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[PATCH 2/2] ARM: mm: hide __start_rodata_section_aligned for non-debug builds

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  [PATCH 2/2] ARM: mm: hide __start_rodata_section_aligned for non-debug builds Arnd Bergmann <arnd@arndb.de> - 2016-02-16 17:10 +0100
    Re: [PATCH 2/2] ARM: mm: hide __start_rodata_section_aligned for  non-debug builds Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2016-02-16 17:20 +0100
      Re: [PATCH 2/2] ARM: mm: hide __start_rodata_section_aligned for non-debug builds Arnd Bergmann <arnd@arndb.de> - 2016-02-16 17:40 +0100
        Re: [PATCH 2/2] ARM: mm: hide __start_rodata_section_aligned for  non-debug builds Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2016-02-16 17:40 +0100

#1335584 — [PATCH 2/2] ARM: mm: hide __start_rodata_section_aligned for non-debug builds

The __start_rodata_section_aligned is only referenced by the
DEBUG_RODATA code, which is only used when the MMU is enabled,
but the definition fails on !MMU builds:

arch/arm/kernel/vmlinux.lds:702: undefined symbol `SECTION_SHIFT' referenced in expression

This hides the symbol whenever DEBUG_RODATA is disabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 64ac2e74f0b2 ("ARM: 8502/1: mm: mark section-aligned portion of rodata NX")
---
 arch/arm/kernel/vmlinux.lds.S | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
index 2d7085ae1c2f..bb12933aee22 100644
--- a/arch/arm/kernel/vmlinux.lds.S
+++ b/arch/arm/kernel/vmlinux.lds.S
@@ -322,12 +322,14 @@ SECTIONS
 	STABS_DEBUG
 }
 
+#ifdef CONFIG_DEBUG_RODATA
 /*
  * Without CONFIG_DEBUG_ALIGN_RODATA, __start_rodata_section_aligned will
  * be the first section-aligned location after __start_rodata. Otherwise,
  * it will be equal to __start_rodata.
  */
 __start_rodata_section_aligned = ALIGN(__start_rodata, 1 << SECTION_SHIFT);
+#endif
 
 /*
  * These must never be empty
-- 
2.7.0

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#1335599 — Re: [PATCH 2/2] ARM: mm: hide __start_rodata_section_aligned for non-debug builds

On 16 February 2016 at 17:03, Arnd Bergmann <arnd@arndb.de> wrote:
> The __start_rodata_section_aligned is only referenced by the
> DEBUG_RODATA code, which is only used when the MMU is enabled,
> but the definition fails on !MMU builds:
>
> arch/arm/kernel/vmlinux.lds:702: undefined symbol `SECTION_SHIFT' referenced in expression
>
> This hides the symbol whenever DEBUG_RODATA is disabled.
>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> Fixes: 64ac2e74f0b2 ("ARM: 8502/1: mm: mark section-aligned portion of rodata NX")
> ---
>  arch/arm/kernel/vmlinux.lds.S | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/arch/arm/kernel/vmlinux.lds.S b/arch/arm/kernel/vmlinux.lds.S
> index 2d7085ae1c2f..bb12933aee22 100644
> --- a/arch/arm/kernel/vmlinux.lds.S
> +++ b/arch/arm/kernel/vmlinux.lds.S
> @@ -322,12 +322,14 @@ SECTIONS
>         STABS_DEBUG
>  }
>
> +#ifdef CONFIG_DEBUG_RODATA
>  /*
>   * Without CONFIG_DEBUG_ALIGN_RODATA, __start_rodata_section_aligned will
>   * be the first section-aligned location after __start_rodata. Otherwise,
>   * it will be equal to __start_rodata.
>   */
>  __start_rodata_section_aligned = ALIGN(__start_rodata, 1 << SECTION_SHIFT);
> +#endif
>

Does

PROVIDE(__start_rodata_section_aligned = xxx);

do the trick as well? If it does, it's a bit cleaner.

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#1335618

On Tuesday 16 February 2016 17:12:52 Ard Biesheuvel wrote:
> > +#ifdef CONFIG_DEBUG_RODATA
> >  /*
> >   * Without CONFIG_DEBUG_ALIGN_RODATA, __start_rodata_section_aligned will
> >   * be the first section-aligned location after __start_rodata. Otherwise,
> >   * it will be equal to __start_rodata.
> >   */
> >  __start_rodata_section_aligned = ALIGN(__start_rodata, 1 << SECTION_SHIFT);
> > +#endif
> >
> 
> Does
> 
> PROVIDE(__start_rodata_section_aligned = xxx);
> 
> do the trick as well? If it does, it's a bit cleaner.
> 

I would assume not, as the problem is that SECTION_SHIFT is a macro that
is not defined when the MMU is disabled.

	Arnd

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#1335619 — Re: [PATCH 2/2] ARM: mm: hide __start_rodata_section_aligned for non-debug builds

On 16 February 2016 at 17:34, Arnd Bergmann <arnd@arndb.de> wrote:
> On Tuesday 16 February 2016 17:12:52 Ard Biesheuvel wrote:
>> > +#ifdef CONFIG_DEBUG_RODATA
>> >  /*
>> >   * Without CONFIG_DEBUG_ALIGN_RODATA, __start_rodata_section_aligned will
>> >   * be the first section-aligned location after __start_rodata. Otherwise,
>> >   * it will be equal to __start_rodata.
>> >   */
>> >  __start_rodata_section_aligned = ALIGN(__start_rodata, 1 << SECTION_SHIFT);
>> > +#endif
>> >
>>
>> Does
>>
>> PROVIDE(__start_rodata_section_aligned = xxx);
>>
>> do the trick as well? If it does, it's a bit cleaner.
>>
>
> I would assume not, as the problem is that SECTION_SHIFT is a macro that
> is not defined when the MMU is disabled.
>

I just tested it myself, and it in fact does work.

PROVIDE() means the expression is evaluated lazily, and SECTION_SHIFT
is just an undefined identifier in the linker script scope if the
preprocessor does not define it.

Anyway, I don't have a strong preference, so ack either way.

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