Re: [PATCH v3 01/17] ARM: add mechanism for late code patching

[Cyril Chemparathy]

On 9/21/2012 2:09 PM, Nicolas Pitre wrote:
> On Tue, 11 Sep 2012, Cyril Chemparathy wrote:
>
> > The original phys_to_virt/virt_to_phys patching implementation relied on early
> > patching prior to MMU initialization.  On PAE systems running out of >4G
> > address space, this would have entailed an additional round of patching after
> > switching over to the high address space.
> >
> > The approach implemented here conceptually extends the original PHYS_OFFSET
> > patching implementation with the introduction of "early" patch stubs.  Early
> > patch code is required to be functional out of the box, even before the patch
> > is applied.  This is implemented by inserting functional (but inefficient)
> > load code into the .runtime.patch.code init section.  Having functional code
> > out of the box then allows us to defer the init time patch application until
> > later in the init sequence.
> >
> > In addition to fitting better with our need for physical address-space
> > switch-over, this implementation should be somewhat more extensible by virtue
> > of its more readable (and hackable) C implementation.  This should prove
> > useful for other similar init time specialization needs, especially in light
> > of our multi-platform kernel initiative.
> >
> > This code has been boot tested in both ARM and Thumb-2 modes on an ARMv7
> > (Cortex-A8) device.
> >
> > Note: the obtuse use of stringified symbols in patch_stub() and
> > early_patch_stub() is intentional.  Theoretically this should have been
> > accomplished with formal operands passed into the asm block, but this requires
> > the use of the 'c' modifier for instantiating the long (e.g. .long %c0).
> > However, the 'c' modifier has been found to ICE certain versions of GCC, and
> > therefore we resort to stringified symbols here.
> >
> > Signed-off-by: Cyril Chemparathy <cyril@xxxxxx>
> > Reviewed-by: Nicolas Pitre <nico@xxxxxxxxxx>
>
> I know I provided review before, but here's another nit I'd like fixed:

Fixed for v4.  Thanks.

> > --- /dev/null
> > +++ b/arch/arm/include/asm/runtime-patch.h
> > @@ -0,0 +1,208 @@
> > +/*
> > + * arch/arm/include/asm/runtime-patch.h
> > + * Note: this file should not be included by non-asm/.h files
> > + *
> > + * Copyright 2012 Texas Instruments, Inc.
> > + *
> > + * This program is free software; you can redistribute it and/or modify it
> > + * under the terms and conditions of the GNU General Public License,
> > + * version 2, as published by the Free Software Foundation.
> > + *
> > + * This program is distributed in the hope it will be useful, but WITHOUT
> > + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> > + * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
> > + * more details.
> > + *
> > + * You should have received a copy of the GNU General Public License along with
> > + * this program.  If not, see <http://www.gnu.org/licenses/>.
> > + */
> > +#ifndef __ASM_ARM_RUNTIME_PATCH_H
> > +#define __ASM_ARM_RUNTIME_PATCH_H
> > +
> > +#include <linux/stringify.h>
> > +
> > +#ifndef __ASSEMBLY__
> > +
> > +#ifdef CONFIG_ARM_RUNTIME_PATCH
>
> [...]
>
> > +#else
> > +
> > +static inline int runtime_patch(const void *table, unsigned size)
> > +{
> > +	return 0;
> > +}
>
> This is wrong.  If runtime_patch() is ever called when
> CONFIG_ARM_RUNTIME_PATCH is not set, then i'd better return an error and
> not pretend it performed the requested action.  Returning -ENOSYS would
> be appropriate.
>
> This is especially important in the following context:
>
> > --- a/arch/arm/kernel/module.c
> > +++ b/arch/arm/kernel/module.c
> > @@ -321,6 +322,12 @@ int module_finalize(const Elf32_Ehdr *hdr, const Elf_Shdr *sechdrs,
> >   	if (s)
> >   		fixup_pv_table((void *)s->sh_addr, s->sh_size);
> >   #endif
> > +	s = find_mod_section(hdr, sechdrs, ".runtime.patch.table");
> > +	if (s) {
> > +		err = runtime_patch((void *)s->sh_addr, s->sh_size);
> > +		if (err)
> > +			return err;
> > +	}
>
> Despite the vermagic check, If ever a .runtime.patch.table section is
> found in a module to be loaded in a kernel with no support for it then
> it is best to return an error than see the kernel crashing later on when
> the fallback stubs have been discarded.
>
>
> Nicolas

--
Thanks
- Cyril
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