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Subject: Re: [PATCH 3/4] LoongArch: Add support for kernel relocation
To:     Jinyang He <hejinyang@loongson.cn>,
        Huacai Chen <chenhuacai@kernel.org>,
        Xi Ruoyao <xry111@xry111.site>
References: <1673255274-18238-1-git-send-email-tangyouling@loongson.cn>
 <1673255274-18238-4-git-send-email-tangyouling@loongson.cn>
 <76395966-35ca-83b5-7d7c-d6717b39e048@loongson.cn>
Cc:     Xuerui Wang <kernel@xen0n.name>, loongarch@lists.linux.dev,
        linux-kernel@vger.kernel.org
From:   Youling Tang <tangyouling@loongson.cn>
Message-ID: <9cd0895e-f27d-5dae-3774-ffdda100281b@loongson.cn>
Date:   Sat, 14 Jan 2023 15:50:16 +0800
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Hi, Jinyang

On 01/14/2023 02:54 PM, Jinyang He wrote:
>
> On 2023-01-09 17:07, Youling Tang wrote:
>> arch/loongarch/kernel/relocate.c contains the functions necessary to
>> relocate the kernel elsewhere in memory.
>>
>> The kernel makes a copy of itself at the new address. It uses the
>> relocation table inserted by the relocs tool to fix symbol references
>> within the new image.
>>
>> If copy/relocation is successful then the entry point of the new kernel
>> is returned, otherwise fall back to starting the kernel in place.
>>
>> Signed-off-by: Youling Tang <tangyouling@loongson.cn>
>> ---
>>   arch/loongarch/Kconfig              | 15 +++++
>>   arch/loongarch/Makefile             |  5 ++
>>   arch/loongarch/kernel/Makefile      |  2 +
>>   arch/loongarch/kernel/head.S        | 18 ++++++
>>   arch/loongarch/kernel/relocate.c    | 96 +++++++++++++++++++++++++++++
>>   arch/loongarch/kernel/vmlinux.lds.S | 11 +++-
>>   6 files changed, 145 insertions(+), 2 deletions(-)
>>   create mode 100644 arch/loongarch/kernel/relocate.c
>>
>> diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
>> index 9cc8b84f7eb0..089a4695b1b3 100644
>> --- a/arch/loongarch/Kconfig
>> +++ b/arch/loongarch/Kconfig
>> @@ -48,6 +48,7 @@ config LOONGARCH
>>       select ARCH_SUPPORTS_ATOMIC_RMW
>>       select ARCH_SUPPORTS_HUGETLBFS
>>       select ARCH_SUPPORTS_NUMA_BALANCING
>> +    select SYS_SUPPORTS_RELOCATABLE
>>       select ARCH_USE_BUILTIN_BSWAP
>>       select ARCH_USE_CMPXCHG_LOCKREF
>>       select ARCH_USE_QUEUED_RWLOCKS
>> @@ -229,6 +230,11 @@ config SCHED_OMIT_FRAME_POINTER
>>   config AS_HAS_EXPLICIT_RELOCS
>>       def_bool $(as-instr,x:pcalau12i \$t0$(comma)%pc_hi20(x))
>>   +config SYS_SUPPORTS_RELOCATABLE
>> +    bool
>> +    help
>> +      Selected if the platform supports relocating the kernel.
>> +
>>   menu "Kernel type and options"
>>     source "kernel/Kconfig.hz"
>> @@ -474,6 +480,15 @@ config PHYSICAL_START
>>         specified in the "crashkernel=YM@XM" command line boot parameter
>>         passed to the panic-ed kernel).
>>   +config RELOCATABLE
>> +    bool "Relocatable kernel"
>> +    depends on SYS_SUPPORTS_RELOCATABLE
>> +    help
>> +      This builds the 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.
>> +
>>   config SECCOMP
>>       bool "Enable seccomp to safely compute untrusted bytecode"
>>       depends on PROC_FS
>> diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
>> index 4402387d2755..27b5a70ff31c 100644
>> --- a/arch/loongarch/Makefile
>> +++ b/arch/loongarch/Makefile
>> @@ -71,6 +71,11 @@ KBUILD_AFLAGS_MODULE        +=
>> -Wa,-mla-global-with-abs
>>   KBUILD_CFLAGS_MODULE        += -fplt
>> -Wa,-mla-global-with-abs,-mla-local-with-abs
>>   endif
>>   +ifeq ($(CONFIG_RELOCATABLE),y)
>> +LDFLAGS_vmlinux            += -static -pie --no-dynamic-linker -z notext
>> +KBUILD_CFLAGS_KERNEL        += -fPIE
>> +endif
>> +
>>   cflags-y += -ffreestanding
>>   cflags-y += $(call cc-option, -mno-check-zero-division)
