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Subject: Re: [PATCH 3/4] LoongArch: Add support for kernel relocation
To:     Huacai Chen <chenhuacai@kernel.org>,
        Xi Ruoyao <xry111@xry111.site>,
        Jinyang He <hejinyang@loongson.cn>
References: <1673255274-18238-1-git-send-email-tangyouling@loongson.cn>
 <1673255274-18238-4-git-send-email-tangyouling@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: <3b281ec1-c432-eb7b-3131-3049a0d0ea95@loongson.cn>
Date:   Wed, 11 Jan 2023 11:18:51 +0800
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On 01/09/2023 05:07 PM, 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
PTR_ADDI	sp, sp, -4 * SZREG      # init stack pointer

This line needs to be removed.

> +
> +	/*
> +	 * 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) {
> +		/* 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)
>