Message-ID: <b52d3d09-c52a-f771-fe40-4628eefae66a@ghiti.fr>
Date:   Sun, 29 Mar 2020 12:15:08 -0400
From:   Alex Ghiti <alex@...ti.fr>
To:     Zong Li <zong.li@...ive.com>
Cc:     Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Anup Patel <anup@...infault.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND v2] riscv: Introduce CONFIG_RELOCATABLE

Hi Zong,

On 3/24/20 3:22 AM, Zong Li wrote:
> On Sun, Mar 22, 2020 at 3:10 PM Alex Ghiti <alex@...ti.fr> wrote:
>>
>> Hi Zong,
>>
>> Sorry for the response delay, please find below my comments.
>>
>> On 3/12/20 1:57 AM, Zong Li wrote:
>>> On Sat, Mar 7, 2020 at 1:58 AM Palmer Dabbelt <palmer@...belt.com> wrote:
>>>>
>>>> On Mon, 02 Mar 2020 21:44:37 PST (-0800), alex@...ti.fr wrote:
>>>>> This config allows to compile the kernel as PIE and to relocate it at any
>>>>> virtual address at runtime: this paves the way to KASLR and to 4-level
>>>>> page table folding at runtime. Runtime relocation is possible since
>>>>> relocation metadata are embedded into the kernel.
>>>>>
>>>>> Note that relocating at runtime introduces an overhead even if the kernel
>>>>> is loaded at the same address it was linked at and that the compiler
>>>>> options are those used in arm64 which uses the same RELA relocation format.
>>>>>
>>>>> Signed-off-by: Alexandre Ghiti <alex@...ti.fr>
>>>>> Reviewed-by: Zong Li <zong.li@...ive.com>
>>>>> Reviewed-by: Anup Patel <anup@...infault.org>
>>>>> Tested-by: Zong Li <zong.li@...ive.com>
>>>>> ---
>>>>> Changes in v2:
>>>>> - Make RELOCATABLE depend on MMU as suggested by Anup
>>>>> - Rename kernel_load_addr into kernel_virt_addr as suggested by Anup
>>>>> - Use __pa_symbol instead of __pa, as suggested by Zong
>>>>> - Rebased on top of v5.6-rc3
>>>>> - Tested with sv48 patchset
>>>>> - Add Reviewed/Tested-by from Zong and Anup
>>>>>
>>>>>    arch/riscv/Kconfig              | 12 +++++
>>>>>    arch/riscv/Makefile             |  5 +-
>>>>>    arch/riscv/boot/loader.lds.S    |  2 +-
>>>>>    arch/riscv/include/asm/page.h   |  5 +-
>>>>>    arch/riscv/kernel/head.S        |  3 +-
>>>>>    arch/riscv/kernel/vmlinux.lds.S | 10 ++--
>>>>>    arch/riscv/mm/Makefile          |  4 ++
>>>>>    arch/riscv/mm/init.c            | 92 ++++++++++++++++++++++++++++-----
>>>>>    8 files changed, 111 insertions(+), 22 deletions(-)
>>>>>
>>>>> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>>>>> index 73f029eae0cc..f5f3d474504d 100644
>>>>> --- a/arch/riscv/Kconfig
>>>>> +++ b/arch/riscv/Kconfig
>>>>> @@ -163,6 +163,18 @@ config PGTABLE_LEVELS
>>>>>         default 3 if 64BIT
>>>>>         default 2
>>>>>
>>>>> +config RELOCATABLE
>>>>> +     bool
>>>>> +     depends on MMU
>>>>> +     help
>>>>> +          This builds a kernel as a Position Independent Executable (PIE),
>>>>> +          which retains all relocation metadata required to relocate the
>>>>> +          kernel binary at runtime to a different virtual address than the
>>>>> +          address it was linked at.
>>>>> +          Since RISCV uses the RELA relocation format, this requires a
>>>>> +          relocation pass at runtime even if the kernel is loaded at the
>>>>> +          same address it was linked at.