>>   diff --git a/arch/loongarch/kernel/Makefile
>> b/arch/loongarch/kernel/Makefile
>> index fcaa024a685e..33787d22e6f4 100644
>> --- a/arch/loongarch/kernel/Makefile
>> +++ b/arch/loongarch/kernel/Makefile
>> @@ -31,6 +31,8 @@ endif
>>   obj-$(CONFIG_MODULES)        += module.o module-sections.o
>>   obj-$(CONFIG_STACKTRACE)    += stacktrace.o
>>   +obj-$(CONFIG_RELOCATABLE)    += relocate.o
>> +
>>   obj-$(CONFIG_PROC_FS)        += proc.o
>>     obj-$(CONFIG_SMP)        += smp.o
>> diff --git a/arch/loongarch/kernel/head.S b/arch/loongarch/kernel/head.S
>> index e8a4bf9d7599..6db1549177ad 100644
>> --- a/arch/loongarch/kernel/head.S
>> +++ b/arch/loongarch/kernel/head.S
>> @@ -88,7 +88,25 @@ SYM_CODE_START(kernel_entry)            # kernel
>> entry point
>>       PTR_ADD        sp, sp, tp
>>       set_saved_sp    sp, t0, t1
>>   +#ifdef CONFIG_RELOCATABLE
>> +    /* Copy kernel and apply the relocations */
>> +    bl        relocate_kernel
>> +
>> +    /* Repoint the sp into the new kernel image */
>> +    PTR_LI        sp, (_THREAD_SIZE - 32 - PT_SIZE)
>> +    PTR_ADD        sp, sp, tp
>> +    set_saved_sp    sp, t0, t1
>> +    PTR_ADDI    sp, sp, -4 * SZREG      # init stack pointer
>> +
>> +    /*
>> +     * relocate_kernel returns the entry point either
>> +     * in the relocated kernel or the original if for
>> +     * some reason relocation failed.
>> +    */
>> +    jr        a0
>> +#else
>>       bl        start_kernel
>> +#endif
>>       ASM_BUG()
>>     SYM_CODE_END(kernel_entry)
>> diff --git a/arch/loongarch/kernel/relocate.c
>> b/arch/loongarch/kernel/relocate.c
>> new file mode 100644
>> index 000000000000..a58551c0698d
>> --- /dev/null
>> +++ b/arch/loongarch/kernel/relocate.c
>> @@ -0,0 +1,96 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +/*
>> + * Support for Kernel relocation at boot time
>> + *
>> + * Copyright (C) 2023 Loongson Technology Corporation Limited
>> + */
>> +
>> +#include <linux/elf.h>
>> +#include <linux/kernel.h>
>> +#include <linux/start_kernel.h>
>> +#include <linux/printk.h>
>> +#include <linux/panic_notifier.h>
>> +#include <asm/bootinfo.h>
>> +#include <asm/inst.h>
>> +#include <asm/sections.h>
>> +
>> +#define RELOCATED(x) ((void *)((long)x + offset))
>> +
>> +extern long __rela_dyn_start;
>> +extern long __rela_dyn_end;
>> +
>> +/*
>> + * Choose a new address for the kernel, for now we'll hard
>> + * code the destination.
>> + */
>> +static inline void __init *determine_relocation_address(void)
>> +{
>> +    return (void *)(CACHE_BASE + 0x02000000);
>> +}
>> +
>> +static inline int __init relocation_addr_valid(void *loc_new)
>> +{
>> +    if ((unsigned long)loc_new & 0x0000ffff) {
>
> Hi, Youling,
>
>
> 12bits-aligned is OK as actually R_LARCH_PCALA_LO12 is absolute.

Yes, 12bits-aligned is enough.

But for the convenience of debugging, is __kaslr_offset more friendly
to align with 1M? (For example, use objdump -d vmlinux --start-address=
(real_addr - __kaslr_offset)).

>
>
> BTW, I think the relocation of kernel looks like the '.so' solve its
> Relocs at user space so that it can be placed any legal address. The
> address is determined first, and relocation make it works on that address.
> Thus, this patch always relocate kernel to 'CACHE_BASE + 0x02000000'
> makes me puzzled. I think the relocation only works when the kernel
> runtime address is different from the link-time address.

IMHO, an immutable offset (eg: 0x1e00000) for the relocation kernel may
not make much sense, it may only be used during debugging to confirm
whether the relocation kernel offset is as expected. (We can remove the
immutable offset code if you think it is unnecessary) A random offset
will be generated when KASLR is enabled.

