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In-Reply-To: <1676018856-26520-5-git-send-email-tangyouling@loongson.cn>
From:   Huacai Chen <chenhuacai@kernel.org>
Date:   Fri, 10 Feb 2023 17:05:52 +0800
Message-ID: <CAAhV-H6NdM014fWuP3=mDMtfroYtfQ9r6orDBXz0D1R99ophQQ@mail.gmail.com>
Subject: Re: [PATCH v4 4/5] LoongArch: Add support for kernel relocation
To:     Youling Tang <tangyouling@loongson.cn>
Cc:     Xi Ruoyao <xry111@xry111.site>, Jinyang He <hejinyang@loongson.cn>,
        Xuerui Wang <kernel@xen0n.name>, loongarch@lists.linux.dev,
        linux-kernel@vger.kernel.org
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Hi, Youling,

On Fri, Feb 10, 2023 at 4:48 PM Youling Tang <tangyouling@loongson.cn> wrote:
>
> This config allows to compile kernel as PIE and to relocate it at
> any virtual address at runtime: this paves the way to KASLR.
> Runtime relocation is possible since relocation metadata are embedded
> into the kernel.
>
> Signed-off-by: Youling Tang <tangyouling@loongson.cn>
> Signed-off-by: Xi Ruoyao <xry111@xry111.site> # Use arch_initcall
> ---
>  arch/loongarch/Kconfig              | 15 ++++++
>  arch/loongarch/Makefile             |  5 ++
>  arch/loongarch/kernel/Makefile      |  2 +
>  arch/loongarch/kernel/head.S        |  5 ++
>  arch/loongarch/kernel/relocate.c    | 78 +++++++++++++++++++++++++++++
>  arch/loongarch/kernel/vmlinux.lds.S | 11 +++-
>  6 files changed, 114 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
We don't need such a Kconfig option since it is always true.

Huacai
>         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 += $(call cc-option, -mno-check-zero-division)
>
> diff --git a/arch/loongarch/kernel/Makefile b/arch/loongarch/kernel/Makefile
> index c8cfbd562921..3341dd5f0926 100644
> --- a/arch/loongarch/kernel/Makefile
> +++ b/arch/loongarch/kernel/Makefile
> @@ -31,6 +31,8 @@ endif
>  obj-$(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 d2ac26b5b22b..499edc80d8ab 100644
> --- a/arch/loongarch/kernel/head.S
> +++ b/arch/loongarch/kernel/head.S
> @@ -86,6 +86,11 @@ SYM_CODE_START(kernel_entry)                 # kernel entry point
>         PTR_ADD         sp, sp, tp
>         set_saved_sp    sp, t0, t1
>
> +#ifdef CONFIG_RELOCATABLE
> +       /* Apply the relocations */
> +       bl              relocate_kernel
> +#endif
> +
>         bl              start_kernel
>         ASM_BUG()
>
> diff --git a/arch/loongarch/kernel/relocate.c b/arch/loongarch/kernel/relocate.c
> new file mode 100644
> index 000000000000..91ce92433bab
> --- /dev/null
> +++ b/arch/loongarch/kernel/relocate.c
> @@ -0,0 +1,78 @@
> +// 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/printk.h>
> +#include <linux/panic_notifier.h>
> +#include <asm/sections.h>
> +
> +#define RELOCATED(x) ((void *)((long)x + reloc_offset))
> +
> +extern long __rela_dyn_start;
> +extern long __rela_dyn_end;
> +
> +static unsigned long reloc_offset;
> +
> +void __init relocate_kernel(void)
> +{
> +       reloc_offset = (unsigned long)_text - VMLINUX_LOAD_ADDRESS;
> +
> +       if (reloc_offset) {
> +               Elf64_Rela *rela, *rela_end;
> +               rela = (Elf64_Rela *)&__rela_dyn_start;
> +               rela_end = (Elf64_Rela *)&__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 =
> +                                       (Elf64_Addr)RELOCATED(relocated_addr);
> +
> +                       *(Elf64_Addr *)RELOCATED(addr) = relocated_addr;
> +               }
> +       }
> +}
> +
> +/*
> + * Show relocation information on panic.
> + */
> +static void show_kernel_relocation(const char *level)
> +{
> +       if (reloc_offset > 0) {
> +               printk(level);
> +               pr_cont("Kernel relocated offset @ 0x%lx\n", reloc_offset);
> +               pr_cont(" .text @ 0x%lx\n", (unsigned long)&_text);
> +               pr_cont(" .data @ 0x%lx\n", (unsigned long)&_sdata);
> +               pr_cont(" .bss  @ 0x%lx\n", (unsigned long)&__bss_start);
> +       }
> +}
> +
> +static int kernel_location_notifier_fn(struct notifier_block *self,
> +                                      unsigned long v, void *p)
> +{
> +       show_kernel_relocation(KERN_EMERG);
> +       return NOTIFY_DONE;
> +}
> +
> +static struct notifier_block kernel_location_notifier = {
> +       .notifier_call = kernel_location_notifier_fn
> +};
> +
> +static int __init register_kernel_offset_dumper(void)
> +{
> +       atomic_notifier_chain_register(&panic_notifier_list,
> +                                      &kernel_location_notifier);
> +       return 0;
> +}
> +
> +arch_initcall(register_kernel_offset_dumper);
> 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)
> --
> 2.37.3
>