>>>>> +
>>>>>    source "arch/riscv/Kconfig.socs"
>>>>>
>>>>>    menu "Platform type"
>>>>> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
>>>>> index b9009a2fbaf5..5a115cf6a9c1 100644
>>>>> --- a/arch/riscv/Makefile
>>>>> +++ b/arch/riscv/Makefile
>>>>> @@ -9,7 +9,10 @@
>>>>>    #
>>>>>
>>>>>    OBJCOPYFLAGS    := -O binary
>>>>> -LDFLAGS_vmlinux :=
>>>>> +ifeq ($(CONFIG_RELOCATABLE),y)
>>>>> +LDFLAGS_vmlinux := -shared -Bsymbolic -z notext -z norelro
>>>>> +KBUILD_CFLAGS += -fPIE
>>>>> +endif
>>>>>    ifeq ($(CONFIG_DYNAMIC_FTRACE),y)
>>>>>         LDFLAGS_vmlinux := --no-relax
>>>>>    endif
>>>>> diff --git a/arch/riscv/boot/loader.lds.S b/arch/riscv/boot/loader.lds.S
>>>>> index 47a5003c2e28..a9ed218171aa 100644
>>>>> --- a/arch/riscv/boot/loader.lds.S
>>>>> +++ b/arch/riscv/boot/loader.lds.S
>>>>> @@ -7,7 +7,7 @@ ENTRY(_start)
>>>>>
>>>>>    SECTIONS
>>>>>    {
>>>>> -     . = PAGE_OFFSET;
>>>>> +     . = CONFIG_PAGE_OFFSET;
>>>>>
>>>>>         .payload : {
>>>>>                 *(.payload)
>>>>> diff --git a/arch/riscv/include/asm/page.h b/arch/riscv/include/asm/page.h
>>>>> index 8ca1930caa44..af5810f9aebd 100644
>>>>> --- a/arch/riscv/include/asm/page.h
>>>>> +++ b/arch/riscv/include/asm/page.h
>>>>> @@ -31,9 +31,9 @@
>>>>>     * When not using MMU this corresponds to the first free page in
>>>>>     * physical memory (aligned on a page boundary).
>>>>>     */
>>>>> -#define PAGE_OFFSET          _AC(CONFIG_PAGE_OFFSET, UL)
>>>>> +#define PAGE_OFFSET          kernel_virt_addr
>>>>
>>>> I assume we want to keep PAGE_OFFSET a constant for the non-relocatable
>>>> systems.  As it currently stands this is imposing a performance hit even when
>>>>
>>>
>>> I had almost done the KASLR implementation on top of this patch.
>>> Actually, PAGE_OFFSET change is unnecessary in KASLR , because we
>>> would move kernel image to a random physical address as well, so the
>>> $pc will go to the relevant random virtual address. We need
>>
>> I don't understand what you mean here, can you explain it a bit more ?
>>
>>> kernel_virt_addr to record the new destination, but keep PAGE_OFFSET
>>> to be CONFIG_PAGE_OFFSET is enough.
>>
>>   From my understanding, PAGE_OFFSET should represent the start of the
>> direct mapping, so in case of a relocatable kernel, its value should
>> reflect the offset too.
>>
> 
> Let me try to use the common RISC-V case to explain that.
> 
> In this v2 version, we could use a random virtual address to map the
> kernel image which is located in 0x8020_0000 physical address, then we
> could achieve the purpose of randomization, but it only happens on
> virtual address, the kernel image is still in 0x8020_0000 physical
> address, so we would also try to move the physical location of kernel
> image by a random offset.
> 
> For example, assume there is a 1G size DDR, the kernel image size is
> 10MB and we get a random kernel offset 0x100_0000, finally, kernel
> image would locate in 0x8120_0000, and run on corresponding virtual
> address 0xffff_ffe0_0100_0000 in CPU view. The direct mapping range is
> still got from 0x8020_0000 to 0xc000_0000, that means, there are
> available free pages from 0x8020_0000 to 0x811f_ffff, and a reserved
> region for kernel image from 0x8120_0000 to 0x81c0_0000, the rest of
> the memory are free pages as well (in fact, there are other reserved
> regions such as dtb, initrd and so on). Hence, we don't change
> PAGE_OFFSET, the PAGE_OFFSET still represent the start of the direct
> mapping as you mentioned, but the location of kernel image which in
> direct mapping would be unpredictable.