Thanks,
Youling

>
>
> Thanks,
> Jinyang
>
>
>> +        /* Inappropriately aligned new location */
>> +        return 0;
>> +    }
>> +    if ((unsigned long)loc_new < (unsigned long)_end) {
>> +        /* New location overlaps original kernel */
>> +        return 0;
>> +    }
>> +    return 1;
>> +}
>> +
>> +void *__init relocate_kernel(void)
>> +{
>> +    Elf64_Rela *rela, *rela_end;
>> +    void *loc_new;
>> +    unsigned long kernel_length;
>> +    long offset = 0;
>> +    int res = 1;
>> +    /* Default to original kernel entry point */
>> +    void *kernel_entry = start_kernel;
>> +
>> +    kernel_length = (long)(_end) - (long)(_text);
>> +
>> +    loc_new = determine_relocation_address();
>> +
>> +    /* Sanity check relocation address */
>> +    if (relocation_addr_valid(loc_new))
>> +        offset = (unsigned long)loc_new - (unsigned long)(_text);
>> +
>> +    if (offset) {
>> +        /* Copy the kernel to it's new location */
>> +        memcpy(loc_new, _text, kernel_length);
>> +
>> +        /* Sync the caches ready for execution of new kernel */
>> +        __asm__ __volatile__ (
>> +            "ibar 0 \t\n"
>> +            "dbar 0 \t\n");
>> +
>> +        rela = (Elf64_Rela *)RELOCATED(&__rela_dyn_start);
>> +        rela_end = (Elf64_Rela *)RELOCATED(&__rela_dyn_end);
>> +
>> +        for ( ; rela < rela_end; rela++) {
>> +            Elf64_Addr addr = rela->r_offset;
>> +            Elf64_Addr relocated_addr = rela->r_addend;
>> +
>> +            if (rela->r_info != R_LARCH_RELATIVE)
>> +                continue;
>> +
>> +            if (relocated_addr >= VMLINUX_LOAD_ADDRESS)
>> +                relocated_addr = RELOCATED(relocated_addr);
>> +
>> +            *(Elf64_Addr *)RELOCATED(addr) = relocated_addr;
>> +
>> +        }
>> +
>> +        /* The current thread is now within the relocated image */
>> +        __current_thread_info = RELOCATED(__current_thread_info);
>> +
>> +        /* Return the new kernel's entry point */
>> +        kernel_entry = RELOCATED(start_kernel);
>> +    }
>> +out:
>> +    return kernel_entry;
>> +}
>> diff --git a/arch/loongarch/kernel/vmlinux.lds.S
>> b/arch/loongarch/kernel/vmlinux.lds.S
>> index 733b16e8d55d..aec0b6567d24 100644
>> --- a/arch/loongarch/kernel/vmlinux.lds.S
>> +++ b/arch/loongarch/kernel/vmlinux.lds.S
>> @@ -70,6 +70,8 @@ SECTIONS
>>       .plt : ALIGN(16) { *(.plt) }
>>       .got.plt : ALIGN(16) { *(.got.plt) }
>>   +    .data.rel : { *(.data.rel*) }
>> +
>>       . = ALIGN(PECOFF_SEGMENT_ALIGN);
>>       __init_begin = .;
>>       __inittext_begin = .;
>> @@ -93,8 +95,6 @@ SECTIONS
>>       PERCPU_SECTION(1 << CONFIG_L1_CACHE_SHIFT)
>>   #endif
>>   -    .rela.dyn : ALIGN(8) { *(.rela.dyn) *(.rela*) }
>> -
>>       .init.bss : {
>>           *(.init.bss)
>>       }
>> @@ -107,6 +107,12 @@ SECTIONS
>>       RO_DATA(4096)
>>       RW_DATA(1 << CONFIG_L1_CACHE_SHIFT, PAGE_SIZE, THREAD_SIZE)
>>   +    .rela.dyn : ALIGN(8) {
>> +        __rela_dyn_start = .;
>> +         *(.rela.dyn) *(.rela*)
>> +        __rela_dyn_end = .;
>> +    }
>> +
>>       .sdata : {
>>           *(.sdata)
>>       }
>> @@ -133,6 +139,7 @@ SECTIONS
>>         DISCARDS
>>       /DISCARD/ : {
>> +        *(.dynamic .dynsym .dynstr .hash .gnu.hash)
>>           *(.gnu.attributes)
>>           *(.options)
>>           *(.eh_frame)