linux-kernel - Re: [PATCH RFC] ARM: option for loading modules into vmalloc area

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Date:	Wed, 19 Nov 2014 18:59:12 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Konstantin Khlebnikov <koct9i@...il.com>,
	Arnd Bergmann <arnd@...db.de>,
	Andrey Ryabinin <a.ryabinin@...sung.com>,
	Konstantin Khlebnikov <k.khlebnikov@...sung.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH RFC] ARM: option for loading modules into vmalloc area

On 19 November 2014 18:12, Nicolas Pitre <nicolas.pitre@...aro.org> wrote:
> On Wed, 19 Nov 2014, Russell King - ARM Linux wrote:
>
>> On Wed, Nov 19, 2014 at 11:57:15AM -0500, Nicolas Pitre wrote:
>> > On Wed, 19 Nov 2014, Russell King - ARM Linux wrote:
>> > > I don't think I ever did, because its pretty much impossible to do as I
>> > > explained in a follow up to this thread.
>> > >
>> > > We _used_ to do this with the userspace insmod methods, but since we got
>> > > this kernel-side linker, it's been pretty much impossible to do without
>> > > rewriting the module code.  That's not going to happen on account of one
>> > > quirky architecture which Linus doesn't particularly like.
>> >
>> > Still... We could try adding a hook in the generic module linker code
>> > for a pre-relocation pass.  Maybe only ARM would use it, but if the need
>> > to load big modules is real then I imagine Linus could be amenable to a
>> > compromise.
>>
>> So, how big a table would you allocate for the trampolines, based upon
>> not knowing anything about the module being loaded?  4K?  8K?  64K?
>
> The idea of a pre-relocation pass is to determine that.  That could be
> something similar to calling apply_relocate() twice: once to determine
> the number of trampoline entries, and a second time to perform the
> actual relocation.
>

Well, the veneers shouldn't take more than 3 words each, right?

ldr ip, [pc]
bx ip
.long symbol

and you would need at most one veneer per unique external symbol
referenced by one or more R_ARM_CALL relocations. Is there no way to
just add that to the static mem footprint as padding, and let the
loader populate it as needed at module relocation time?

-- 
Ard.
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