> 
>> Is there any issue with having PAGE_OFFSET equal to kernel_virt_addr ?
> 
> If we make PAGE_OFFSET be the new random kernel start address, the
> system would lose the part of memory after we also moving kernel image
> to a new physical location, that means, the memory which prior of new
> random kernel start address is disappeared, because some reasons like
> it only creates page table start from new physical address, the
> memblock only sees the memory from __pa(PAGE_OFFSET), and etc.
> 
>>
>> If we use both, we will need to know precisely when we should use
>> kernel_virt_addr or PAGE_OFFSET, which I think will be painful and error
>> prone.
> 
> There is a use case, we could get kernel offset by the expression
> 'kernel_virt_addr - PAGE_OFFSET'.
> 
> It seems to me that I could post the RFC patches first, but it depends
> on other patch series, include your relocation implementation.
> 

You are totally right, thanks for your explanation. I'll fix that in v3 
then.

I'll take a look at your KASLR implementation soon,

Thanks,

Alex

>>
>>>
>>>>> -#define KERN_VIRT_SIZE (-PAGE_OFFSET)
>>>>> +#define KERN_VIRT_SIZE               (-_AC(CONFIG_PAGE_OFFSET, UL))
>>>>
>>>> This seems like it would cause issues if the kernel is relocated to high enough
>>>> addresses that "kernel_virt_addr+KERN_VIRT_SIZE" overflows.
>>>>
>>>
>>> Based on the same reason, keep KERN_VIRT_SIZE to be -PAGE_OFFSET is good.
>>>
>>>>>    #ifndef __ASSEMBLY__
>>>>>
>>>>> @@ -97,6 +97,7 @@ extern unsigned long pfn_base;
>>>>>    #define ARCH_PFN_OFFSET              (PAGE_OFFSET >> PAGE_SHIFT)
>>>>>    #endif /* CONFIG_MMU */
>>>>>
>>>>> +extern unsigned long kernel_virt_addr;
>>>>>    extern unsigned long max_low_pfn;
>>>>>    extern unsigned long min_low_pfn;
>>>>>
>>>>> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
>>>>> index 271860fc2c3f..d792912c2da3 100644
>>>>> --- a/arch/riscv/kernel/head.S
>>>>> +++ b/arch/riscv/kernel/head.S
>>>>> @@ -131,7 +131,8 @@ clear_bss_done:
>>>>>    #ifdef CONFIG_MMU
>>>>>    relocate:
>>>>>         /* Relocate return address */
>>>>> -     li a1, PAGE_OFFSET
>>>>> +     la a1, kernel_virt_addr
>>>>> +     REG_L a1, 0(a1)
>>>>>         la a2, _start
>>>>>         sub a1, a1, a2
>>>>>         add ra, ra, a1
>>>>> diff --git a/arch/riscv/kernel/vmlinux.lds.S b/arch/riscv/kernel/vmlinux.lds.S
>>>>> index 1e0193ded420..5bf69e9b91e6 100644
>>>>> --- a/arch/riscv/kernel/vmlinux.lds.S
>>>>> +++ b/arch/riscv/kernel/vmlinux.lds.S
>>>>> @@ -4,7 +4,7 @@
>>>>>     * Copyright (C) 2017 SiFive
>>>>>     */
>>>>>
>>>>> -#define LOAD_OFFSET PAGE_OFFSET
>>>>> +#define LOAD_OFFSET CONFIG_PAGE_OFFSET
>>>>>    #include <asm/vmlinux.lds.h>
>>>>>    #include <asm/page.h>
>>>>>    #include <asm/cache.h>
>>>>> @@ -71,9 +71,11 @@ SECTIONS
>>>>>
>>>>>         EXCEPTION_TABLE(0x10)
>>>>>
>>>>> -     .rel.dyn : {
>>>>> -             *(.rel.dyn*)
>>>>> -     }
>>>>> +        .rela.dyn : ALIGN(8) {
>>>>> +             __rela_dyn_start = .;
>>>>> +                *(.rela .rela*)
>>>>> +             __rela_dyn_end = .;
>>>>> +        }
>>>>
>>>> It looks like the indentation is screwed up here: I see a mix of tabs/spaces
>>>> that doesn't match the rest of the file.
