linux-kernel - Re: arm: module: Add apply_relocate

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Date:   Wed, 18 Jan 2017 11:37:49 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     Jessica Yu <jeyu@...hat.com>
cc:     Abel Vesa <abelvesa@...ux.com>, linux@...linux.org.uk,
        jpoimboe@...hat.com, jikos@...nel.org, pmladek@...e.com,
        rostedt@...dmis.org, mingo@...hat.com, gregkh@...uxfoundation.org,
        geert+renesas@...der.be, davem@...emloft.net,
        akpm@...ux-foundation.org, emil.l.velikov@...il.com,
        mchehab@...nel.org, linux@...ck-us.net, ard.biesheuvel@...aro.org,
        jens.wiklander@...aro.org, jean-philippe.brucker@....com,
        viro@...iv.linux.org.uk, stefano.stabellini@...citrix.com,
        chris.brandt@...esas.com, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, live-patching@...r.kernel.org
Subject: Re: arm: module: Add apply_relocate_add

On Mon, 16 Jan 2017, Jessica Yu wrote:

> +++ Abel Vesa [06/12/16 17:06 +0000]:
> > It was only added to fix compiler error. It is not implemented
> > yet.
> > 
> > Signed-off-by: Abel Vesa <abelvesa@...ux.com>
> > ---
> > arch/arm/kernel/module.c | 9 +++++++++
> > 1 file changed, 9 insertions(+)
> > 
> > diff --git a/arch/arm/kernel/module.c b/arch/arm/kernel/module.c
> > index 4f14b5c..bf94922 100644
> > --- a/arch/arm/kernel/module.c
> > +++ b/arch/arm/kernel/module.c
> > @@ -52,6 +52,15 @@ void *module_alloc(unsigned long size)
> > #endif
> > 
> > int
> > +apply_relocate_add(Elf32_Shdr *sechdrs, const char *strtab,
> > +		   unsigned int symindex, unsigned int relindex,
> > +		   struct module *module)
> > +{
> > +	/* Not implemented yet */
> > +	return 0;
> > +}
> 
> Are SHT_RELA relocation sections actually supported on 32-bit arm? It looks
> like there's only support for SHT_REL.
> 
> arch/arm/Kconfig:84:	select MODULES_USE_ELF_REL
> 
> If we support SHT_REL sections, the correct fix is to probably have
> klp_write_object_relocations() check if the relocation section is
> SHT_REL or SHT_RELA, and call the appropriate function (apply_relocate
> or apply_relocate_add), similar to how the module loader does it.

Agreed. According to arch/Kconfig and definitions you can use only one of 
MODULES_USE_ELF_REL and MODULES_USE_ELF_RELA. arm arch uses the former. So 
yes, I think we should use the same if/else construct from the module 
loader in klp_write_object_relocations()... and it should go with this 
patch set, because arm is the first arch to actually use SHT_REL and not 
SHT_RELA (iiuc). x86, s390 and powerpc (archs we support) use SHT_RELA.

Miroslav