>> > On Tue, Jan 30, 2018 at 12:49:00AM +0100, Anders Roxell wrote:
>> >> While testing multi_v7_defconfig with LOCKDEP enabled, the kernel
>> >> fails to load simple modules, as reported by kselftest:
>> >>
>> >> [   34.107620] test_printf: section 4 reloc 2 sym 'memset': relocation
>> >> 28 out of range (0xbf046044 -> 0xc109f720)
>> >> selftests: printf.sh [FAIL]
>> >>
>> >> The problem that is seen when LOCKDEP is enabled without
>> >> ARM_MODULE_PLTS, is that LOCKDEP eats so much memory that the top of the
>> >> kernel gets out of reach from the bottom of the module area.
>> >
>> > This really doesn't follow IMHO - enabling various features can cause
>> > this, and we're not going to end up stuffing the Kconfig full of these
>> > select statements each time we find a combination of Kconfig symbols
>> > that cause it.
>> >
>> > lockdep isn't that special - I can (and do) build kernels with lockdep
>> > enabled, with modules, and I do not run into this problem.  So it's
>> > not as simple as you make out in this commit description.
>> >
>> > It's likely that you have either a fairly full kernel configuration (it
>> > must be to place memset() more than 16MB) or you are not placing the
>> > kernel at 0xc0008000 due to memory reservations in the low memory.
>> >
>> >> Suggested-by: Arnd Bergmann <arnd@xxxxxxxx>
>> >
>> > I guess this was discussed privately with Arnd, since there's no record
>> > of the discussion on the lists - which is even more reason why the
>> > commit message needs to describe better why you need this change.
>>
>> I don't remember the original discussion exactly, but it's clear that
>> the addition of LOCKDEP to the build is what caused the size to
>> grow dramatically and prevent module loading.
>
> As I say above, it does not follow "lockdep is enabled" therefore "we
> need module plts" so I don't accept this argument.
>
> Yes, if you have a lot of features built in, then its entirely possible
> that enabling lockdep will cause the kernel to overflow the non-plt
> boundary.  It will also be true that enabling a few more other features
> will also cause the kernel to overflow the non-plt boundary as well.
> Should we add those other features to a Kconfig expression too?  At
> what point do we stop adding these?
>
>> There are two alternative ways to do this without forcing
>> ARM_MODULE_PLTS on all the time (which also triggered
>> the 0day bot warning):
>
> Yes, 0day is pointing out yet again what a silly idea it is to select
> symbols that (a) have dependencies and (b) are user visible, something
> that I've long disagreed about.
>
>> a) we could make ARM_MODULE_PLTS default to 'y' when
>> LOCKDEP is anbled, making it a more reasonable default while
>> also letting users turn it off when the lockdep-enabled kernel
>> is still small enough
>
> As I've said, I don't believe "LOCKDEP" therefore need "MODULE_PLTS"
> follows.  It's just a symptom of an already large kernel.  I suspect
> without lockdep, Ander's kernel is already approaching the
> problematical 16MB mark.
>
> Another option that causes the kernel to grow by a few megabytes is
> the kernel protection options (ronx etc).  I suspect if Anders built
> a kernel that had lockdep enabled and ronx etc disabled, that there
> would be no need for module plts.  Should we turn on module plts if
> lockdep or ronx is enabled?
>
> I don't think there's a _sane_ solution to this other than defaulting
> ARM_MODULE_PLTS to 'y' without any of these dependencies, and if people
> want to turn it off, they still can if they're sure they won't run into
> this situation.

I'll send out a v2 with this change shortly.

Thank you all for your comments.

Cheers,
Anders