>>>>
>>>>>
>>>>>         _end = .;
>>>>>
>>>>> diff --git a/arch/riscv/mm/Makefile b/arch/riscv/mm/Makefile
>>>>> index 50b7af58c566..27593d362248 100644
>>>>> --- a/arch/riscv/mm/Makefile
>>>>> +++ b/arch/riscv/mm/Makefile
>>>>> @@ -1,6 +1,10 @@
>>>>>    # SPDX-License-Identifier: GPL-2.0-only
>>>>>
>>>>>    CFLAGS_init.o := -mcmodel=medany
>>>>> +ifdef CONFIG_RELOCATABLE
>>>>> +CFLAGS_init.o += -fno-pie
>>>>> +endif
>>>>> +
>>>>>    ifdef CONFIG_FTRACE
>>>>>    CFLAGS_REMOVE_init.o = -pg
>>>>>    endif
>>>>> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
>>>>> index 965a8cf4829c..428aee2669aa 100644
>>>>> --- a/arch/riscv/mm/init.c
>>>>> +++ b/arch/riscv/mm/init.c
>>>>> @@ -12,6 +12,9 @@
>>>>>    #include <linux/sizes.h>
>>>>>    #include <linux/of_fdt.h>
>>>>>    #include <linux/libfdt.h>
>>>>> +#ifdef CONFIG_RELOCATABLE
>>>>> +#include <linux/elf.h>
>>>>> +#endif
>>>>>
>>>>>    #include <asm/fixmap.h>
>>>>>    #include <asm/tlbflush.h>
>>>>> @@ -28,6 +31,9 @@ EXPORT_SYMBOL(empty_zero_page);
>>>>>    extern char _start[];
>>>>>    void *dtb_early_va;
>>>>>
>>>>> +unsigned long kernel_virt_addr = _AC(CONFIG_PAGE_OFFSET, UL);
>>>>> +EXPORT_SYMBOL(kernel_virt_addr);
>>>>> +
>>>>>    static void __init zone_sizes_init(void)
>>>>>    {
>>>>>         unsigned long max_zone_pfns[MAX_NR_ZONES] = { 0, };
>>>>> @@ -132,7 +138,8 @@ void __init setup_bootmem(void)
>>>>>                 phys_addr_t end = reg->base + reg->size;
>>>>>
>>>>>                 if (reg->base <= vmlinux_end && vmlinux_end <= end) {
>>>>> -                     mem_size = min(reg->size, (phys_addr_t)-PAGE_OFFSET);
>>>>> +                     mem_size = min(reg->size,
>>>>> +                                    (phys_addr_t)-kernel_virt_addr);
>>>>
>>>> PAGE_OFFSET is kernel_virt_addr, so I don't see any reason to change these --
>>>> they account for a significant fraction of the diff.
>>>>
>>>
>>> kernel_virt_addr would be assigned to a random destination by KASLR,
>>> but here still should be PAGE_OFFSET rather than kernel_virt_addr as
>>> mentioned above.
>>>
>>>>>                         /*
>>>>>                          * Remove memblock from the end of usable area to the
>>>>> @@ -269,7 +276,7 @@ static phys_addr_t __init alloc_pmd(uintptr_t va)
>>>>>         if (mmu_enabled)
>>>>>                 return memblock_phys_alloc(PAGE_SIZE, PAGE_SIZE);
>>>>>
>>>>> -     pmd_num = (va - PAGE_OFFSET) >> PGDIR_SHIFT;
>>>>> +     pmd_num = (va - kernel_virt_addr) >> PGDIR_SHIFT;
>>>
>>> Here is the same, please use PAGE_OFFSET instead of kernel_virt_addr.
