[PATCH 1/2] ARM: mmu64: allow to disable null pointer trap on zero page

[Ahmad Fatoum]

On 10/14/20 5:08 PM, Michael Tretter wrote:
> Barebox uses the zero page to trap NULL pointer dereferences. However,
> if the SDRAM starts at address 0x0, this makes the first page of the
> SDRAM inaccessible and makes it impossible to load images to offset 0x0
> in the SDRAM.
> 
> Trapping NULL pointer dereferences on such systems is still desirable.
> Therefore, add a function to disable the traps if accessing the zero
> page is necessary and to re-enable the traps after the access is done.

Can't we map the (phy_addr_t)0 at some higher virtual address and
change uimage to use phys_to_virt() ?

Something like 

static inline void *phys_to_virt(unsigned long phys)
{
	if (!IS_ENABLED(CONFIG_ARM_MACH_CUSTOM_MAPPING) || !arm_mach_phys_to_virt)
		return (void *)phys;
	return arm_mach_phys_to_virt(phys);
}


> 
> Signed-off-by: Michael Tretter <m.tretter at pengutronix.de>
> ---
>  arch/arm/cpu/Kconfig  |  1 +
>  arch/arm/cpu/mmu_64.c | 13 ++++++++++++-
>  include/zero_page.h   | 40 ++++++++++++++++++++++++++++++++++++++++
>  lib/Kconfig           |  3 +++
>  4 files changed, 56 insertions(+), 1 deletion(-)
>  create mode 100644 include/zero_page.h
> 
> diff --git a/arch/arm/cpu/Kconfig b/arch/arm/cpu/Kconfig
> index f9f52a625260..ca3bd98962e2 100644
> --- a/arch/arm/cpu/Kconfig
> +++ b/arch/arm/cpu/Kconfig
> @@ -89,6 +89,7 @@ config CPU_V8
>  	select ARM_EXCEPTIONS
>  	select GENERIC_FIND_NEXT_BIT
>  	select ARCH_HAS_STACK_DUMP
> +	select ARCH_HAS_ZERO_PAGE
>  
>  config CPU_XSC3
>          bool
> diff --git a/arch/arm/cpu/mmu_64.c b/arch/arm/cpu/mmu_64.c
> index 7e9ae84810f6..bd15807f9160 100644
> --- a/arch/arm/cpu/mmu_64.c
> +++ b/arch/arm/cpu/mmu_64.c
> @@ -10,6 +10,7 @@
>  #include <init.h>
>  #include <mmu.h>
>  #include <errno.h>
> +#include <zero_page.h>
>  #include <linux/sizes.h>
>  #include <asm/memory.h>
>  #include <asm/pgtable64.h>
> @@ -168,6 +169,16 @@ static void mmu_enable(void)
>  	set_cr(get_cr() | CR_M | CR_C | CR_I);
>  }
>  
> +void zero_page_disable(void)
> +{
> +	create_sections(0x0, 0x0, PAGE_SIZE, CACHED_MEM);
> +}
> +
> +void zero_page_enable(void)
> +{
> +	create_sections(0x0, 0x0, PAGE_SIZE, 0x0);
> +}
> +
>  /*
>   * Prepare MMU for usage enable it.
>   */
> @@ -194,7 +205,7 @@ void __mmu_init(bool mmu_on)
>  		create_sections(bank->start, bank->start, bank->size, CACHED_MEM);
>  
>  	/* Make zero page faulting to catch NULL pointer derefs */
> -	create_sections(0x0, 0x0, 0x1000, 0x0);
> +	zero_page_enable();
>  
>  	mmu_enable();
>  }
> diff --git a/include/zero_page.h b/include/zero_page.h
> new file mode 100644
> index 000000000000..d8dd07cfe959
> --- /dev/null
> +++ b/include/zero_page.h
> @@ -0,0 +1,40 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +#ifndef __ZERO_PAGE_H
> +#define __ZERO_PAGE_H
> +
> +#include <common.h>
> +
> +#if defined CONFIG_ARCH_HAS_ZERO_PAGE
> +
> +/*
> + * zero_page_enable - enable null pointer trap
> + */
> +void zero_page_enable(void);
> +
> +/*
> + * zero_page_disable - disable null pointer trap
> + *
> + * Disable the null pointer trap on the zero page if access to the zero page
> + * is actually required. Disable the trap with care and re-enable it
> + * immediately after the access to properly trap null pointers.
> + */
> +void zero_page_disable(void);
> +
> +#else
> +
> +static inline void zero_page_enable(void)
> +{
> +}
> +
> +static inline void zero_page_disable(void)
> +{
> +}
> +
> +#endif
> +
> +static inline bool zero_page_contains(unsigned long addr)
> +{
> +	return addr < PAGE_SIZE;
> +}
> +
> +#endif /* __ZERO_PAGE_H */
> diff --git a/lib/Kconfig b/lib/Kconfig
> index 887f50ff003f..e5831ecdb9a7 100644
> --- a/lib/Kconfig
> +++ b/lib/Kconfig
> @@ -182,6 +182,9 @@ config ARCH_HAS_STACK_DUMP
>  config ARCH_HAS_DATA_ABORT_MASK
>  	bool
>  
> +config ARCH_HAS_ZERO_PAGE
> +	bool
> +
>  config HAVE_EFFICIENT_UNALIGNED_ACCESS
>  	bool
>  
> 

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