linux-kernel - Re: [PATCH 3/5] arm: decompressor: define a new zImage tag

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Date:   Mon, 01 Jun 2020 18:19:52 +0200
From:   Lukasz Stelmach <l.stelmach@...sung.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Enrico Weigelt <info@...ux.net>,
        Kees Cook <keescook@...omium.org>,
        Ingo Molnar <mingo@...nel.org>,
        Ben Dooks <ben-linux@...ff.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [PATCH 3/5] arm: decompressor: define a new zImage tag

It was <2020-06-01 pon 15:55>, when Russell King - ARM Linux admin wrote:
> On Mon, Jun 01, 2020 at 04:27:52PM +0200, Łukasz Stelmach wrote:
>> Add DCSZ tag which holds dynamic memory (stack, bss, malloc pool)
>> requirements of the decompressor code.
>
> Why do we need to know the stack and BSS size, when the userspace
> kexec tool doesn't need to know this to perform the same function.


kexec-tools load zImage as low in DRAM as possible and rely on two
assumptions:

    + the zImage will copy itself to make enough room for the kernel,
    + sizeof(zImage+mem) < sizeof(kernel+mem), which is true because
      of compression.

       DRAM start
       + 0x8000

zImage    |-----------|-----|-------|
            text+data   bss   stack 

                 text+data           bss   
kernel    |---------------------|-------------------|


initrd                                              |-initrd-|-dtb-|


My code on the other hand tries to load the zImage high enough to make
room for the kernel without copying the zImage.


            text+data           bss   
kernel    |-----------|-------------------|

zImage                |-----------|-----|-------|
                        text+data   bss   stack 

initrd                                          |-initrd-|-dtb-|


In such case the second assumption would be

    sizeof(zImage+mem) < sizeof(kernel bss)

and I can't tell for sure it would always be true. That is why we need
detailed information about decompressor memory requirements.


>> 
>> Signed-off-by: Łukasz Stelmach <l.stelmach@...sung.com>
>> ---
>>  arch/arm/boot/compressed/vmlinux.lds.S |  9 ++++++++-
>>  arch/arm/include/asm/image.h           | 13 +++++++++++++
>>  2 files changed, 21 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/arm/boot/compressed/vmlinux.lds.S b/arch/arm/boot/compressed/vmlinux.lds.S
>> index 308e9cd6a897..dcfdb3209c90 100644
>> --- a/arch/arm/boot/compressed/vmlinux.lds.S
>> +++ b/arch/arm/boot/compressed/vmlinux.lds.S
>> @@ -39,6 +39,11 @@ SECTIONS
>>      LONG(ARM_ZIMAGE_MAGIC3)
>>      LONG(ZIMAGE_MAGIC(__piggy_size_addr - _start))
>>      LONG(ZIMAGE_MAGIC(_kernel_bss_size))
>> +    LONG(ZIMAGE_MAGIC(5))
>> +    LONG(ARM_ZIMAGE_MAGIC4)
>> +    LONG(ZIMAGE_MAGIC(_end - __bss_start))
>> +    LONG(ZIMAGE_MAGIC(_stack_end - _stack_start))
>> +    LONG(ZIMAGE_MAGIC(_malloc_size))
>>      LONG(0)
>>      _table_end = .;
>>    }
>> @@ -108,10 +113,12 @@ SECTIONS
>>    . = BSS_START;
>>    __bss_start = .;
>>    .bss			: { *(.bss) }
>> +  . = ALIGN(8);		/* the stack must be 64-bit aligned and adjoin bss */
>>    _end = .;
>>  
>> -  . = ALIGN(8);		/* the stack must be 64-bit aligned */
>> +  _stack_start = .;
>>    .stack		: { *(.stack) }
>> +  _stack_end = .;
>>  
>>    PROVIDE(__pecoff_data_size = ALIGN(512) - ADDR(.data));
>>    PROVIDE(__pecoff_end = ALIGN(512));
>> diff --git a/arch/arm/include/asm/image.h b/arch/arm/include/asm/image.h
>> index d5c18a0f6a34..624438740f23 100644
>> --- a/arch/arm/include/asm/image.h
>> +++ b/arch/arm/include/asm/image.h
>> @@ -15,6 +15,7 @@
>>  #define ARM_ZIMAGE_MAGIC1 ZIMAGE_MAGIC(0x016f2818)
>>  #define ARM_ZIMAGE_MAGIC2 (0x45454545)
>>  #define ARM_ZIMAGE_MAGIC3 ZIMAGE_MAGIC(0x5a534c4b)
>> +#define ARM_ZIMAGE_MAGIC4 ZIMAGE_MAGIC(0x5a534344)
>>  
>>  #ifndef __ASSEMBLY__
>>  
>> @@ -43,6 +44,18 @@ struct arm_zimage_tag {
>>  	} u;
>>  };
>>  
>> +struct arm_zimage_tag_dc {
>> +	struct tag_header hdr;
>> +	union {
>> +#define ZIMAGE_TAG_DECOMP_SIZE ARM_ZIMAGE_MAGIC4
>> +		struct zimage_decomp_size {
>> +			__le32 bss_size;
>> +			__le32 stack_size;
>> +			__le32 malloc_size;
>> +		} decomp_size;
>> +	} u;
>> +};
>> +
>>  #endif /* __ASSEMBLY__ */
>>  
>>  #endif /* __ASM_IMAGE_H */
>> -- 
>> 2.26.2
>> 
>> 

-- 
Łukasz Stelmach
Samsung R&D Institute Poland
Samsung Electronics

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