>>>
>>>>>         BUG_ON(pmd_num >= NUM_EARLY_PMDS);
>>>>>         return (uintptr_t)&early_pmd[pmd_num * PTRS_PER_PMD];
>>>>>    }
>>>>> @@ -370,6 +377,54 @@ static uintptr_t __init best_map_size(phys_addr_t base, phys_addr_t size)
>>>>>    #error "setup_vm() is called from head.S before relocate so it should not use absolute addressing."
>>>>>    #endif
>>>>>
>>>>> +#ifdef CONFIG_RELOCATABLE
>>>>> +extern unsigned long __rela_dyn_start, __rela_dyn_end;
>>>>> +
>>>>> +#ifdef CONFIG_64BIT
>>>>> +#define Elf_Rela Elf64_Rela
>>>>> +#define Elf_Addr Elf64_Addr
>>>>> +#else
>>>>> +#define Elf_Rela Elf32_Rela
>>>>> +#define Elf_Addr Elf32_Addr
>>>>> +#endif
>>>>> +
>>>>> +void __init relocate_kernel(uintptr_t load_pa)
>>>>> +{
>>>>> +     Elf_Rela *rela = (Elf_Rela *)&__rela_dyn_start;
>>>>> +     uintptr_t link_addr = _AC(CONFIG_PAGE_OFFSET, UL);
>>>>> +     /*
>>>>> +      * This holds the offset between the linked virtual address and the
>>>>> +      * relocated virtual address.
>>>>> +      */
>>>>> +     uintptr_t reloc_offset = kernel_virt_addr - link_addr;
>>>>> +     /*
>>>>> +      * This holds the offset between linked virtual address and physical
>>>>> +      * address whereas va_pa_offset holds the offset between relocated
>>>>> +      * virtual address and physical address.
>>>>> +      */
>>>>> +     uintptr_t va_link_pa_offset = link_addr - load_pa;
>>>>> +
>>>>> +     for ( ; rela < (Elf_Rela *)&__rela_dyn_end; rela++) {
>>>>> +             Elf_Addr addr = (rela->r_offset - va_link_pa_offset);
>>>>> +             Elf_Addr relocated_addr = rela->r_addend;
>>>>> +
>>>>> +             if (rela->r_info != R_RISCV_RELATIVE)
>>>>> +                     continue;
>>>>
>>>> This should at least provide a warning when it encounters an unresolvable
>>>> relocation.  Is it currently stands this just ignores all other runtime
>>>> relocations, and while I can buy the argument there shouldn't be any (though
>>>> I'd expect R_RISCV_{32,64} to show up?) we certainly shouldn't just silently
>>>> skip them.
>>>>
>>>>> +
>>>>> +             /*
>>>>> +              * Make sure to not relocate vdso symbols like rt_sigreturn
>>>>> +              * which are linked from the address 0 in vmlinux since
>>>>> +              * vdso symbol addresses are actually used as an offset from
>>>>> +              * mm->context.vdso in VDSO_OFFSET macro.
>>>>> +              */
>>>>> +             if (relocated_addr >= link_addr)
>>>>> +                     relocated_addr += reloc_offset;
>>>>> +
>>>>> +             *(Elf_Addr *)addr = relocated_addr;
>>>>> +     }
>>>>> +}
>>>>> +#endif
>>>>> +
>>>>>    asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>>>>>    {
>>>>>         uintptr_t va, end_va;
>>>>> @@ -377,9 +432,20 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>>>>>         uintptr_t load_sz = (uintptr_t)(&_end) - load_pa;
>>>>>         uintptr_t map_size = best_map_size(load_pa, MAX_EARLY_MAPPING_SIZE);
>>>>>
>>>>> -     va_pa_offset = PAGE_OFFSET - load_pa;
>>>>> +     va_pa_offset = kernel_virt_addr - load_pa;
>>>>>         pfn_base = PFN_DOWN(load_pa);
>>>>>
>>>>> +#ifdef CONFIG_RELOCATABLE
>>>>> +     /*
>>>>> +      * Early page table uses only one PGDIR, which makes it possible
>>>>> +      * to map 1GB aligned on 1GB: if the relocation offset makes the kernel
>>>>> +      * cross over a 1G boundary, raise a bug since a part of the kernel
>>>>> +      * would not get mapped.
