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

From: Konstantin Khlebnikov
Date: Wed Nov 19 2014 - 10:52:45 EST

On Wed, Nov 19, 2014 at 5:54 PM, Ard Biesheuvel
<ard.biesheuvel@xxxxxxxxxx> wrote:
> On 19 November 2014 14:40, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>> On Tuesday 18 November 2014 21:13:56 Konstantin Khlebnikov wrote:
>>> On 2014-11-18 20:34, Russell King - ARM Linux wrote:
>>> > On Tue, Nov 18, 2014 at 08:21:46PM +0400, Konstantin Khlebnikov wrote:
>>> >> Usually modules are loaded into small area prior to the kernel
>>> >> text because they are linked with the kernel using short calls.
>>> >> Compile-time instrumentation like GCOV or KASAN bloats code a lot,
>>> >> and as a result huge modules no longer fit into reserved area.
>>> >>
>>> >> This patch adds option CONFIG_MODULES_USE_VMALLOC which lifts
>>> >> limitation on amount of loaded modules. It links modules using
>>> >> long-calls (option -mlong-calls) and loads them into vmalloc area.
>>> >>
>>> >> In few places exported symbols are called from inline assembly.
>>> >> This patch adds macro for such call sites: __asmbl and __asmbl_clobber.
>>> >> Call turns into single 'bl' or sequence 'movw; movt; blx' depending on
>>> >> context and state of config option.
>>> >>
>>> >> Unfortunately this option isn't compatible with CONFIG_FUNCTION_TRACER.
>>> >> Compiler emits short calls to profiling function despite of -mlong-calls.
>>> >> This is a bug in GCC, but ftrace anyway needs an update to handle this.
>>> > It also isn't compatible with the older architectures which don't have
>>> > "blx".
>>> Ok, I'll add "depends on CPU_V6 || CPU_V7" I don't think that it is
>>> necessary for older cpus.
>> Why not just use a different branch instruction for the older CPUs?
> ARMv6 doesn't support movw/movt so this will only work on v7.
> What about doing 'mov lr, pc; ldr pc,=symbol' instead? You clearly
> don't care about performance in this case, so the performance hit (due
> to the dcache access and interfering with the return stack predictors)
> should be tolerable. The only thing to be careful about is thumb2
> kernels: you would need to set the thumb bit in lr manually but only
> if the call is made /from/ thumb. You would probably be better off
> just depending on !THUMB2_KERNEL.

Do you mean ldr pc, =symbol ?

In this case I get this error:

/tmp/ccAHtONU.s: Assembler messages:
/tmp/ccAHtONU.s:220: Error: invalid literal constant: pool needs to be closer

Probably constant pool doesn't work well in inline assembly.

Something like this seems work:

add lr, pc, #4
ldr pc, [pc, #-4]
.long symbol

> Ard.
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