>>>>> +      */
>>>>> +     BUG_ON(SZ_1G - (kernel_virt_addr & (SZ_1G - 1)) < load_sz);
>>>>> +     relocate_kernel(load_pa);
>>>>> +#endif
>>>>> +
>>>>>         /*
>>>>>          * Enforce boot alignment requirements of RV32 and
>>>>>          * RV64 by only allowing PMD or PGD mappings.
>>>>> @@ -387,7 +453,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>>>>>         BUG_ON(map_size == PAGE_SIZE);
>>>>>
>>>>>         /* Sanity check alignment and size */
>>>>> -     BUG_ON((PAGE_OFFSET % PGDIR_SIZE) != 0);
>>>>> +     BUILD_BUG_ON((_AC(CONFIG_PAGE_OFFSET, UL) % PGDIR_SIZE) != 0);
>>>>>         BUG_ON((load_pa % map_size) != 0);
>>>>>         BUG_ON(load_sz > MAX_EARLY_MAPPING_SIZE);
>>>>>
>>>>> @@ -400,13 +466,13 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>>>>>         create_pmd_mapping(fixmap_pmd, FIXADDR_START,
>>>>>                            (uintptr_t)fixmap_pte, PMD_SIZE, PAGE_TABLE);
>>>>>         /* Setup trampoline PGD and PMD */
>>>>> -     create_pgd_mapping(trampoline_pg_dir, PAGE_OFFSET,
>>>>> +     create_pgd_mapping(trampoline_pg_dir, kernel_virt_addr,
>>>>>                            (uintptr_t)trampoline_pmd, PGDIR_SIZE, PAGE_TABLE);
>>>>> -     create_pmd_mapping(trampoline_pmd, PAGE_OFFSET,
>>>>> +     create_pmd_mapping(trampoline_pmd, kernel_virt_addr,
>>>>>                            load_pa, PMD_SIZE, PAGE_KERNEL_EXEC);
>>>>>    #else
>>>>>         /* Setup trampoline PGD */
>>>>> -     create_pgd_mapping(trampoline_pg_dir, PAGE_OFFSET,
>>>>> +     create_pgd_mapping(trampoline_pg_dir, kernel_virt_addr,
>>>>>                            load_pa, PGDIR_SIZE, PAGE_KERNEL_EXEC);
>>>>>    #endif
>>>>>
>>>>> @@ -415,10 +481,10 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
>>>>>          * us to reach paging_init(). We map all memory banks later
>>>>>          * in setup_vm_final() below.
>>>>>          */
>>>>> -     end_va = PAGE_OFFSET + load_sz;
>>>>> -     for (va = PAGE_OFFSET; va < end_va; va += map_size)
>>>>> +     end_va = kernel_virt_addr + load_sz;
>>>>> +     for (va = kernel_virt_addr; va < end_va; va += map_size)
>>>>>                 create_pgd_mapping(early_pg_dir, va,
>>>>> -                                load_pa + (va - PAGE_OFFSET),
>>>>> +                                load_pa + (va - kernel_virt_addr),
>>>>>                                    map_size, PAGE_KERNEL_EXEC);
>>>>>
>>>>>         /* Create fixed mapping for early FDT parsing */
>>>>> @@ -457,9 +523,9 @@ static void __init setup_vm_final(void)
>>>>>                         break;
>>>>>                 if (memblock_is_nomap(reg))
>>>>>                         continue;
>>>>> -             if (start <= __pa(PAGE_OFFSET) &&
>>>>> -                 __pa(PAGE_OFFSET) < end)
>>>>> -                     start = __pa(PAGE_OFFSET);
>>>>> +             if (start <= __pa_symbol(kernel_virt_addr) &&
>>>>> +                 __pa(kernel_virt_addr) < end)
>>>>> +                     start = __pa_symbol(kernel_virt_addr);
>>>
>>> Here is the same, please use PAGE_OFFSET instead of kernel_virt_addr.
>>>
>>>>>
>>>>>                 map_size = best_map_size(start, end - start);
>>>>>                 for (pa = start; pa < end; pa += map_size) {
>>
>> Thanks,
>>
>> Alex

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